CHINAMPA AGRICULTURE - cIRcle UBC

CHINAMPA AGRICULTURE: THE OPERATION OF AN
INTENSIVE PRE INDUSTRIAL RESOURCE
SYSTEM IN THE VALLEY OF MEXICO
by
ALASTAIR J . ROBERTSON
B.A., The University of British Columbia, 1976
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF ARTS
in
THE FACULTY OF GRADUATE STUDIES
(Department of Geography)
We accept this thesis as conforming
to the required standard
THE UNIVERSITY OF BRITISH COLUMBIA
October 1983
©
Alastair J. Robertson
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ABSTRACT
During the 14th and 15th Centuries the complex of marshes and
shallow lakes that occupied the low elevations of the Valley of Mexico
were thoroughly modified by the construction of thousands of small
agricultural platforms known as chinampas, and of an elaborate network
of dykes, canals and aqueducts.
A quantified model of the hydrological
systems of the Valley indicates that under natural conditions water
levels fluctuated to such an extent that the productivity of chinampa
platforms would have been limited.
The larger hydraulic installations
smoothed these fluctuations in the upstream portions of the lake complex
and so permitted the agricultural exploitation of these areas.
On the
platforms seedbedding and other intensive horticultural techniques were
employed which allowed the maximum utilization of the controlled hydraulic environment.
Canals were exploited not only for irrigation water
but also as reservoirs for plant nutrients, which promoted a conservative use of materials in the system as a whole.
Communities of non-
crop plants were maintained on the platform border and these protected
crops from climatic extremes, and may also have been important in regulating populations of insect pests.
The hydraulic installations were
managed by high-ranking officials of the Aztec state, while the chinampa
platforms were to a large extent managed by their cultivators. Although
managerial decisions were made by individuals of widely different rank,
there was a tendency for decision-making to occur at the lowest hierarchic level compatible with the ability of the manager to command the
ii
labour and materials necessary to implement the decision.
This allowed
managers to respond rapidly and efficiently to small variations in
the conditions of the physical systems that they managed.
The available
data indicate that the chinampas produced high and sustained yields
per unit area and did so without requiring large subsidies of energy
and materials.
The physical, ecological and managerial principles of
chinampa agriculture are therefore relevant to the design of modern
agricultural systems that seek to exhibit these general properties.
iii
TABLE OF CONTENTS
Page
ABSTRACT
ii
LIST OF TABLES
vii
LIST OF FIGURES
ix
ACKNOWLEDGEMENT
xi i i
CHAPTER I.
INTRODUCTION
CHAPTER II.
CHAPTER III.
1
THE HYDROLOGY OF THE BASIN OF MEXICO
9
THE COLONIZATION OF THE LAKE COMPLEX
44
Chinampa Agriculture without Water Level Control
44
Chinampa Agriculture and Water Level Control in
Limited Areas
59
Chinampa Agriculture and Water Level Control
in Large Areas
65
Late Aztec Hydraulic Installations
CHAPTER IV.
THE ECOLOGY OF CHINAMPA AGRICULTURE
The Chinampa Platforms
73
92
92
Flora and Fauna
101
Seedbeds and Transplanting
120
Weeding and Pest Control
126
Material Cycles
130
Agricultural Calendar
136
iv
Paje
Yields and Energy Ratios
145
The Principles of Chinampa Agriculture
150
CHAPTER V. THE MANAGEMENT OF CHINAMPA AGRICULTURE
The City State
158
The Confederation and the Tribute State
162
The Lake Aztec State
166
The Construction of Hydraulic Installations
172
The State and its Hydraulic Installations
178
The Management of Agriculture
183
The Evaluation of Aztec Management
188
CHAPTER VI. SUMMARY AND CONCLUSIONS
APPENDIX.
RECONSTRUCTION OF THE 16TH CENTURY HYDROLOGY OF
THE BASIN OF MEXICO
Part I.
211
The Use of Modern Data in Reconstructing 16th
Century Conditions
Part II.
198
212
Derivation of Mean Annual Values for the 16th
Century Hydrological System
222
1.
Drainage Basins
222
2.
Physiographic Regions
222
3.
Precipitation
222
4.
Evapotranspiration
223
5.
Agricultural Evaporation Supplement
224
v
Page
6.
Surface Discharge
225
7.
Undifferentiated Runoff
226
8.
Lake Discharge
226
9.
Mean Annual Totals
226
Estimation of Mean Monthly Distributions
229
1.
Monthly Distribution of Precipitation
229
2.
Monthly Distribution of Surface Discharge
230
3.
Primary and Secondary Evaporation
233
4.
Monthly Distribution of Evaporation
238
5.
Monthly Distribution of Surplus
238
6.
Monthly Distribution of Undifferentiated Runoff
244
7.
Lake Tlahuac and Lake Chalco
251
Estimates of Variation in Lake Stage
262
1.
Assumptions
262
2.
Estimated Variations of Lake Stage with Normal
Part III.
. Part IV.
Precipitation
3.
262
Estimates of Variations of Lake Stage with
Precipitation at Exceptionally High or Low Values
BIBLIOGRAPHY
264
266
vi
LIST OF TABLES
Page
TABLE I.
Absolute Chronology and Period Designations
for the Valley of Mexico
TABLE II.
45
Widths of Canals in the Chinampas of San Luis
and San Gregorio
99
TABLE III.
Common Aquatic Plants of the Chinampas
103
TABLE IV.
Edible and Medicinal Plants of the Chinampas
108
TABLE V.
Common Highland Mesoamerican Crops
112
TABLE VI.
Consumptive Use of Water in Centimeters for
Localities at Low Elevations in the South of
the Valley of Mexico
132
TABLE VII.
Nutrient Content of Organic Fertilizers
134
TABLE VIII.
Aztec Agricultural and Ritual Calendars
139
TABLE IX.
Yields of Shelled Corn, Mixquic
146
TABLE X.
Yields of Saleable Portions of Selected
Vegetables, Mixquic
TABLE XI.
147
Summary of Estimates of 16th Century Mean
Annual Water Balance, Basin of Mexico
TABLE XII.
Stations Used in Calculating Monthly Values
of Climatic Variables
TABLE XIII.
227
231
Primary Evaporation as a Proportion of Total
Evaporation
237
vii
Stations Used in Estimation of Distribution
of Secondary Evaporation
Estimated 16th Century Mean Monthly Water
Balance, Basin of Mexico
Assumed Areas and Elevations of the Antique
Lakes
viii
LIST OF FIGURES
No.
1.
Page
The watershed of the Basin of Mexico in the 16th
Century
2.
10
Estimated annual water transfers in the Basin of
Mexico during the 16th Century
3.
13
Subsidiary drainage basins and physiographic regions
in the Basin of Mexico in the 16th Century
15
4.
Mean annual isohyets in the Basin of Mexico, 1920-1959
17
5.
Mean monthly precipitation averaged for seven stations
throughout the Basin of Mexico
19
6.
Average mean isotherms in the Basin of Mexico, 1920-1958
20
7.
Average monthly temperatures, Moyoguarda, 1951-1975
21
8.
Average monthly potential evaporation, Moyoguarda,
9.
10.
1951-1975
21
Estimated monthly variation in stage of Antique lakes
21
Average minimum isotherms in the Basin of Mexico,
1920-1958
11.
22
Maximum, mean and minimum temperatures, Moyoguarda,
1951-1975
24
12.
Frequency of frosts, Moyoguarda, 1951-1975
25
13.
Mean adjusted potential evaporation in the Basin of
14.
Mexico
27
Daily discharge of Rio Amecameca, 1947
30
ix
No.
Page
15.
Schematic section of spring in Sierra Ajusco
32
16.
Annual precipitation at Tacubaya, 1878-1975
36
17.
Variability and precipitation for selected stations
in the Basin of Mexico
18.
37
Annual values for adjusted potential evaporation and
precipitation at Moyoguarda, 1951-1975
19.
Estimated variations in stage of Lake Chalco for
varying annual precipitation values
20.
50
Estimated variations in the extent and location of
the inundated margins of Lake Chalco
21.
40
52
Length to area ratios of hypothetical dykes in Lakes
Xochimilco and Chalco
64
22.
Chinampa platforms in the vicinity of Xochimilco
67
23.
Chinampas of San Gregorio Atlapulco, on the southern
shore of Lake Xochimilco
69
24.
Lake Aztec hydraulic installations
74
25.
Reconstructions of 16th Century aqueducts
76
26.
The effects of dyking on normal variations in the stage
of the Central and Southern Lakes
27.
Estimated normal variations in the stage of the Southern
and Central Lakes with water levels controlled
28.
81
84
Estimated interannual variations of stage in the
controlled lake complex, without centralized management
x
85
No.
29.
Page
Estimated interannual variations of stage of the
controlled lake complex, with centralized management
30.
Schematic representation of nutrient transport in
chinampa agriculture
31.
137
Principal settlements of the Valley of Mexico
during the 16th Century
32.
89
159
Changing lake levels and precipitation in the Basin
of Mexico, 1600 B.C. to present
213
33.
Geological sections of the Basin of Mexico
218
34.
Location of meteorological stations in the Basin of
Mexico
35.
232
Precipitation and discharge for Xochimilco and
Chalco basins
36.
234
Water balance diagrams for selected stations in the
Basin of Mexico
37.
240
Surplus and discharge for the Basin of Rios Magdalena
and Esclava, Basin of Mexico
38.
242
Surplus and Discharge for Xochimilco and Chalco basins,
Basin of Mexico
39.
243
Estimated mean monthly discharge of Rios de la Compana
and San Francisco, Valley of Mexico
40.
245
Estimated mean monthly discharge of Rio Amecameca,
Chalco basin
246
xi
No.
41.
Page
Observed yields of springs in the Xochimilco and
Chalco basins, 1903-04
42.
248
Graphs used for estimates of monthly distributions of
infiltrated flows (I) and indirect flows (J)
43.
Estimated monthly distribution of surplus and runoff,
Chalco basin
44.
250
250
Observed stage of Lake Tlahuac and estimated inputs of
Lake Chalco
261
xii
ACKNOWLEDGEMENT
I would like to thank the many individuals who have helped me
in the course of this work.
In particular I would like to thank Susan
Penner and Susan Leslie for their unfailing encouragement, and Irene
Hull for preparing the manuscript under difficult conditions.
I would
like to thank Dr. Alfred H. Siemens and Dr. Michael Church who have
been enormously supportive of my work: much of what may be of value
in this thesis stems from their good counsel, while I alone am responsible for its shortcomings.
My special thanks are also due to Alfred
Siemens and to Rick Clark for their personal concern and great efforts
on my behalf:
without them I could not have completed the work and I
am extremely grateful to them.
Finally, I would like to thank those
inhabitants of the chinampa villages who made me welcome and who
instructed me in the intricacies of their ancient and beautiful
landscape.
xi i i
1
CHAPTER I
INTRODUCTION
The industrialized countries have been remarkably successful in
developing agricultural systems with high returns on investments of
scarce inputs.
In the United States, where labour has historically
been in short supply, the productivity of labour in agriculture has
increased enormously in the last century:
in 1880 about five people
could be supplied with food and fibre from the work of a single agricultural labourer, while today the figure is close to 80.
At the same time
there has been a large increase in the output for each unit input of
land:
since 1950 the productivity of land planted to wheat in the
United States has increased twofold, and that of corn by a factor of
2.4.
Similar increases have been accomplished in Western Europe and
other developed regions of the world (Rasmussen 1982:80).
These gains in productivity have been achieved by the massive
substitution of machinery for labour, the extensive use of artificial
fertilizers and biocides, and by the development of varieties of crop
species that thrive in the controlled conditions of modern agriculture.
In order to produce these materials and transport them to the fields,
to maintain and operate machinery and to apply agricultural chemicals
large investments of energy are required.
It is true that compared
to the total amount of energy consumed in the developed nations, energy
use in agriculture is quite small, amounting in the United States to
only 2.8% of the total consumption, and in Western Europe to 4.9%.
Where energy is in adequate supply these investments in agriculture are
2
well justified (F.A.O. 1977:95).
However, there are two major problems
associated with the practice of subsidising agriculture with large
amounts of commercial energy.
First, the cost of energy is increasing
and, to the extent that it is doing so, the high levels of use in
agriculture are becoming increasingly unacceptable.
In the capital-
short regions of the world this factor is already critical, and the
peasant farmers who constitute the majority of the world's agriculturalists are simply unable to pay for the energy that is required to develop
the productivity of their agricultural systems.
Second, although the
yields of agriculture in the developed countries have risen with increasing investments of energy, the rate of increase in production has not
kept pace with the rate of increase of energy use.
In the opinion of
some specialists the current line of development of Western agriculture
has reached the point of diminishing returns, and some alternative line
of development is required if agricultural production is to continue
to increase (Kinne & McClure 1977:24).
For both these reasons, there is a growing interest in certain
aspects of pre-industrial agriculture, in particular, in the fact that
such systems tend to be more efficient in their use of energy than are
those of the developed nations.
As an example, maize production in the
United States returns about 3? units of energy for each unit of energy
invested, while maize production by traditional methods in Mexico and
Guatemala returns about 30 units of energy for each one invested (F.A.O.
1977:93; Fluck & Baird 1980:47).
However, any advantage that these
traditional systems might offer in terms of their efficient use of energy
are offset by the fact that their returns per unit investment of both
land and labour are generally quite low.
The traditional systems of
3
Middle America produce about ? ton/hectare, when fallow land is included
in the calculation, and something of the order of 1 to H tons per
agricultural worker per year (Redfield & Villa 1934:52, 55; Stadelman
1940:103, 147, 149; Steggerda 1941:118; Villa 1945:60, 61). Commercial
corn production in the United States, in contrast, produces about
5 tons/hectare, and about 10 times that much per agricultural labourer
(F.A.O. 1977:95).
Low yields are characteristic of many traditional systems, but
not of all of them.
Some intensive pre-industrial systems have not only
the relatively high energy efficiency that is generally characteristic
of traditional systems, but also yields per unit area that compare
favourably with those of modern agriculture.
It is systems of this
type that are of particular interest because they provide indications
of how modern agriculture might be developed in an environment of scarce
or expensive energy resources.
It is one such system, chinampa cultivation in the Valley of
Mexico, that is the subject of the present thesis.
"Chinampa" is an
indigenous Nahautl term that denotes a small rectangular platform,
constructed of soil and decayed vegetable matter, typically about 10
meters by 100 meters and surrounded on three or four sides by canals or
ditches.
The sides of the platforms are held in place by living willow
trees, and the surfaces are intensively cultivated.
Thousands of these
chinampas were constructed by the Aztecs and their predecessors in the
Valley of Mexico during the 14th and 15th Centuries, together with the
complex hydraulic installations that acted to maintain the water in the
canals and ditches at a more or less constant level.
The chinampas
yielded between three and four metric tons of maize per hectare, as well
4
as a significant quantity of horticultural products, and these yields
compare favourably with those of modern agriculture. Furthermore,
chinampa cultivation had an estimated energy efficiency of between
10:1 and 15:1 which, although lower than the figures for other traditional
systems, is still significantly larger than the 3.5:1 efficiency of modern
practices (derived from Parsons 1976:245).
This thesis is intended to explain how the Aztecs contrived to
produce high yields and, at the same time, to make an efficient use of
energy in chinampa agriculture.
A part of the answer is sought in
purely technical aspects of the system, and the hydrology and ecology of
the chinampas are discussed at length.
However, the question cannot be
resolved solely in technical terms, and certain aspects of Aztec social
and political organization are also considered, with a view to elucidating
the role of management in the accomplishments of chinampa agriculture.
The frame of reference is broad, including elements from both
sides of the traditional division between "man" and "land", but it is
not unbounded. The scope of the thesis is limited by considering the
chinampas as a physical-cultural resource system with the essential
function of transforming the "natural stuff" in the environment of a
culture into materials that are required or desired by the participants
in that culture (Zimmerman 1964:17).
Thus, although the discussion
impinges upon certain aspects of the physical landscape of the Valley
of Mexico, it is restricted to those aspects that have a direct bearing
on the production of crops in the chinampas.
Similarly, discussion of
the social, political and religious life of the Aztecs is limited to
those aspects that were directly related to agriculture.
5
The chinampa system falls quite readily into three major subsystems, namely, the lake complex of the Valley of Mexico and the
hydraulic installations by means of which its function was modified;
the ecological system by means of which nutrients and water available
in the lake complex were transformed into useful agricultural products,
and a management system by means of which the structure of the two
physical subsystems was modified and their functions controlled. Each
-
of these three subsystems is treated in turn, and in each case the
structure of the subsystems is- described and the efficiency with which
they performed their functions is evaluated.
There are numerous problems with the systemic approach to the
study of interactions between a culture and its physical surroundings.
There are few well established research procedures, and the models and
constructs that are employed have not been as thoroughly tested by
experience as would be desirable.
The purview includes subject matter
that has traditionally been the province of both the sciences and the
arts, and there are difficulties in combining the languages,, methodologies and theories of these disparate approaches within a single study.
However, there is one major advantage to the systemic approach that
more than outweighs these disadvantages.
The traditional formulation
for the discussion of any instance of man's use of the land has been to
conceive of the two entities as being distinct and externally related,
and to study the relations between the two.
As Geertz has pointed out,
this formulation must finally result in the gross question of the
extent to which each entity conditions the function of the other, and
elicits the equally gross response:
(Geertz 1971:1-11).
"To a degree, but not completely"
The systemic approach overcomes this problem by
6
conceiving of any instance of man's use of the land not as a relationship between two separate entities, but as a single entity which contains
both physical and cultural elements, which has a coherent organization,
and which performs a discernible function.
The advantage of this formu-
lation over the traditional one is that the question of how a complex
system such as an agricultural system works can be rendered quite incisive.
By conceiving of the combined physical and cultural system as a
single entity one can ask such relatively precise questions as
"How is
the system organized?"; "To what extent and in what manner is it stable?";
"To what degree is it efficient in its use of energy and other inputs?"
or "What mechanisms regulate the functioning of the system?".
The present
paper asks these questions of the chinampa system of cultivation.
A great deal of the material that follows concerns the centralized
and bureaucratic apparatus of the Aztec state and its role in the function
of chinampa agriculture, and this obviously has a relevance to the discussion of Wittfogel's theories of "oriental despotisms" (Wittfogel:1957).
However, in the present work, although note is taken of the work of
Wittfogel and other researchers in this domain, the question of whether
or not the state owes its origins to the development of hydraulic agriculture is not pursued. Nor in fact could this issue be resolved with
reference to chinampa agriculture alone.
The chinampas were not con-
structed until quite late in the history of the Valley of Mexico,
when many of the institutions of the state were already in place.
If
the question was to be pursued with reference to the Valley of Mexico,
then the discussion should include the development of Teotihuacan and
flood-water irrigation and, quite probably, the development of drained
field agriculture in the Lowlands of Mexico as well.
Some aspects of
7
this large issue are relevant to an understanding of the function of
chinampa agriculture and are discussed where appropriate, but the
larger issue itself is not treated explicitly in the present work.
The chinampas are discussed as they existed in the late 15th
and early 16th Centuries, in the period immediately prior to the Spanish
Conquest of Tenochtitlan in 1521.
Evidence for their condition at this
time is drawn from the few small areas of debilitated chinampas that
s t i l l persist in the Valley of Mexico; from those modern data on the
physical condition of the Valley that can be extrapolated back to the
16th Century with some assurance; from the practices of contemporary
Indian agriculturalists in Mexico and Guatemala; from early Spanish
colonial documents and from ethnohistorical and archaeological sources.
There are many problems with these sources.
The extant chinampas are
particularly frustrating as a source of information on the 16th Century
chinampas, because although one knows that some of the forms and practices that can be observed today are the same as ones that existed in
the 16th Century, it is often difficult to distinguish truly indigenous
features from those which have developed since the Spanish Conquest.
Modern data have been used extensively in the reconstruction of the 16th
Century hydrology of the Valley, but because the Valley has undergone
considerable physical modification in the past half millennium modern
data are frequently unrepresentative of past conditions.
The problem
of using these modern data is discussed at length in the appendix to
this thesis.
The other sources of information on pre-Hispanic condi-
tions are frequently unreliable or fragmentary, and on some aspects of
chinampa agriculture we have virtually no information at a l l .
However,
the combination of modern climatic dates of historical records of
8
drainage and land use patterns, and of assumptions based on these
records has allowed a quantified and consistent reconstruction of 16th
Century hydrological conditions.
There is sufficient information on
agricultural practices to allow a broad description of the ecology of the
chinampas, and, although the sources on Aztec management practices are
poor, they are at least sufficient to sketch in the main characteristics
of the organization and administration of chinampa agriculture. The
available sources provide a sufficient basis for a description of the
structure and function of chinampa agriculture, and an explanation of
how the chinampas worked.
9
CHAPTER II
THE HYDROLOGY OF THE BASIN OF MEXICO
The chinampas were constructed in the complex of lakes and
marshes that occupied the low elevations of the Basin of Mexico. In
order to understand the problems of constructing and operating the
chinampas and their associated hydraulic installations, it is necessary
to understand something of the behaviour of the lake complex and of the
hydrological system of which they were a part.
The Valley of Mexico is located on the Mexican Mesa Central,
which is an immense uplifted sedimentary block, split with fractures
and faults.
Since the Lower Tertiary Era, igneous materials have been
extruded through these lines of weakness, covering the fault block, and
dividing it into a number of more or less distinct compartments, of
which the Basin of Mexico is one (Figure T).
It is separated from the
Lerma Valley to the east by the Sierra de Las Cruces, and from the
Puebla Valley to the west by Sierras Rio Frio and Nevada. The south of
the Valley is bounded by Sierra Ajusco, and the north by low ranges of
discontinuous hills.
These Sierras are all the products of vulcanism
that has occurred sporadically since the Mid Tertiary, and that continues
to the present.
Their peaks are generally composite volcanoes, while
their foothills and lower slopes are formed of overlapping and interpenetrating layers of andesites, basalts and volcanic ash.
The floor of
the Valley has filled with alluvial materials, consisting largely of
bentonitic clays with intercalated layers of ash and lava.
10
Figure 1.
The Watershed of the Basin of Mexico in the
16th Century (C.H.C.V.M.1964; Sanders,
Parsons, Santley 1979:Map 20).
11
The Valley is a closed drainage basin, with a total area close
to 9,600 square kilometers.
The peaks that define its boundary are
generally between 3,000 and 4,000 meters above sea level, but are
slightly lower in the north, and in the southeast they reach a maximum
of 5,452 meters in the peak of Popocatepetl.
Above the 2,700 meter
contour approximately, the Basin is truly mountainous, with steep slopes
and thin and stony soils.
Below the 2,700 meter contour the land is
generally hilly, but with several quite large peripheral valleys, including the Avenidas de Pachuca in the north, the Teotihuacan Valley,
and numerous smaller valleys and pockets of level ground. At the
2,300 meter contour there is an abrupt change in relief as the foothills give way to the alluvial plains of the main Valley floor.
These
decline to a minimum elevation of 2,237 meters above sea level, and
comprise about 20 percent of the area of the Basin.
The hydrology of the Basin has been enormously modified since the
16th Century.
It now drains through a breach in its northern wall into
the Rio Lerma system, but at the time of the Conquest it was an entirely
closed drainage basin.
Water that was precipitated on the surrounding
mountains and hills drained towards the lowlying central areas where
it was stored in a complex of lakes and marshes until it was lost from
the system by evaporation.
The main floor of the Basin was occupied
by six more or less distinct bodies of water, known collectively to the
Aztecs.as the Lakes of the Moon.
These lakes were at slightly differ-
ent elevations, with Lakes Zumpango and Xaltocan in the North, and
Lakes Chalco and Xochimilco in the South, all draining into Lake
Texcoco in the centre of the Basin.
The Lake of Mexico in the western
12
part of Lake Texcoco was formed artificially by the construction of the
16 kilometer dyke known as the Albarradon de Nezahualcoyotl.
All of
the.lakes lost water by evaporation, but Texcoco did so at a rather
greater rate than the others and because it was never flushed through
with fresh water it became quite saline.
Figure 2 is a schematic diagram that summarizes the main characteristics of the hydrology of the Basin of Mexico.
It shows estimates
of the volumes of water that in the course of a normal year during the
late 15th and early 16th Centuries passed through the Basin. The
seven major divisions of the chart correspond to the seven major drainage basins that together constitute the Basin of Mexico, and these are
mapped in Figure 3.
These basins are natural watersheds except that
the Mexico-Texcoco basin has been divided into a western and an eastern
section, corresponding with the artificial separation of the Lake of
Mexico from Lake Texcoco.
Inputs of precipitation (P) are indicated on
the left side of individual basins, and outputs by evaporation (E) on
the right.
Each watershed is divided into three physiographic regions,
each of which had quite distinct drainage and evaporation characteristics:
mountains (m), valleys (v) or lakeshore plains (p), and lakes, both
extant and reconstructed (1) are distinguished.
The flow of water
between these various watersheds and physiographic regions is represented
by lines whose thicknesses are directly proportional to the volumes of
water transmitted in the course of a normal year.
The separate basins
were linked in the manner shown except that some seepage may have
occurred between Lakes Chalco and Xochimilco, on one hand, and Lake
Texcoco, on the other, and that the Aztec hydraulic installations allowed
the discharge of water from Lake Xochimilco either directly into Lake
C h a l c o
X o c h i m i l c o
B a s i n
70U
Qm
36
29
658
Rv
353
Qm
12
8
336
628
Qv U l
17 J0|N\
Rv
323
Ev
H i.86
, 769
Sgw,l
©H
30
655
N o r t h e a s t e r n
B a s i n s
El
10 h ©
U
513
P a c h u c a
1282
B a s i n
M e x i c o
T e x c o c o
Figure 2.
Q Pach.
252
PI
o f
B a s i n
Estimated Annual
Water Transfers in
the Basin of Mexico
during the 16th
Century.
For Legend and Notes, see page
following.
Q N.E.
156
El
53
7757
L a k e
Rm
250
775
511
Qv
3 5
Sgw,l
t h e
363
Rm
M5
Pv
326
576
o f
Em
516
Pm
778
547
Pm
+>! 868
Pv
370
B a s i n
B a s i n
C u a u t i t l a n
B a s i n
14
Figure 2:
Legend and Notes
Symbols
(+)
Inputs of water
Basin Boundaries
0
Outputs of water
Direction of Flow
123
Volume in m x 10
123
Depth per unit area in mm
3
6
Large Volume Storage
Line thickness proportional to volume of water.
Variables
P
Precipitation
E
Evapotranspiration
Q
Surface Runoff
A
Consumptive use of
water in agriculture
R
Subsurface and
Undifferentiated Runoff
Physiographic Regions
m
mountains
P
lakeshore plains
v
valleys
1
lakes
Notes
1)
For Precipitation (P), Evapotranspiration (E) and, in the Xochimilco
and Chalco basins only, Surface Runoff (Q). The values shown are
directly confirmed by C.H.C.V.M. publications for basins and
physiographic regions.
2)
For Surface Runoff (Q) in the remaining five basins, the distribution is directly continued by C.H.C.V.M. publications for basins
but not for physiographic regions.
3)
The value for the agricultural evaporation supplement is available
only for the Basin of Mexico as a whole, and is distributed between
subsidiary basins and physiographic regions in a manner not directly
continued by C.H.C.V.M. publications.
4)
The values shown for Runoff (R) are inferred, and are not directly
continued by C.H.C.V.M. publications.
5)
The sources used, and the means by which values for the diagram
were calculated, are described in Appendix Part II.
15
'99"W
Figure 3.
Subsidiary Drainage Basins and Physiographic
Regions of the Basin of Mexico in the 16th
Century (modified from C.H.C.V.M. 1964 V I I I :
333).
16
Texcoco, or via the Lake of Mexico. The chart is based on modern
hydrological and climatic data for the Valley of Mexico, and the problems associated with the use of these data to describe past conditions
are discussed in Part I of the Appendix. Part II of the Appendix gives
an account of the calculations employed in producing the chart.
The Basin is located between 19° and 20° north, and its climate
is essentially tropical, but modified by the effects of elevation.
During the summer months it is supplied with water from the Gulf of
Mexico, transported by the North East Trades, and precipitated by
orographic and convectional lifting mechanisms.
During the winter,
the Gulf is cooler, and the air passing over it is more stable, with
the result that the Trades are deflected to the south by the mountain
barrier, and the Valley receives relatively little moisture. Such
precipitation as does occur during the winter months is the product of
frontal lifting mechanisms, caused by occasional inclusions of cold
polar air.
The distribution of precipitation within the Valley is shown
in the map of mean annual isohyets (Figure 4).
Values range from less
than 500 millimeters to more than 1300 millimeters, with a general
tendency for the lower figures to occur in low elevations towards the
north of the Valley, and the higher figures at high elevations in the
south.
This distribution is clearly reflected in the chart, Figure 2,
where it is evident that the bulk of the water entering the system
occurs as precipitation on the mountains, and that, on a unit area
basis, the mountains in the south receive rather more precipitation
than those in the North.
Precipitation shows a strong seasonal distribution, which is
typical of tropical climates, and this is evident in the plot of mean
Figure 4.
Mean Annual Isohyets in the Basin of Mexico,
1920-1959 (C.H.C.V.M. 1964 VIII:243).
18
monthly values, averaged for a number of stations throughout the Basin
(Figure 5).
The frontal lifting mechanisms of the winter months produce
very little of the year's total.
Values increase steadily through the
Spring, until May, when large volumes of warm, moist air arrive from the
Gulf, and the ground is warm enough for convectional lifting mechanisms
to operate.
Values are high from June through to September, during
which four months a little over 70 percent of the year's total precipitation occurs.
The rainy season is interrupted by a brief, relatively
dry spell during August, which is known locally as the "veranillo" or
"little summer", and which is common throughout eastern and central
parts of Mexico. This phenomenon is related to an interruption of the
Trade Winds by the expansion of low pressure cells lying off the eastern
seaboard of the United States.
During September the sun moves southwards,
the Gulf cools, the Trades cease to ascend the mountains, and precipitation begins its decline to low winter values.
As would be expected at these elevations, temperatures are rather
lower than is normal in the Tropics.
The average mean isotherms show
values ranging from more than 16° to less than 7° (Figure 6).
Again,
the distribution is related to relief and latitude, and temperatures
are generally lower at high elevations, and towards the south of the
Valley.
Temperatures fluctuate in the course of the year with a cycle
that corresponds approximately with the passage of the sun overhead, but
with the summer peaks smoothed somewhat by the effects of the rainy
season (Figure 7).
Mean minimum temperatures in the Valley quite
regularly fall below freezing (Figure 10).
The low values at high
elevations are accounted for by altitude, but they also occur at low
elevations when pockets of cold, stable air are trapped on the Valley
floor.
19
M
M
J
J
0
Figure 5. Mean Monthly Precipitation, averaged for 7
stations throughout the Basin of Mexico
(C.H.C.V.M. Boletfn Hidrologico).
N
D
Figure 6.
Average Mean Isotherms in the Basin of Mexico,
1920-1958 (C.H.C.V.M. 1964 V I I I : 2 4 9 ) .
21
Figure 8. Average Potential Evaporation, Moyoguarda, 1951-1975
(C.H.C.V.M. Boletin Hidrologico n.d.).
E
2241
M
Figure 9.
A
M
J
J
A
S
O
N
Estimated Variations in Stage of Antique Lakes
(Appendix).
D
Figure 10.
Average Minimum Isotherms in the Basin of
Mexico, 1920-1958 (C.H.C.V.M. 1964 VIII:253).
23
Temperatures in the Valley also vary from year to year.
Figure 11
shows data from Moyoguarda, and it can be seen that maximum and minimum
temperatures have a range of 7° and 7.5°, respectively, and that mean
temperatures do not vary by more than 3° for the period shown. The
seasonal distribution of temperatures also varies from year to year, and
because maize is a crop that is particularly susceptible to frost damage,
minimum temperatures are of considerable importance.
Figure 12 shows
the frequency of freezing temperatures during the winter months, again
at Moyoguarda, and it can be seen that frosts are virtually certain
during January and February, that there is a better than even chance
that they occur in November and March, and that occasionally they occur
as early as October or as late as April.
Water is transferred from the ground to the atmosphere either
by evaporation from exposed surfaces or by transpiration from growing
plants.
The two processes together are known as evapotranspiration.
Because the transfer is diffuse, and because it is determined by a large
number of factors relating to the capacity of the atmosphere to absorb
water, the properties of the evaporating surfaces and vegetation from
which water is lost and the availability of water it is extremely difficult to make estimates of the rates at which water is actually lost by
evapotranspiration.
The procedure that has proved most satisfactory
in the Basin of Mexico involves the calculation of actual evapotranspiration rates from daily precipitation and potential evaporation data.
Precipitation is employed as an indicator of the amount of water available for evaporation.
Potential evaporation is conceived of as the
volume of water that would be evaporated i f water was freely available,
and it is approximated as the volumes lost from standard evaporating
Figure 11.
Maximum, Mean and Minimum Temperatures,
Moyoguarda, 1951-1975 (C.H.C.V.M. Boletfn
Hidrologico n.d.).
100
90
~
80
CD
§ 70
60
50
o
c
Q)
Z3
cr
CO
1_
LL
40
30
20
10
0
Figure 12.
0
N
D
J
M
Frequency of Frosts, Moyoguarda, Basin of
Mexico 1951-1975 (C.H.C.V.M. Boletin
Hidrologico n.d.).
S/
26
pans.
The evaporation climate of the mountains and the valleys of the
Basin are quite different, and the C.H.C.V.M. employs different methods
for estimating actual evapotranspiration losses in the two regions.
In
the present reconstruction of 16th Century conditions, C.H.C.V.M.
estimates of actual evapotranspiration losses have been retained in the
mountains throughout the Basin, in the valleys of the Pachuca and Northeastern basins and in the Teotihuacan Valley.
In those areas of the
Basin that were occupied by the complex of lakes and marshes modern
estimates of actual evapotranspiration losses have been replaced with
values for an adjusted potential evaporation rate, which is employed by
the C.H.C.V.M. as an approximation of actual evapotranspiration losses
from areas of open water and saturated soils.
In addition to these
various categories of evaporation, the C.H.C.V.M. makes use of an
agricultural evaporation supplement, intended as an expression of the
consumptive use of water in agriculture.
On the grounds that Aztec
agriculture, although less profligate in its use of water than modern
irrigated agriculture, was more widespread, this supplement has been
retained.
The procedures employed both by the C.H.C.V.M. and the
present writer are described more fully in the Appendix.
Figure 13 shows mean annual values for the adjusted potential
evaporation rate used as an approximation of losses from open water
and saturated soils.
far,
Like the other climatic parameters discussed so
it is evident that evaporation rates are strongly influenced by
both elevation and latitude:
values are lowest at high elevations in
the south of the Basin, and highest at low elevations in the centre and
north of the Basin, reaching a maximum of more than 1500 millimeters
in the areas of Lake Texcoco and Lakes Zumpango and Xaltocan. Actual
27
Figure 13. Mean Annual Adjusted Potential Evaporation in
the Basin of Mexico.
Note: Adjusted Potential Evaporation is equal to 70%
of Potential Evaporation (C.H.C.V.M. 1964 V I I I : 2 6 3 ) .
28
evapotranspiration shows a similar pattern of distribution, but the
range of values is rather greater.
In the mountains where temperatures
are low and drainage is effective in removing water values are between
270 and 425 millimeters, with the higher values in the north.
Values
in the lowlying areas of the Basin are generally between 800 and 1000
millimeters, but reach a maximum of more than 1500 millimeters in the
small areas of open water that remain in the Northeastern basins
(C.H.C.V.M. 1964 VIII:333).
Evaporation rates vary seasonally and Figure 8 (page 21 above)
shows mean monthly potential evaporation rates at Moyoguarda, a station
at low elevations in the Xochimilco basin.
Values are at a minimum
during the winter months when both temperature and precipitation values
are low, and they increase through the spring as temperatures increase.
Peak values occur in April and May, when temperatures are quite high,
but before the rainy season has begun.
During the rainy season evapora-
tion rates remain high but, because the air is moist and has a low
absorption capacity, they do not increase in proportion to the increase
in temperature.
In September and October both precipitation and tempera-
ture values decline, and evaporation rates fall to their low winter
values.
The effects of these evaporation losses on the distribution of
water within the Basin are evident in Figure 2.
The Pachuca and North-
Q
eastern basins receive about 2.3 x 10 cubic meters of precipitation in
the course of a normal year but, as about 90% of this volume is lost by
local evaporation, only modest volumes drain southwards into Lakes
Zumpango and Xaltocan.
These volumes are augmented by drainage off the
slopes of the Sierra de las Cruces, in the Cuautitlan basin but, because
29
evaporation rates are high in Lakes Zumpango and Xaltocan, virtually
all of the incoming volumes are lost, and a meagre 13 x 10 cubic
meters drain southwards into Lake Texcoco.
The Chalco, Xochimilco
q
and Mexico basins receive about 2.4 x 10 cubic meters of precipitation
per year, which is only slightly larger than the figure for the two
northern basins, but evaporation losses are lower, and about 25%
of this volume drains west, and then north, into Lake Texcoco.
The mountains and hills of the Basin are drained by numerous
rivers and streams that converge on the Valley floor.
None of these
watercourses are large, and most descend through narrow, elongated
watersheds as directly to the Valley floor as the tortured topography
of the region allows.
Figure 14 shows daily discharge data for Rio
Amecameca, which drains the slopes of Popocatepetl in the extreme
southeast of the Valley.
There is very little discharge during the
winter months, when precipitation values are low, and in the spring
the flow is intermittent.
Precipitation during the summer months is
convectional and the discharge data show a series of distinct hydrographs, each one of which marks the passage of runoff from an individual
storm.
The catchment area of Rio Amecameca is large compared to other
watercourses in the Basin, and the frequency of storms within the catchment area is relatively high.
individual storms overlap,
out the rainy season.
Because of this the hydrographs of
and Rio Amecameca discharges water through-
In addition, there may be a small baseflow
resulting from snowmelt on Popocatepetl, but this is not evident in this
graph.
In smaller watersheds where the frequency of storms is lower,
and where there is no discernible baseflow, discharge tends to be intermittent during the rainy season, and to cease altogether during the dry
season.
31
It is evident from Figure 2 that measured surface discharge
accounts for only a small part of the volumes of water that drained
from the mountains.
Large areas of the Basin are composed of fractured
basalts and andesites which are extremely porous, and in these areas
most of the drainage is subterranean.
Many streams lose water by
seepage, others disappear entirely into fissures and cracks, and large
volumes of water percolate into the underlying rocks without entering
channeled flow at a l l .
In the Valley today these infiltrated volumes
are extracted by mechanical pumping, but in the past they seeped into
aquifers in the Valley floor, or supplied the numerous springs that
occurred along the margins of the lake complex.
Springs were particu-
larly common along the western shores of the Lake of Mexico, and along
the southern shores of Lakes Chalco and Xochimilco. One of the latter
is shown schematically in Figure 15.
Measurements made in 1903 and
1904, before the aquifers were entirely depleted by pumping, show that
some of the larger springs had minimum yields of more than 800 l.p.s.
(Marroquin y Rivera 1914:151-54).
The subsurface hydrology of the Basin has been enormously modified by pumping, and modern data are of little value in describing preHispanic conditions.
The present reconstruction makes the assumption
that volumes of groundwater stored in the Valley remained approximately
constant over long periods, and from this it follows that all of the
water that percolated into aquifers at higher elevations sooner or later
made its way by seepage or springflow to the Valley floor, and became
available for loss by evaporation.
This is discussed more fully in
Part I of the Appendix, but the assumption is made that infiltration
represented a shift of water within the Basin, and not a loss from it.
Figure 15.
Schematic Section of Spring in Sierra Ajusco
(C.H.C.V.M. 1964 1:27).
33
This assumption is carried through in Figure 2, where all of the
water available in the mountains (defined as Pm - Em - Am) is shown
draining into the lakes (1), lakeshore plains (p) or valleys (v).
The transfer occurs either by surface discharge (Qm) or by undifferentiated runoff (Rm).
For the whole of the Basin, surface discharge
accounts for only 19% of the water that drains off the mountains,
and values for subsidiary basins range from 46% in the Mexico basin,
where the underlying rocks are relatively impermeable, to less than
7% in the combined Xochimilco and Chalco basins, where infiltration
capacities are extremely high.
The remaining volumes are transferred
by undifferentiated runoff (Rm), which is defined as the difference
between the volumes of water available in the mountains and the volumes transferred by surface discharge (Rm = Pm - Em - Am - Qm).
Because undifferentiated runoff is defined in this way, it includes
volumes of water that drain on the surface in overland flow or in
channels that are too small to merit metering, as well as volumes that
drain beneath the surface.
Although the proportion of subsurface flow
to the total flow of undifferentiated runoff cannot be determined with
any precision, the indications are that it is substantial and that a
large part of the drainage of the Valley occurs beneath the surface.
This characteristic of the hydrology of the Basin is important because
it affects lake recharge patterns.
Subsurface flows are very much
slower than those that occur on the surface and, to the extent that the
lakes were recharged by subsurface flows, inputs would have responded
in a very dampened and delayed fashion to variations in precipitation.
Water from the hillsides drained towards the centre of the Basin
and accumulated in the Lake complex.
The supply of water varied from
34
season to season and so, under natural conditions, the levels of the
lakes also fluctuated.
By means of the calculations described in the
Appendix, Part III, it has been possible to produce a simple model of
the normal fluctuations of inputs and outputs of the lake complex
and, from this, to make a rough estimate of normal seasonal variations
in stage.
These estimates are shown for Lakes Chalco, Xochimilco and
Mexico-Texcoco in Figure 9 (page 21 above).
Because so great a propor-
tion of the inputs of the lake complex were in the form of infiltrated
flows, and because these flows are assumed to have lagged behind precipitation, fluctuations in stage were quite out of phase with fluctuations
in precipitation.
During the spring, when evaporation rates and inputs
from the preceding year's precipitation were declining, lake levels
declined, and during the summer at the height of the current year's
rainy season, lake levels were at a minimum.
At the end of the rainy
season evaporation rates declined and water from the current year's
precipitation began to accumulate in the lakes, with the result that
stage increased.
During the winter, when evaporation rates were at
their lowest, inputs peaked, and lake levels reached their maximum.
Once within the lake complex water that was not lost by evaporation from the upstream lakes drained towards Lake Texcoco.
The Chart
clearly indicates that most of the water precipitated on the Northern
basins was lost to the system either by local evaporation or by evaporation from Lakes Zumpango and Xaltocan, and that in normal years, only
small volumes were discharged into Texcoco.
In contrast, relatively
little of the precipitation in the Southern basins was lost by evaporation and large volumes of water drained off the slopes of Sierras Nevada,
Ajusco and Las Cruces into the Southern lakes, and were subsequently
35
discharged into Lake Texcoco.
With its relatively large area and with
evaporation rates estimated in excess of 1600 millimeters per annum, an
estimated 777 million cubic meters of water were lost annually from the
latter lake, and the loss was sufficient to balance the budget of this
closed hydrological system.
Thus far the climate and hydrology of the Basin have been discussed with reference only to mean values but in fact the climate of
the Basin, and the hydrological variables that are derived from i t ,
differ considerably from year to year.
Figure 16 shows annual precipi-
tation figures for Tacubaya. The mean value for the period shown is
699 millimeters, but values in individual years range from a low of
397 millimeters, or 57% of the mean, to a high of 1098 millimeters, or
157% of the mean.
Tacubaya is in no way exceptional in this respect.
Figure 17 illustrates a moderate positive correlation between mean
annual precipitation and precipitation variability.
Not only does the
annual rainfall of the Basin vary considerably from year to year, but
the variation tends to be greater where precipitation values are high.
The variation is not entirely random, as is evident in the 10year overlapping mean for the Tacubaya figures, which show a trend
towards a recurring cycle of about 50 years.
These relatively wet and
dry spells correspond with irregularities in the behaviour of the Thermal
Equator, which are not entirely understood but which modify the behaviour
of the North East Trades, and so modify precipitation in the Valley.
Even although these longer term fluctuations have some effect in
reducing the differences between extreme values in short periods, consecutive years can have quite large differences in annual values.
The year
1957, for example, was a particularly dry one with precipitation only
37
13001
1100
E
JE
c
o
9001
Q.
O
x
x
X
700
500
100
200
Standard
Figure 17.
300
Deviation
Variability and Precipitation, for selected
stations in the Basin of Mexico (calculated
from C.H.C.V.M. Boletfn Hidrologico n.d.).
400
38
72% of the mean, while the following year was unusually wet, with a
value of 149% of the mean.
The variability of precipitation can be expressed as the probability of a given percentage of the mean actually occurring in any
individual year.
These values have been calculated for the central
highlands of Mexico for different precipitation classes (Wallen 1955:81).
The mean annual precipitation for the entire area of the Basin of Mexico
is close to 700 millimeters, and WalleVs calculations indicate that in
any individual year there is a 46% probability that precipitation will
actually equal the mean, a 9% probability that precipitation will equal
71% of the mean, and a 5% probability that it will equal 125% of the
mean.
In other words, precipitation over long periods can be expected
to equal mean values about once every other year, but to be about 25%
above or below mean values about once every six or seven years.
Precipitation varies not only with respect to the total volumes
precipitated within the year, but also with respect to the time of year
at which it occurs.
If the rainy season is defined as comprising those
months during which precipitation is equal to or greater than 10% of
the year's total, then at Tacubaya it normally lasts for four months.
But in 6% of years, it lasts for three months or less, in 30% of years
it lasts for five months, and in 9% of years it lasts for six months.
There is no clear correlation of a long rainy season with high annual
precipitation:
both 1957, which was a dry year, and 1975, which was a
wet one, had rainy seasons of six months.
The starting date of the
rainy season is critical to some aspects of agriculture, and this too
is quite variable.
Data from 15 stations in the south of the Basin
show that in 48% of years the rainy season, as defined, starts in June,
39
but it starts in May in 30% of years, in July in 15% of years, and
occasionally it starts as late as August, or as early as April
(C.H.C.V.M. Boletin Hidrologico n.d.).
Actual evapotranspiration rates appear to vary significantly
from year to year, but the characteristics of the variation are quite
different in different physiographic regions.
The lowlying areas of
the Basin are poorly drained, and because water is freely available it
is the absorption capacity of the atmosphere that is likely to limit
evaporation losses.
In these areas, actual rates can approach potential
rates, and these are likely to be higher in warm dry years than in cool
moist ones.
confirm this.
Some actual evapotranspiration data are available that
The data are for 1957 and 1958, which were exceptionally
dry and wet years, respectively (C.H.C.V.M. 1964 V:257, 259).
In the
valleys, lakeshore plains and lakes there was a difference of 13% of
the mean annual value between the two years, and losses were higher in
the dry year than in the wet one.
Some data for the adjusted potential
evaporation rate that is used to estimate losses from marshes and lakes
are also available, and are shown in Figure 18, together with annual
precipitation values.
The evaporation values, with a standard deviation
of 165.5 vary more widely about the mean than do precipitation values,
which have a standard deviation of 133.3.
The two sets of data are
positively correlated, but with a coefficient of 0.21, the correlation
is extremely weak.
In the area of the lake complex actual evapotrans-
piration losses are approximated by the adjusted potential rate shown
in the figure, and in other lowlying regions of the Basin the actual
rates may be quite close to this adjusted potential rate.
If
Moyoguarda is at all representative of conditions on the valley floor,
Figure 18.
Annual values for adjusted potential evaporation
and precipitation at Moyoguarda, 1951-1975
(C.H.C.V.M. Boleti'n Hidrologico n.d.).
41
then the figures indicate that actual evapotranspiration losses may vary
quite widely from year to year, and that the variation is unrelated to
variations in precipitation.
The situation in the mountains is very different.
Drainage is
efficient in these areas, and it is likely to be the availability of
water rather than the absorption capacity of the atmosphere that limits
evapotranspiration losses.
Under these circumstances, losses would be
higher in wet years than in dry ones, and this is confirmed by the data
for 1957 and 1958 when there was a difference of 21% of the mean between
the two years and losses were consistently highest during the wet year.
Data are not available for prolonged periods, but the indications are
that actual evapotranspiration losses in the mountains may vary considerably from year to year, and that the variability is positively related
to variability in precipitation.
The relationship between annual values of precipitation and of
actual evapotranspiration is important because it is the difference
between these two that determines the volumes of water available for
soil moisture recharge and runoff.
There is, however, very little data
available and it is not possible to describe the relationship in any
detail or with an assurance.
The data available for 1957 and 1958 show
that over the entire area of the Basin there was very little difference
between total actual evapotranspiration losses in the two years, and
amounted to 2.5% of the mean.
In the calculations of annual variability
in lake stage that are described in the appendix, evaporation rates have
been held constant and precipitation has been taken as the source of
variability of inputs to the lake complex.
42
The inputs to the lakes may have been quite highly variable,
but this would not have resulted in correspondingly large variations
in lake stage.
The lakes occupied shallow, saucer shaped depressions
that in the present reconstruction are modeled as shallow spherical
segments (Appendix, Part IV). As a consequence of this geometry, there
would have been a tendency for any increase in lake inputs to be translated into increases in the area of the lake, as well as increases in
stage.
Both stage and area would have been more responsive to variations
in inputs when water levels were low than they would have been when water
levels were high.
During dry periods, relatively modest inputs would
have caused the area of the lake to expand, and stage to increase, but
during wet periods, when the area of the lakes was large, any input
volumes would have been distributed over the whole of the lake, and
increases in stage would have been quite small.
At the same time, the
outputs of the lake were proportional to stage.
Any increases in input
that resulted in an increase of stage would also have resulted in an
increase in output, and in all the lakes except Texcoco, this too would
have acted to reduce the variability of lake stage relative to the
variability of inputs.
These mechanisms would have dampened fluctuations
in inputs, but they would not have eliminated them.
Calculations carried
out in the Appendix indicate that there was a significant seasonal variation in the stage of the lakes, and a rather greater interannual variation.
The chinampas were constructed in the complex of lakes and marshes
that occupied the lowlying areas of the Valley of Mexico, and the problems
of constructing and operating the chinampas were intimately related to
the behaviour of the lake complex and of the hydrological system of
which it was a part.
Virtually all aspects of the hydrological system
43
had some consequence for the development and operation of the chinampas,
but two features were of fundamental importance.
First, inputs of water
varied considerably both from season to season and from year to year.
Much of this variability was translated into fluctuations in stage in
the lake complex.
Agriculture requires as constant an environment as
possible, and the early phases of the development of chinampa agriculture
were centred on the problems of smoothing fluctuations in lake stage.
Second, the Valley of Mexico was a closed drainage basin within which
excess volumes of water could be relocated, but from which they could
not be discharged.
Any reduction in fluctuations in one part of the
lake complex necessarily resulted in increased fluctuations in some
other part of the complex.
As the areas of the chinampas were extended,
they began to impinge on this attribute of the hydrological system,
and the later phases of the development of the chinampas were centred
on the problem of the optimum distribution of fluctuations within the
lake complex.
A great deal of what follows is essentially concerned
with the problem of controlling variability in the closed drainage basin
of the Valley of Mexico.
44
CHAPTER III
THE COLONIZATION OF THE LAKE COMPLEX
It is useful to distinguish three phases in the development of
the chinampas, namely, chinampa agriculture without water level control,
with water level control in limited areas, and with water level control
in whole lakes.
Chronologically these three stages are not well defined,
and they undoubtedly overlapped and merged into each other.
However, the
categories are useful because each stage involved the solution to quite
distinct sets of technical and administrative problems, and each impinged
on quite different aspects of the function of the lake complex.
It should be noted that there is really very little information
on the development of chinampa agriculture.
Much of what follows is
based not on direct evidence, but on inferences from quite general
characteristics of the hydrology and history of the Valley of Mexico.
The new period designations developed by Parsons and others are
used in this thesis, and these are shown together with an absolute
chronology in Table I.
Chinampa agriculture without water level control
The practice of transforming marshes and shallow lakes into
agricultural platforms is unlikely to have originated in the Valley of
Mexico.
There is mounting evidence from the Yucatan that chinampas,
or something very similar, were an important component of Classic
Mayan subsistence and, if this is the case, then the form probably
originated in the lowlands of Tabasco and Veracruz in the remote past,
45
TABLE I
ABSOLUTE CHRONOLOGY AND PERIOD DESIGNATIONS FOR THE
VALLEY OF MEXICO
Approx. Absolute
New Period
Old Period
Chronology
Designation
Designation
Late Horizon
Second Intermediate
Phase 3
Late Aztec
Early Aztec
Second Intermediate
Phase 2
Late Toltec
Second Intermediate
Phase 1
Early Toltec
Middle Horizon
Late Classic
First Intermediate
Phase 4
Early Classic
First Intermediate
Phase 3
Terminal
Formative
First Intermediate
Phase 2
Late
Formative
First Intermediate
Phase 1
Middle
Formative
Early Horizon
Early
Formative
1520
1400
1200
950
700
400
100
A. C.
B. C.
200
500
800
1100
Source:
Parsons 1974:90.
46
and was introduced to the Valley of Mexico, along with so many other
cultural traits, sometime during the Early Horizon (Mathney 1978).
There is some direct evidence that raised platforms may have been
used for agriculture in marshy areas of the Valley during the First
Intermediate, Phase 1.
At a site dating from this period, near Tlaltenco
on the western shores of Lake Texcoco there are the remains of a platform
which was about 2 meters higher than the contemporary lake bed, founded
on a layer of rocks, and held in place by stakes driven into the lake
floor (Palerm 1973:83).
The evidence is not conclusive, but the platform
may have been used for agriculture, and been a precursor of chinampa
cultivation.
There is rather more substantial evidence that chinampa
cultivation was practiced during the Middle Horizon and in Lake Chalco,
near Xico, in Lake Xochimilco, near Culhuacan, and on the eastern
shores of Lake Texcoco there are the remains of a number of platforms
which were very probably used for agriculture, all dating from this
period (Apenes 1940:30; West and Armillas 1950:169; Nunley 1967:578;
Armillas 1971:658).
These scattered pieces of evidence suggest that the practice of
cultivating poorly drained land by use of drainage ditches or raised
platforms may have been known at quite an early date, but this does
not imply that the form was also important at an early date.
Parsons'
thorough analysis of settlement data in the Basin indicates that the
Valley floor was not colonized to any significant extent prior to the
Second Intermediate Phase 1 (Parsons 1974:81-108).
This is the period
immediately following the decline of Teotihuacan, and it is characterized by the dispersal of population throughout the Valley, into areas that
47
had only been sparsely settled during the Middle Horizon.
Two quite
large centres were established on Lake Chalco during this period, one
one the northwest shore of Cona Xico in the centre of the lake, and the
other about 2 kilometers to the southeast of modern Chalco.
In addition
to these, some smaller settlements developed at the foot of Vulcan
Tecutli and around the periphery of Lake Xochimilco. All of these
settlements were at elevations well below the 2250 meter contour, and
well within the area of the lake complex.
Of the centres that were established in the Valley during the
SI-1, Tula became the most important, and during the SI-2 it came to
dominate the Valley.
Populations in the southern and central parts of
the Valley declined during this period, and the sites on Lakes Chalco
and Xochimilco were abandoned.
The reasons for this decline are obscure,
but they may have been associated with the relations between Tula, in the
northwest corner of the Valley of Mexico, and the equally commanding
centre of Cholula in the Puebla Valley, which reduced the southern and
central portions of the Valley of Mexico to a buffer zone between two
powerful and contentious states (Parsons 1974:107).
Towards the end of the SI-2 Tula declined in importance, and the
site was sacked and abandoned sometime during the early part of the
13th Century. This apparently removed a constraint on population
growth in the southern and central parts of the Basin of Mexico and a
number of large settlements were established on and around the shores
of the lake complex, including most of the towns with which we are
familiar through the accounts of the early Spanish colonists.
It is clear that the low elevations of Lake Chalco were first
colonized during the SI-1, but this does not necessarily indicate that
48
the early development of chinampas also occurred at that time. Lorenzo
has presented evidence that mean water levels in the lake complex were
declining during the SI-1 and that by the end of the period they were
considerably lower than they had been three-hundred years earlier
(Lorenzo 1956, cited in Sanders 1970:88).
Because the floor of the
Chalco basin has such gentle gradients, even slight reductions of the
mean water level would have rendered large areas of the lakeshore
plain secure from inundation.
Under these circumstances it could be
that the colonization of the lakeshore plains during the SI-1 consisted
of nothing more than the extension of existing techniques of agriculture
onto the emerging areas of the lakeshore plain.
If this is not the case,
then it is hard to understand why refugees from Teotihuacan, dispersing to areas that were only sparsely populated, should have developed
novel and extremely labourious techniques for cultivating swamps, rather
than simply settling at slightly higher elevations and cultivating
adequately drained land by techniques with which they were already
quite familiar.
The situation during the SI-3 was quite different.
Lorenzo's
researches indicate that water levels were rising, and it would have
been necessary to undertake some form of drainage work even to maintain
existing areas of cultivation.
At the same time the population of the
Valley was increasing quite rapidly, and this would have impeded a
retreat from the Valley floor and stimulated the development of techniques for cultivating high water table land (Parsons 1974:103). For
these reasons it seems probable that chinampa agriculture was developed
during the SI-3, rather than during the SI-1, even although the lowlying
areas of the basin had been colonized at the earlier date.
49
In its natural state the lake complex consisted of a series of ecological
zones arranged more or less concentrically around the centres of the
various lakes, and the character of these zones was broadly defined by
the intersection of hydrological variables with variations in relief.
On the basis of the calculations described in the Appendix it has been
possible to estimate the extent of fluctuations in Lake Chalco, under
uncontrolled conditions.
Figure 19 shows variation in stage over a 10
year period in which years 2 and 8 had precipitation 25% above the mean,
years 4 and 6, 25% below the mean and the remaining years had normal
values.
As the data cited for Tacubaya indicate, these extremes of
precipitation are quite realistic, but their frequency is overrepresented in the figure.
On average the extremes depicted would have been
more likely to have occurred over a 20 or 25 year period, rather than
over the 10 years shown here.
The regime of the lake was dominated by
the Tag time implicit in the infiltrated flows by means of which the lake
acquired the bulk of its input water.
Seasonal maxima and minima in
stage lagged some 5 or 6 months behind precipitation maxima and minima,
with the result that fluctuations in stage and precipitation were almost
exactly out of phase.
The lag times involved also meant that the effects
of a particular year's precipitation became evident only towards the
end of that year, and persisted over the subsequent two or three years.
In this example the high precipitation values in year 2 results in maximum values for stage during the winter of years 3 and 4, and they are
s t i l l evident in the above average values of year 5.
Similarly, the
low precipitation values in year 6 hold stage at below average values
until year 9.
The effects of the geometry of the lake depressions in
smoothing variations when water levels are high is also evident in the
E l e v a t i o n a b o v e
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51
figure.
During year 7 when the mean annual water level was estimated as
2239.2 meters, the range of monthly values was 1.36 meters, while in
year 3 when the mean annual value was 2241.4 meters, the range was only
0.31 meters.
Although the values shown in the figure are only estimates,
it is clear that the regime of the lake was characterized by pronounced
seasonal variations in stage, and by rather larger inter-annual variations, and that the variations tended to be dampened during wet periods
when stage was high, and emphasized during dry periods, when stage was
low.
The operational environment of the early colonists was defined by
the relationship between these fluctuating water levels and the gentle
gradients of the Valley floor.
These interacting variables are shown
schematically in Figure 20, where values for stage are the mean and
extreme values show in Figure 19, and the profile shown for the lake
follows from the assumptions stated in the Appendix, Part IV.1. Under
normal conditions the central portion of the lake would have contained
open water, and would have done so throughout the year.
During dry
periods the area of this standing water would have diminished and
during exceptionally dry periods it would have disappeared altogether.
During wet periods the lake would have expanded.
Surrounding the lakes
there would have been a series of marshy margins.
Those closest to the
lake would have been exposed only during the summers of exceptionally
dry periods.
Those at a slightly greater remove would normally have
been flooded during the winter and exposed during the summer,.but during
exceptionally dry or exceptionally wet periods they would have been either
exposed or inundated throughout the year.
The outer margins would
normally have been exposed throughout the year, but they would have been
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53
flooded during the winters of exceptionally wet periods.
Given the
extremely gentle gradients of the region, these margins would have been
quite wide.
The calculations suggest that, on average, a strip of land
about 700 meters wide would regularly have been liable to inundation,
and beyond this there would have been a further 1000 meters that would
have been flooded occasionally.
On the eastern shores of Lake Chalco,
where gradients are more gentle than the average, the periodically
inundated margins would have been rather wider.
The lake margins would
have been surrounded by a zone of high water tables that was not liable
to inundation and this would have merged into the well-drained outer
lakeshore plain.
The margins and the lakeshore plains were poorly drained and
liable to inundation, but they did offer two great advantages to agriculturalists.
First, they had nutrient-rich soils covered with a
light growth of vegetation that could be removed with neolithic tools.
The most common soils in the Basin are the Chestnuts that are distributed
over the lower slopes of the hillsides.
These are loams or sandy loams,
with a moisture retention capacity in the vicinity of 250 millimeters per
meter depth.
In areas where they are not heavily eroded they are gener-
ally about 1 - 3 meters deep, and rather deeper where they have accumulated
in depressions in the hillsides.
They are quite good agricultural soils
and are amenable to improvement by cultivation practices.
The soils that
occur in the larger subsidiary valleys and in the area of the old lakeshore plains are Chernozems. These are clay loams with a crumbed structure, a moisture retention capacity of about 300 millimeters per meter
depth, and they are high in organic matter.
They are generally quite deep,
between 7 and 8 meters at the mouth of the Teotihuacan Valley, and they
54
are good agricultural soils (Sanders 1970:73).
It is difficult to dis-
tinguish the effects of variation in soil quality on yields from the
effects of differing cultivation practices, but in the Teotihuacan
Valley today the Chernozems of the Valley floor can yield about twice
as much as the Chestnuts of the lower slopes under similar dry fallowing systems, and this gives some indication of the advantage of the
soils of the Valley floor (Charlton 1970:88).
The second advantage of the lake margins for agriculturalists
was that water was present in large volumes at times when precipitation
values were low.
Maize is a crop of the humid lowlands and, where its
cultivation is entirely dependent on rainfall, production is marginal
in areas where precipitation is less than 600 millimeters per year
(Sanders et al. 1979:222-24).
The southern part of the Basin is usually
quite adequately watered and the mean annual precipitation in the areas
of Lakes Xochimilco and Chalco is upwards of 700 millimeters, but in
the variable conditions of the region, values occasionally fall below
600 millimeters.
The Moyoguarda data cited above show values of less
than 600 millimeters in four of the 25 years of the record (Figure 18).
If Lorenzo's evidence is correct and the climate of the Basin was
passing through a dry phase during the Second Intermediate, then the
frequency of dry years may have been higher than this.
In addition,
the cyclical fluctuation that is evident in the precipitation data for
Tacubaya suggests that there would have been periods of twenty or thirty
years when the frequency of dry years would have been higher than usual,
alternating with similar periods of lower than normal frequencies.
Under
these circumstances, some form of irrigation would have been valuable in
reducing the frequency of crop failure due to droughts.
55
Irrigation can be valuable not only in augmenting the volumes of
water available for agriculture but also in adjusting the seasonal distribution of water availability.
Maize is a crop that is particularly
susceptible to damage by frost and it was noted that frosts are common
even at low elevations in the south of the Valley.
Under normal cir-
cumstances maize is planted in April or May at a time of year when
precipitation values are increasing and it is harvested in September
before there is any great danger of frost.
However, the rains are often
late and if planting is delayed until the start of the rains, then the
probability of frost damage at the end of the six-month growing season
increases.
If it so happens that the rains are late in the same year that
frosts are early, the conjunction can be ruinous.
Irrigation is valuable
in these circumstances because it permits planting to take place in April
or May regardless of whether or not the rains have started, and the
harvest can take place six months later before there is any great probability of damage by frost (Sanders et al. 1979:222-24).
The initial phase of the colonization of the Valley floor undoubtedly consisted of the extension of existing techniques of agriculture
onto the relatively well-drained areas of the outer lakeshore plains, but
as the area under cultivation extended into the marshy margins of the lake
complex, novel techniques would have to have been developed for dealing
with the problems of high water tables and occasional inundations.
It might have been possible to cultivate maize on the seasonally
inundated margins of the lakes without any modification at all to local
drainage patterns.
In modern Tabasco there is a seasonally inundated
area of river flood-plain that supports an agricultural system known
56
locally as the "marceno" or "March planting" (Orozco-Segovia and Gliessman
1979: 11 ).
Maize is planted on areas that have been exposed by declining
water levels and it is harvested six months later just before water
levels rise and the ground is again inundated. On occasions the water
levels rise a little early in the season and flood the land before the
corn is harvested, but this appears to create no major difficulties and
the harvest is carried out from boats.
The system is extraordinarily
productive and yields from a single planting are normally between 4 and 5
tons per hectare, and are occasionally double that amount (Orozco-Segovia
and Gliessman 1979:11).
In Tabasco the high yields are a consequence of
the high fertility of the s i l t in which the maize is cultivated.
The practice is not in any way confirmed for the Valley of Mexico
but something similar might have been possible.
Maize could have been
planted quite close to the April shoreline of the lake and, because water
levels declined during the summer and tended not to return to April levels
until the following September, the crop could have been clear of the water
for the period of the growing season and could have been harvested before
the rising water flooded the land.
Apart from access to nutrient-rich
soils the practice would also have allowed an April planting in moist
soils, even if the rains were late.
If one assumes on the evidence of
the modern chinampas that the planting took place on land that was
between 25 centimeters and 50 centimeters above April water levels,
then the schematic diagram indicates that a strip of land about 300
meters.wide would have had the appropriate conditions.
The location and
dimensions of this zone would have been determined by the location of
the April shoreline in any particular year, but a zone with these conditions would have existed in every year.
57
Something like the marceno system may have been practiced in the
Valley of Mexico, but its usefulness would, have been limited by poor
drainage and soil aeration.
The most simple means of reducing these
problems would have been the excavation of trenches.
The configuration
of the chinampas and the practice in marshy areas in other parts of
Middle America today suggest that the ditches would have been excavated
at right angles to the shoreline and about 10 meters or so apart
(West and Armillas 1950:172-73; Vanegas 1978:5).
Ditches would have
permitted the accelerated drainage of water from the lake margins and
by this function alone would have reduced the problems of waterlogging
and increased the utility of seasonally or intermittently inundated
land.
They would have had the additional function of increasing the
area of land in which pre-season irrigation was available: by excavating them to a depth a few centimeters below the mean April water-level,
ditches could have provided standing water for"bucket irrigation in the
drier areas of the outer margins and inner lakeshore plains.
How far
it would have been worth extending these trenches away from the lake
would have depended in part on an evaluation of the cost of excavating
the ditches, in terms of the effort required, against the benefits of
pre-season irrigation.
If one assumes that 20 centimeters of water in
the bottom of a ditch was sufficient for bucket irrigation, that this
depth was required in April, and that it was not worth the effort of
excavating ditches more than one meter deep, then in the conditions
shown in the schematic diagram, it would have been worth extending the
ditches to a distance of about 1 kilometer from the mean shoreline.
However, a further consideration is introduced as soon as any
permanent installation is associated with water levels that vary from
58
year to year.
The ditches could only have been fully utilized when
April water levels were at or close to mean values.
In dry periods,
April water levels would have fallen below the level of the bottom of
the ditches and water would not have been available.
The problem could
have been overcome by increasing the depth of the ditches, but this
would have added to their cost.
In wet periods those areas close to the
mean shoreline would have been flooded and the labour expended in excavating ditches there would have been wasted.
Because water levels varied
from year to year the return on effort expended in excavating ditches
would have been reduced.
The area of cultivable land could also have been increased by
raising soil surfaces in the more frequently inundated margins of the
lake.
The benefits of such an undertaking would have been much the same
as the benefits of excavating ditches, but the costs would have been
higher and would have been exacted in the form of the work required to
transport the necessary volumes of material.
If one assumes that plat-
forms 10 meters wide were separated by ditches 1 meter wide, that the
surfaces of the platforms were 30 centimeters above mean April water
levels and that ditch bottoms were 20 centimeters at least below mean
April water levels, then in the conditions shown in the diagram a strip
of land 10 meters wide, extending 200 meters on either side of the mean
shoreline, would have required the transport of about 670 cubic meters
of material to raise the surface to the necessary elevation.
This work
may have been carried out in some areas but, because the platforms would
have been liable to flooding during wet periods and would have been
without water during dry ones, it seems unlikely that large areas of
raised platforms would have been constructed, and that the early
59
colonization of high water table land would have been largely restricted
to the excavation of ditches.
Chinampa agriculture and water level control in limited areas
The value of permanent installations on the margins of the lakes
was limited by the large year-to-year variations in water levels.
If
water levels could have been held constant, the value of ditches and
agricultural platforms could have been enormously increased.
Lake
stage is a function of the difference between input and output rates
and it can be held constant only when the two rates are matched. In
the categories used in the present discussion, lake inputs were by
direct precipitation, by surface and subsurface discharge from the
hillsides and, in all the lakes except Chalco and Zumpango, by discharge from lakes upstream.
Lake outputs were by evaporation and, in
all the lakes except Texcoco, by discharge to lakes downstream.
Most of these variables were not amenable to regulation.
cipitation is quite beyond control.
Pre-
Evaporation and subsurface drainage
could be modified to some extent by controlling vegetation cover, slope
gradients and soil quality, but the flows are too diffuse to allow any
precise control.
Streamflow could be regulated by check dams but this
would have required the construction and integrated operation of a large
number of small installations and, in all the basins except Mexico and
Cuautitlan, streamflow is too small a proportion of the total input to
the lake complex to have allowed anything but slight modifications to
water levels.
The discharge of upstream lakes was technically amenable to control but, because it involved the exploitation of upstream storage
60
capacity in a manner that was not of benefit to the inhabitants of
upstream watersheds, it also required a degree of coercive political
control.
The small polities of the early colonists were unable to
exercise this type of control over their upstream neighbours and,
because of this, they would not have been able to regulate the discharge of upstream lakes.
This aspect of the development of chinampa
agriculture is discussed more fully below with reference to the growth
of the Mexica state and the management of chinampa agriculture. The
only remaining variable by means of which the level of water in the
lakes could have been controlled was discharge as an output, and it
was this variable that virtually all of the purely hydraulic installations of the lake complex were intended to regulate.
Essentially, lake
levels could be held constant by insuring that all inputs of water were
translated into outputs as rapidly as possible.
To accomplish this
two types of installation were necessary, namely, dykes and canals.
We have very little direct information on early hydraulic installations in the Basin, but it is possible to make some quite plausible
assumptions from more general characteristics of the region. Drainage
canals were probably constructed by straightening and deepening natural
watercourses, and it is possible that the major canals of the 16th
Century were in much the same locations as the original watercourses.
The Canal Amecameca ran along the southern shores of Lakes Xochimilco
and Chalco draining the discharge of Rio Amecameca and other smaller
streams in the south and southeast of the Basin, as well as the discharge of the many springs along the southern shoreline of the two
lakes.
At a point near Xochimilco the canal turned north and flowed
into Lakes Mexico and Texcoco, collecting as it passed the discharge
61
of rivers in the southwestern corner of the Valley (Figure 24).
A second
large canal ran along the northern shores of Lakes Chalco and Xochimilco,
but this may have been a Colonial rather than a pre-Hispanic installation.
Many of the streams that drained the slopes of the Sierra de Las Cruces
were canalized, and.their discharge directed through the Lake of Mexico
into Lake Texcoco (Palerm 1973:113 note 48).
In theory, water levels could have been held constant by the use
of canals above.
If the canals were excavated down to the same level as
the minimum stage of a lake, excess volumes would simply drain out of
the lake and the level would not fluctuate.
In practice, there were
three reasons why water levels would not have been maintained at minimum
values and why fluctuations in stage would not have been controlled by
canals alone.
First, water was required for irrigation, and i f the lakes
were maintained at low levels, water would have been available only in
small areas in the centre of the lakes.
Second, it was noted above,
with reference to the regime of Lake Chalco, that the geometry of the
lake basins acted to dampen, fluctuations in both the stage and the
extent of the lake when water levels were high, and to emphasize them
when water levels were low.
As the aim of the hydraulic installations
was to minimize fluctuations in stage, this property of the lakes of
the complex would have provided an incentive for maintaining stage at
higher rather than lower values.
Finally, in the south of the Valley
at least, it would not have been possible to hold water levels at
minimum values.
The difference in elevation between Lakes Chalco and
Xochimilco, on the one hand, and Lake Texcoco, on the other, was so
slight that high water levels in Lake Texcoco were above the low water
levels in the other two lakes.
Thus, when the levels of Texcoco rose
62
to the level of the water in Lakes Chalco and Xochimilco, the gradient
between them would have been eliminated and the levels of all three
lakes would thereinafter have risen together.
For these reasons, dykes
as well as canals were required for the control of fluctuations in stage.
With dykes, water could be backed up and maintained sufficiently high
above the levels of downstream lakes that the gradient persisted, and
drainage could occur even when stage downstream increased.
With high
water levels, the natural tendency for fluctuations to decrease with
increase in stage would reinforce the effects of the dyke in holding
levels constant and, in the large areas behind the dyke, water could be
stored for irrigation.
The locations of the earliest dykes in the lake complex are
unknown.
There are mentions of early hydraulic works in the vicinity
of Culhuacan and Xochimilco and at^ Xico and Tlapacoya on Lake Chalco
(Palerm 1973:113 note 48).
In addition, the early importance of sites
along the western shores of Lake Mexico-Texcoco suggest that dykes may
have been constructed across the mouths of the many small streams and
rivers that drained the slopes of the Sierra de la Cruces.
Although
there is no more, detailed evidence than this, some sites in the lake
complex were more suitable for dyking than others, and it is probable
that these were the first to be adapted.
The earliest dykes were prob-
ably constructed across the mouths of small indentations in the shoreline of the lake complex and, given the minute scale of traditional
agricultural practices, they were probably only a few meters long.
There are a number of locations in the southern lakes that would have
been suitable for larger dykes.
If one assumes dykes were approximately
the same height, the costs of the dyke in terms of the amount of effort
63
required to construct it would have been roughly proportional to its
length, and its benefits in terms -of the control of fluctuations would
have been roughly proportional to the area contained by the dyke. Thus
the ratio
of the length of the dyke to the area contained is an indica-
tion of the efficiency of any particular installation.
Figure 21 shows
a number of sites in the southern half of the Basin, evaluated in these
terms.
Smooth or protruberant sections of shoreline are clearly unsuit-
able (Dyke No. 1).
Islands near the shoreline and small bays would have
been appropriate for dyking and the hydraulic works mentioned in the
vicinity of Xochimilco and Tlapacoya may have exploited these topographic features (Dykes Nos. 3 and 5).
The dykes across the mouths of
lakes, Nos. 7 and 8, are technically more complex than those with lower
efficiency ratios, and are to be discussed in the following section.
Within the areas contained by the dykes, water could held at a
constant level.
Ditches could be excavated with the assurance that they
would contain water for bucket irrigation in April in all but extraordinarily dry years and,with the assurance that flooding would occur
only in exceptionally wet years, it would have been worth the effort
required to raise soil surfaces above the levels of the water.
To the
extent that dykes and canals were successful in holding water levels
constant the problems of the earlier colonists would have been resolved
and the potential benefits of chinampa agriculture could be realized.
It is not possible to put any precise dates on the construction
of these earlier installations and the development of chinampa agriculture
in restricted areas of the lake complex.
Some small bays and inlets may
have been colonized as early as the Second Intermediate Phase One and
64
No.
Dyke
Length
Enclosed
Area
Ratio
km
1 .
2.
34.
56.
78.
Figure 21.
7-3
3-9
2.3
4.0
3-2
0.8
3-3
2.5
4.6
3.9
3-9
10.0
6.9
3-6
116.8
98.8
0.6
1 . 0
1.7
2.5
2.2
4.5
35-4
39-5
Length to Area ratios of hypothetical
dykes in Lakes Xochimilco and Chalco.
65
others may not have been dyked until after the emergence of the Aztec
state in the Late Horizon.
However, it seems likely that the develop-
ment occurred largely during the Second Intermediate Phase Three, when
population was pressing on carrying capacity at existing technical
levels and when there was an incentive to.solve subsistence problems
with novel agricultural practices.
Chinampa agriculture and water level control in large areas
Dykes across the mouths of bays and inlets would have allowed
control of water levels in limited areas and within these areas chinampas
could have been constructed and operated.
However, these were not the
most suitable sites for dyking and it is evident from Figure 21 that
dykes across the discharge points of lakes would have been very much
more efficient.
In the final phase of the development of chinampa
agriculture, dykes were constructed that controlled water levels in
large areas and that substantially modified the function of the whole of
the lake complex.
Lake Chalco was the most suitable of the lakes for colonization
and it was almost undoubtedly the first of the lakes to be fully adapted
for chinampa agriculture. The lake was shallow and it had a well
defined mouth with a small island at Cuitlahuac that would have facilitated the construction of a dyke.
There was only a slight gradient
between Chalco and Xochimilco so that there was no great natural head of
water to contend with, but it was elevated sufficiently high above the
level of Lake Texcoco so that a drainage gradient persisted in all but
unusually wet periods and water could be discharged whenever it was
expedient to do so.
In general, Lake Chalco bore a basic functional
66
resemblance to the small, independent and open drainage systems that had
been colonized at an earlier date and, apart from issues arising from
the fact that a rather greater area was involved, the colonization of
Lake Chalco did not require the solution to any novel technical problems.
Lake Xochimilco had good water supply and drainage characteristics and it too was well suited to conversion to chinampa agriculture,
but it may not have been quite as well suited as Lake Chalco.
It was
about 1.5 meters above the level of Lake Texcoco and, although the difference in elevation is not great, it would have necessitated the construction of installations that were considerably larger and more robust
than those constructed at Cuitlahuac. Certainly, at the time of the
Conquest the Aztecs had found it necessary to construct two, and possibly
as many as four, separate dykes between the Ixtapalapa peninsula and the
shoreline at Coyoacan.
For this technical reason and for reasons associated with the
failure of the inhabitants of the Xochimilco basin to form a unified
administrative apparatus prior to the ascendancy of the Mexica it is
likely that the mouth of Lake Xochimilco was not dyked until the 15th
Century.
This is not to suggest that Lake Xochimilco was without
chinampas at an earlier date.
Ethnohistorical sources indicate that
there were chinampas in the vicinity of Culhuacan during the early 14th
Century, and the large areas of chinampa cultivation in the vicinity
of Xochimilco were undoubtedly constructed before the Mexica came to
power (Peterson 1962:93; Vaillant 1966:110; Davi.es 1977:37-38). Figure
22 shows some of the extant chinampas in the vicinity of Xochimilco and,
although they may not have been installed prior to the rise of the
Mexica, they are probably among the oldest of the extant chinampas.
Figure 22.
Chinampa platforms in the vicinity of Xochimilco
(CIA Mexicana Aerofoto SA 1977).
68
However, these areas of chinampas are probably best understood as the
product of incremental extensions to controlled areas along the lakeshore, rather than as the infilling of areas contained behind a dyke
at the mouth of the lake.
Even i f the Aztecs were not responsible for
the dyking of the mouth of Lake Xochimilco, they did complete the colonization of the lake, and all the indications are that during the 15th
Century the area was thoroughly reworked.
Existing chinampas were
realigned and tidied up and any areas that had not previously been
colonized were converted to the orderly and uniform chinampas, some of
which can be seen in the vicinity of San Gregorio today (Figure 23).
Because the growth of modern Mexico City has obscured so many
physical traces we know very little of the history of settlement and
chinampa agriculture in that region, but it is clear that the combined
Mexico-Texcoco was of all the lakes in the Basin the least suitable for
conversion to chinampa agriculture.
The model indicates that because
of its greater area natural seasonal fluctuations in stage may have been
less than in the other lakes.
But because above-normal volumes of water
would have been disproportionately accommodated in Lake Mexico-Texcoco
it is likely that the range in values for stage over a period of years
was greater than was the case in the other lakes.
At the same time the
lake lacked a drainage outlet so that all losses were by evaporation,
with the result that the waters of the lake became quite brackish. The
salinity may have been unequally distributed and it is possible that
there was a sufficient flow of fresh water from the slopes of Sierra de
las Cruces and from Lake Xochimilco that the salts did not accumulate in
the western parts of Mexico-Texcoco to the extent that they did elsewhere.
Figure 23.
Chinampas of San Gregorio Atlapulco, on the
southern shore of Lake Xochimilco (CIA Mexicana
Aerofoto SA 1977).
70
The most feasible method of claiming brackish marshes and lakes
for agriculture would have been to construct dykes across the inlets
along the foot of Sierra de las Cruces.
These would have been filled
with the discharge of freshwater from the streams flowing off the h i l l sides and, during periods of seasons of low water levels, the water
could have been discharged into the main body of the lake, flushing the
areas contained by the dykes.
If the operation was repeated sufficiently
often, most of the salts could be washed from the soils, and the area
could be used for agriculture.
The dykes could have been extended in
increments to form quite large compartments that could be flushed in this
manner.
Given the historical importance of the settlements on the west-
ern shores of Lake Mexico-Texcoco it is likely that at least a few
sections of the shoreline had been colonized in this manner prior to
the foundation of Tenochtitlan in the mid 14th Century.
There are also
some indications that parts of the lake itself had been settled at quite
an early date.
Archaeological evidence from Tlatelolco indicates that
the site may have been settled in the 13th Century and there are some
ethnohistorical indications that the island upon which Tenochtitlan was
established was inhabited before the Mexica arrived (Davies 1977:37).
The traditional date for the founding of Tenochtitlan is 1325,
but there has been some confusion concerning the transposition of the
Nahua calendar and it is now thought that a more probable date is 1345
(Davies 1977:37).
Soon after it was founded, dykes were constructed
connecting the new city to the shoreline at Tlacopan, Tepeyac, Ixtapalapa and Coyoacan, with construction probably proceeding in that
order (Madrid-Mendizabel 1946:3).
These radiating dykes divided the
western part of Mexico-Texcoco. into a number of large sectors, each of
71
which could have been subdivided by smaller dykes, filled with fresh
water and claimed for agriculture.
The final stage in this line of
development was the construction of the Albarradon de Nezahualcoyotl
in 1449 which extended for 16 kilometers from a point on the shoreline
to the northeast of Tenochtitlan to the Ixtapalapa peninsula (MadridMendizabel 1946:21).
This strategy for claiming saline portions of the lake complex
could have succeeded as long as adequate supplies of fresh water were
available.
The smaller compartments on the western shores of the lake
could have been supplied by streamflow from Sierra de las Cruces, but
the larger compartments and sectors may have required additional supplies.
The Aztecs constructed a number of aqueducts which may have
been intended to fulfill this function.
These included aqueducts from
the springs of Atzcapotzalco which may have been completed at quite an
early date, the Chapultepec aqueduct, the remains of which can still
be seen in modern Mexico City, and an aqueduct from the springs of
Coyoacan, which was completed in the last years of the 15th Century
(Madrid-Mendizabel 1946:21).
The largest supply of fresh water to the
Lake of Mexico was from Lake Xochimilco. The canals that directed this
water north to Tenochtitlan were important as transport routes, but the
configuration of the system suggests that an equally important function
was the provision of fresh water to Tenochtitlan, both for drinking and
for the operation of chinampa agriculture in the constituent compartments
of the Lake of Mexico.
With these supplements of water, levels in the compartments of
the Lake of Mexico could be maintained above the levels of Lake Texcoco,
but only so long as the level of water in Lake Texcoco did not rise too
72
high.
However, the levels of water in Lake Texcoco fluctuated consider-
ably.
Calculations with the model of the hydrological system suggest
that in normal years the levels may have varied by as much as 70 centimeters and it is recorded that on at least one occasion the level of
Texcoco approached that of Chalco, which was about 2 meters above the
level of the lower lake (see Figure 28 below).
Even under less extreme
circumstances, dykes in the Lake of Mexico would have to have been
constructed to very large tolerances i f they were to remain above the
levels of Lake Texcoco in the majority of years.
The only method of reducing variations in the stage of Lake
Texcoco was to exploit the upstream storage capacity of the lake
complex.
It would have been possible to avert dangerously high water
levels in Lake Texcoco by holding back water in the southern lakes and
in Lakes Zumpango and Xaltocan in the north.
The latter lakes were
more important in this respect than the mean annual figures charted in
the diagram of the hydrological system of the Basin suggest (see Figure
2 above).
In normal years very little water was discharged from the
northern lakes but calculations with the model indicate that in wet
years significantly large volumes did drain southward into Lake Texcoco.
This finding is confirmed by early Spanish colonial documents that single
out Rio Cuautitlan as one of the major causes of flooding in Mexico City
(Madrid-Mendizabel 1946:21).
For these reasons it would have been impor-
tant for the inhabitants of Tenochtitalan to exploit the upstream storage
capacity of the system.
The existence of a dyke at Ecatepec, across the
discharge point of the northern lakes, is not entirely substantiated
for the period prior to the Conquest, but such an installation would have
been important in preventing floods in Tenochtitlan during wet years.
73
There were at least two dykes across the mouth of Lake Xochimilco and
these too would have been valuable in averting damaging rises in the
level of Lake Texcoco.
The fact that these dykes would have been
critical to the development of the Lake of Mexico, while they would
merely have been advantageous to the development of Lake Xochimilco,
suggests that they were constructed in the 14th Century by the Mexica
rather than at an earlier date by the Xochimilca or the Culhua of the
Ixtapalapa peninsula.
Because the Lake of Mexico was the least suitable of the lakes
for conversion to chinampa agriculture and because its colonization
required the solution of quite novel technical problems, it was the last
of the lakes to be reclaimed.
It was settled at a later date than the
other lakes of the system and the work on hydraulic installations continued right up until the time of the Conquest.
The problems of creat-
ing and then reclaiming the Lake of Mexico were enormous, and it is
probable that the Lake was never fully claimed for chinampa agriculture.
Late Aztec Hydraulic Installations
Figure 24 shows the major dykes, aqueducts and canals of the lake
complex as they were in the early part of the 16th Century. The Nahua
word for "dyke" or "causeway" is "cuepotli" which translates literally
as "roadway of turf," and suggests how the earlier and smaller dykes
may have been constructed (de Lameiras 1974:29).
were more, elaborate.
Later installations
The Albarrad6n de Nezahualcoyotl was formed by
driving two parallel rows of stakes into the lakebed and by filling
the space between them with earth, rubble and.aquatic vegetation
(Palerm 1973:83).
The dykes radiating from Tenochtitlan were made of
A.
B.
C.
D.
E.
F.
G.
H.
I.
J,
K.
Tenochtitlan
Tlatelolco
Tepeyac
Culhuacan
Ixtapalapa
Churubusco
Albarradon de Ahuizotl
Albarradon de Nezahualcoyotl
Atzcapotzalco aqueduct
Chapultepec aqueduct
Coyoacan aqueduct
dyke
canal
aqueduct
16thc. lake shore
2250 m contour
Figure 24.
Late Aztec Hydraulic
Installations (Palerm
1973).
75
similar materials, but they were faced with stone and mortar and may
have been plastered as well (Palerm 1973:83).
Many of the dykes served
not only to control water levels but also as transport routes.
The
dyke at Cuitlahuac was wide enough for four horsemen to ride abreast,
and the dyke running from Ixtapalapa to Tenochtitlan was twice as wide
as that (Palerm 1973:46, 49).
We have little direct information on the 16th Century canals, but
the network probably bore a general resemblance to the waterways in
the extant chinampas and consisted of a few larger canals formed by
straightening and deepening natural watercourses and numerous smaller
canals for local drainage, irrigation and transport.
There were a number of aqueducts in the system which consisted of
ditches mounted on causeways.
Figure 25A shows a reconstruction of the
aqueduct from Atzcapotzalco to Tenochtitlan which was built between
1417 and 1427, and which was made of earth and stone held in place by
wooden stakes.
Figure 25B shows, a section of the Chapultepec aqueduct
which took thirteen years to construct, and.which was completed in 1466.
The structure was of wood and stone, held in place with mortar, and
provided with a double channel so that each in turn could be cleaned
without interrupting the flow of water (C.H.C.V.M. 1964 1:149-50; Palerm
1973:52).
The aqueducts were fed from springs and, from the descrip-
tion of the disasters associated with the Coyoacan aqueduct which was
completed in 1497 or 1498, it is evident that the discharge was fed
first from the spring into a pool or reservoir formed by the construction of a retaining wall around the spring and thence to the head of
the aqueduct.
The impression given in the sources is that the retaining
wall was quite high and the reservoir quite deep (Palerm 1973:85).
76
B.
Figure 25.
Chapultepec Aqueduct.
Reconstructions of 16th Century Aqueducts
(C.H.C.V.M. 1964 1:149, 150).
77
At various points in the Lake of Mexico and probably in the other
lakes as well, dykes and aqueducts were breached to allow the passage of
water and, in some of the larger canals, of canoes as well.
In some
instances, the openings were no more than unadorned interruptions in the
line of the dyke, as was the case in the Albarradon de Nezahualcoyotl
(Palerm 1973:34).
Dykes that also served transport needs were equipped
with wooden bridges over the openings as was the case with the dykes
that converged on Tenochtitlan (Palerm 1973:32).
We also have a des-
cription of the Chapultepec aqueduct in which a conduit "as wide as
an ox" conducted the flow of the aqueduct over the water below (Palerm
1973:52).
The dykes maintained water at different levels.
This feature
of the system is most dramatically illustrated by an incident during the
battle for Tenochtitlan when the Aztecs deliberately breached dykes
in the hope that the Spanish invaders would be drowned in the sudden
rush of water that followed (Diaz 1976:317).
Even without such inci-
dents as examples, it is clear from the configuration of the system that
the dykes maintained differences in stage, and it seems probable that
the Aztecs could control these levels quite accurately (Palerm 1973:49).
The sources provide no clear descriptions of the devices used to regulate water levels.
Weirs are technically quite straightforward and
these may have been employed by the Aztecs.
There are some modern
examples of Indian hydraulic systems in which water levels are controlled
by moving and replacing heavy stones in apertures at different elevations in a dyke.
Early Spanish-Nahua dictionaries have quite a large
vocabulary relating to the opening or the blocking-up of apertures, to
the materials employed for this purpose, and to the individuals whose
78
job it was to perform the task, and this suggests that similar devices
may have been used in the lake complex in the 16th Century (de Lameiras
1974:31-34).
Some of the dykes were breached by canals and we know that there
was a great deal of canoe traffic between the various lakes.
This
implies the existence of sluices or locks that would permit the passage
of canoes and, at the same time, maintain differences in water levels,
but the sources offer no adequate descriptions of these mechanisms.
The only direct reference is in a passage of one of Cortez letters,
1
which reads:
Y ya junto la ciudad esta una puente de madera de diez
pasos de anchura y por allf esta abierta la calzada porque
tenga lugar el agua de entrar y salir, porque crece y mengua,
y tambien por fortaleza de la ciudad.porque quitan y ponen
algunas vigas muy leungas y anchas de que la dicha puente
esta hecha, todas las veces que quieren; y de estas hay
muchas por la ciudad.
(Quoted in Palerm 1973:52)
The text is not clear.
The last lines would.seem to suggest some kind
of a drawbridge by. means of which access to the city could be restricted,
but the first lines associate the opening of the bridge with allowing
the water to "come and go," because it "rises and falls," and this would
seem to indicate the existence of some kind of sluice gate. Whatever
the installations were, there were many of them throughout the city.
The configuration of the hydraulic system and the direct evidence
that we have indicate that the Aztecs had some means of regulating
water levels and, at the same time, permitting the passage of canoes.
But without any concrete information we are left to speculate on the
nature of these devices.
79
The chinampa platforms themselves were constructed in the areas
where water levels were controlled and we know that virtually the whole
area of both Lakes Xochimilco and Chalco were converted to chinampa
agriculture.
Chinampas were also constructed in the Lake of Mexico, but
because the area is now concealed by Mexico City it is not clear how
thoroughly the lake was colonized (Calnek 1972).
Sections along the.
western shoreline and in the vicinity of Tenochtitlan and other towns
on the lake were converted to chinampas for both agricultural and
residential purposes.
The description of the bridge in the Chapultepec
aqueduct includes the note that the water in the canal below the aqueduct was saline, which suggests that something less than the whole of
the lake was colonized (Palerm 1973:52).
In addition to the areas in
the southern lakes, there are indications that chinampa agriculture
was practiced on the eastern shores on Lake Texcoco, at the mouth of
the Teotihuacan Valley, and in restricted areas in Lakes Zumpango and
Xaltocan. (Apenes 1943; Gibson 1964:268; Palerm 1973).
The: map of the hydraulic installations of the Basin at the time
of the Conquest is.in effect a diagram of the structure of the system.
Taken in conjunction with the model of the hydrological system of the
Basin that is described in the Appendix it is possible to infer a great
deal about the manner in which these various installations functioned.
The dykes across the mouths of Lakes Chalco and Xochimilco and
the Albarradon de Nezahualcoyotl did not simply maintain upstream water
levels constant as did the earlier dykes across the mouths of small bays
and inlets, but they quite radically altered the function of the lake
complex as a whole.
Water levels were a function of the volumes of
water stored within the complex.
These volumes were determined by
80
differences in the rates of precipitation and evaporation for in the
whole of the Basin of Mexico they were entirely beyond control.
Levels
in upstream portions of the lake complex could be controlled by passing
surplus volumes downstream, but the surplus volumes were not eliminated
from the lake complex, they were only relocated within it and caused
the water levels to rise in the downstream lakes.
Any decrease in the
fluctuations in one part of the system could only be accomplished by
increasing fluctuations in some other part.
Figure 26A shows an estimate of normal seasonal fluctuations in
the four southern lakes of the complex under natural and uncontrolled
conditions.
Figure 26B shows exactly the same data, but with the addition
of a dyke at Cuitlahuac which held levels in Lake Chalco as close to
constant as possible.
Because evaporation rates in Lake Chalco during
the early part: of the summer were slightly higher than the sum of all
inputs, levels-declined in spite of the dyke, but the decline was only
slight.
The water that in uncontrolled conditions had been stored in
Lake Chalco was now distributed between the other three lakes, with the
result that their levels fluctuated more widely than they had before
the dyke was in place.
Figure 26C shows the same data again, but with
the addition of a further dyke at the mouth of Lake Xochimilco.
Because
evaporation rates in Xochimilco are at no season of the year greater
than input rates, the levels in that lake could be held quite constant.
Lakes Mexico and Texcoco, on the other hand, had to accommodate the
excess from both of the upstream lakes and their levels fluctuated yet
more widely.
Figure 26D shows the effect of the construction of the
Albarradon de Nezahualcoyotl on the function of the system.
It can be
seen that the dyke would only have been marginally effective in controlling
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The Effects of Dyking on Normal Variations
in the stage of the central and southern
lakes (continued on following page).
(Appendix Parts III and IV).
82
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83
fluctuations in the Lake of Mexico.
Evaporation rates in the Lake of
Mexico are high; during the spring and summer they greatly exceed inputs
with the result that the levels fluctuated considerably.
Lake Texcoco,
the final destination of all the excess water in the system, fluctuated
very widely.
The lakes were linked in series and, with the exception
of Lake Chalco, it was technically possible to control levels in any
of the lakes not only by regulating outputs downstream, but also by
regulating inputs from lakes upstream.
Figure 26E, for example, indi-
cates that it would have been possible to hold the levels of Mexico
quite constant by exploiting the storage capacity of Lake Xochimilco.
Figure 27 shows the same data as Figure 26D but with an indication of the relative elevations of the four lakes.
The differences in
elevation are taken from.18th Century figures, but they do not conflict
with what we do know of the lake complex in the 16th Century (Sanders
1976).
The dyke at Cuitlahuac held median levels of Lake Chalco some
50 centimeters above the level of Lake Xochimilco, and Lake Xochimilco
was held 1.5 meters above the level of the Lake of Mexico. The Albarradon de Nezahualcoyotl held the Lake of Mexico 50 centimeters above the
level of Lake Texcoco.
In normal years quite low dykes would have been sufficient to
maintain differences in water levels, but in wet years the complex was
liable to flooding.
Figure 28 shows the same four lakes and their
relative elevations, but it shows the variations in lake levels over a
period of three consecutive years, in which year 1 had precipitation 25%
in excess of normal values, year 2 had precipitation 25% below normal
values, and year 3 had normal precipitation.
The period is assumed
to have been preceded by a series of normal years.
The variations
84
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2240
X o c h i m i l c o
2239 H
M e x i c o
UJ 2238 A
T e x c o c o
2237
1
J
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Figure 27.
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1
A
1
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Estimated Normal Variations in the Stage of the
Southern and Central Lakes with water levels
controlled (Appendix Parts III and IV).
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Chalco mean stage
Xochimilco dyke
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Estimated Inter-annual variations of stage in the
controlled Lake Complex without centralized Management
(Appendix Parts III and IV).
CO
CJ1
86
in precipitation are large, but they are not entirely unrealistic, and
the extremes shown would probably have occurred in the course of a 27
year period.
It is also assumed that the dykes projected 50 centimeters
above median upstream water levels and that they could accommodate a
rise of this much before their storage capacity was exhausted.
It is
assumed that by means of the canal network linking Lakes Xochimilco,
Mexico and Texcoco the discharge of the former could have been delivered
to either of the latter at the discretion of the managers of the system.
Lakes Zumpango and Xaltocan in the north of the lake complex are not
shown, but the discharge of the Cuautitlan Basin is included in the
inputs of Lake Texcoco (Appendix Parts III and IV).
At outset the level of Texocois at its normal highwater mark
and the levels of the other three lakes are at their median values.
During the spring and early summer of year 1 , when evaporation rates
are high and before the current year's precipitation has taken effect,
levels in all the lakes except Xochimilco decline, with Chalco reaching
minimum levels in May, and Mexico and Texcoco in June.
In June or July
the effects of the current year's precipitation begin to take effect and
levels begin to rise.
In July the levels in Chalco and Mexico reach
their median values, and excess volumes are passed downstream to Texcoco,
whose levels rise accordingly.
In September the level of Texcoco rises
to the median value of Mexico; thereinafter the gradient between the
two is eliminated, and their levels rise in tandem.
In the same month
the storage capacity of the two northern lakes is exceeded and the
discharge of the Cuautitlan basin drains into Texcoco.
The excessive
precipitation of year 1 continues to accumulate in the lake complex and
water levels continue to rise.
In October of year 1 the levels of
87
Mexico and Texcoco rise above the level of the Albarradon de Nezahualcoyotl and the saline waters of Texcoco can mingle with the fresh waters
of the Lake of Mexico. Levels continue to rise during the fall and
winter of year 1 . In February of year 2, five months after the peak in
precipitation, lake storage reaches its maximum and the levels of Mexico
and Texcoco come very close to the level of Xochimilco. In April of
year 2 evaporation rates exceed input rates and levels begin to decline.
In May the levels of Chalco fall slightly and reach a minimum in June,
after which water is retained until the median level is reestablished.
Levels in Mexico and Texcoco decline steadily during the spring of
year 2, but the rate of decline decreases as the effects of the precipitation begin to accumulate in the lake complex and, during the last part
of year 2 and the early part of year 3, the rate of decline is very
low.
As a result of the low precipitation of year 2, inputs to the lake
complex in the early part of year 3 are not sufficiently large to
offset evaporation losses.
In Chalco stage declines earlier than in
the preceding year and to a lower value, and in Xochimilco there is a
slight fluctuation in stage.
The stage of Mexico and Texcoco declines
quite steeply, and in June they fall below the elevation of the dyke
separating the two.
Stage is at a minimum in July, at which time the
current year's precipitation begins to take effect, and levels rise
towards their winter peak.
During the three-year period shown in Figure 28, the level of
each lake was held as close to median values as possible, and the
managers of the individual basins were assumed to have acted in an
uncoordinated and self-interested fashion.
Managers in Chalco and
Xochimilco simply discharged unwanted volumes downstream, and managers
88
in Mexico could do little other than tolerate the resulting floods.
Figure 29 assumes the same precipitation values as in the preceding
figure but, in this case, the managers of the Mexico basin are assumed
to have control of the upstream lakes, and the storage capacity of
Xochimilco and Chalco are exploited to reduce flooding, in Mexico. The
results are similar to those shown in the preceding figure, but there
are some significant differences.
In the first year, the stage of Mexico is held constant for
9 months rather than 5 as in the preceding diagram.
In the second year
the stage of Mexico and Texcoco is above the level of the dyke that
separates them for 16 rather than 22 months, and the maximum height of
the flood is some 30 centimeters less than in the preceding figure.
The differences are not very great, but in less extreme conditions than
those depicted here, the capacity to increase the time during which the
stage of Mexico could have been held constant and to reduce the height
and duration of flooding may have been critical for the inhabitants of
the Lake of Mexico.
The diagrams, and the model from which they are derived are only
approximations of 16th Century conditions, but they do provide an insight
into the functioning of Aztec hydraulic installations.
evidently quite complex:
The system was
in the model there are several alternatives
available to the manager with regard to the routing of water, the location
1
of storage and the timing of discharge; these provided a number of
options for dealing with any particular contingency.
The real system was
very much more complex and the range of options open to the Aztec managers
correspondingly larger.
The system required a considerable level of
Chalco
Chalco dyke
2241
CD
>
Chalco mean stage
Xochimilco dyke
Xochimilco
o
CD
CO
c
o
]- Xochimilco mean stage
2240
CD
E
CD
>
O
JD
O
c
o
I- Mexico dyke
"a 2239
>
LU
[• Mexico mean stage
U
2238
J FMAMJ
Figure 29.
J ASONDJFMAMJ
JASONDJ
FMAMJ
Texcoco mean stage
JASOND
Estimated Interannual Variations in Stage of the Controlled
Lake Complex, with Centralized Management (Appendix, Parts
III and IV).
CO
90
experience and skill on the part of the managers.
This involved not
only experience of the consequences of particular decisions on the
function of the system, but also experience in relating present observations to the future behaviour of the system.
Because there was a lag
between precipitation and the resulting variations in lake storage the
possibility of anticipating future stage from present precipitation
existed.
Because stage was not simply determined by precipitation, but
related as well to volumes of water already within the hydrological
system, skill was needed in making inferences of the moisture content
of the Valley from such indicators as spring flow or the condition of
plants, and in relating these to the future condition of the lake
complex.
The system also required decision-making not on the basis of
achieving simple or unique goals, but on the evaluation of the relative
costs.and benefits of different strategies.
In the simplified and
extreme situation shown in the diagram, the major choice was between
flooding Mexico to a great depth for a long period, but maintaining the
production of Xochimilco largely uninterrupted or reducing the extent
of flooding in Mexico, but increasing the extent of the disruption of
production in Xochimilco. The Aztec managers would have had to reconcile
a very much more complicated set of costs and benefits.
The diagram makes it clear that floods occurred when the storage
capacity of the lake complex was exhausted, and it was the availability
of storage that finally set limits on the expansion of chinampa agriculture.
In this closed drainage basin the area within which water levels
were controlled could only be increased by reducing the areas in which
water levels could fluctuate freely, by reducing the storage capacity of
the system and by increasing the probability of flooding.
Chinampa
91
agriculture could only be extended to the point where the benefits of
increased production were offset by the increased probability of flooding.
Whether or not the Aztecs reached this point is uncertain. At
the time of the Conquest the Valley had a population of about 300,000,
which was very much higher than it had ever been before.
The historical
record shows a high incidence of famines and it has been estimated that
a serious disruption of production occurred about once every four years
(Harris 1977:147ff).
The indications are that the population was press-
ing very hard on the carrying capacity of the region and, under these
circumstances, it is at least possible that the Aztecs were tempted to
increase the area of chinampa agriculture up to or beyond the limits
imposed by available storage capacity and that they, like so many of
their unfortunate descendants, had no alternative but to overexploit
their resource base.
92
CHAPTER IV
THE ECOLOGY OF CHINAMPA AGRICULTURE
The modified lake system was one of the two major physical
subsystems of chinampa agriculture.
individual chinampa platforms.
The otherwas the multitude of
The basic characteristics of these
major subsystems are quite different.
The lake complex was a large and
unique entity in which each of the lakes had different characteristics
and each of the major dykes or canals performed distinctly different
functions.
The chinampa platforms, in contrast, were small and numer-
ous and, in terms of their micro-relief and biota, each one was very
similar to all the others.
Where the focus of the preceding sections
was on the large areas of the lake complex, the focus of the following
pages is on the ecology of a typical, small chinampa platform.
The Chinampa Platforms
Prescott, in his Conquest of Mexico, published in 1843 has this
to say about the chinampas:
The chinampas, that archipelago of wandering islands . . . have
nearly disappeared. These had their origin in the detached
masses of earth, which, loosened from the shores, were still
held together by the fibrous roots with which they were penetrated. The primitive Aztecs,.in their poverty of land, availed
themselves of the hint thus afforded by nature. They constructed
rafts of reeds, rushes and other fibrous materials, which, tightly
knit together, formed a sufficient basis for the sediment that
they drew from the bottom of the lake. Gradually islands were
formed, two or three hundred feet in length, and three or four
feet in depth, with a rich stimulated soil, on which the economical Indian raised his vegetables and flowers for the market of
Tenochtitlan. Some of these chinampas were firm enough to allow
93
the growth of small trees, and to sustain a hut for the residence
of the person that had charge of i t , who with a long pole, resting
on the sides or bottom of the shallow basin, could change the
position of his little territory at pleasure which with its rich
freight of vegetable stores was seen moving like some enchanted
island over the water.
(Cited in Willey 1939:86).
The idea that the chinampas once floated, but have since become rooted,
is widespread in both popular and academic literature.
In parts of
the lake complex today there are dense mats of floating vegetation and,
i f the load is distributed with boards or bundles of twigs, it is
possible to walk on them, and so it may be true that nature offered a
hint to the primitive Aztecs.
However, the idea that anything resemb-
ling the extant chinampa platforms once floated is certainly untrue.
Some of the early Spanish visitors to the Valley of Mexico
described rafts with flowers and vegetables growing on them. They
may have been describing canoes loaded with seedlings that were being
transported from one place to another, or they may have been describing
floating seedbeds as opposed to floating chinampa platforms.
Floating
seedbeds are quite feasible, and in Burma today plants are propagated
on a thin layer of soil distributed over the surface of mats of floating
aquatic vegetation.
Although the practice is not known in the modern
chinampas, it could have occurred in the past (National Geographic 1974:835
-838). Clavijero, writing his Historia de Mexico in 1780 made use of
the earlier sources, but he confused descriptions of boats loaded with
seedlings or of floating seedbeds with descriptions of the chinampas
themselves and generated a description of the chinampas that is the
basis of Prescott's account.
The misunderstanding was endorsed by no
less an authority than Humboldt in his
Political Essay,
and therein-
after it became firmly entrenched in the literature on the Valley of
94
Mexico.
Leicht and Wi1 ley have both shown how the confusion arose, and
several modern authors have demonstrated that the chinampas did not
float and could never have done so, but the idea is appealing and
tourists continue to be lured to the "Jardines Flotantes" in Xochimilco
and continue to be disappointed when they find that the gardens do not
float (Leicht 1937; Wilken 1979).
Chinampas are no longer constructed in the Valley of Mexico, but
it is probable that the earliest chinampas were formed by the excavation
of ditches into high water table land, with the spoil from the ditches
being distributed over the intervening soil surfaces.
Palerm distin-
guishes chinampas formed in this fashion as "chinampas tierra adentro"
or inland chinampas (Palerm 1973:238).
As the cultivable area was extended into increasingly high water
table land, spoil from the ditches became insufficient to raise the
soil surface to the necessary elevation, and volumes were supplemented
by additions of the water weed and soil.
The weeds grew in dense and
interwoven mats, known as "cespedes", and sections of these mats were
cut and towed to the site of the chinampas where they were sunk with
loads, of earth and turf ferried out from the shore.
The operation was
repeated until the material accumulated to a sufficient height about
the level of the water (Armillas 1971:653; West and Armillas 1950:174-76).
The further the chinampas were extended from the shoreline, the greater
the volume of the material required, and where water was a meter or so
deep the chinampas were in effect artificial islands resting on the bed
of the lake.
This type of chinampa Palerm distinguishes as "laguna
adentro" (Palerm 1973:238).
This description is from historical records,
but it is plausible and confirmed by the practice of modern chinamperos
95
who raise the surface of chinampa platforms with alternating layers of
mud and aquatic weeds.
Once the necessary volume of material had been accumulated the
sides of the chinampas were held in place with "ahuejote" trees.
These
may have been inserted as stakes or piles driven into the lake bed,
which happened to take root and.grow into trees, or they may have been
propagated deliberately by cuttings, as they are in the chinampas today.
Whether the majority of the chinampas were originally of the
"laguna adentro" or the "tierra adentro" form is uncertain and depends
very much on the depth of the lake at the time that the chinampas were
constructed.
It has been suggested that the lakes were extremely shal-
low and best described as marshes rather than as open bodies of water,
and that the chinampas were predominantly of the "tierra adentro" type
(Sanders et al. 1979:280).
However, if water levels were maintained
artificially high, as was suggested in the preceding sections of this
work, the islands may have been formed very largely of imported materials, and been of the "laguna adentro" type.
It is equally possible
that the chinampas were initially formed by excavation, but with the
development of dyking they were subsequently raised.
Whether "laguna"
or "tierra adentro," the resulting landscape was very much the same.
Chinampas are generally rectilinear, but they occur in a variety
of shapes.
Air photographs of Xochimilco show square chinampas, long
narrow chinampas, and chinampas of irregular compound shapes (Figure
22, page 67 above).
Their dimensions vary considerably as well and
range from minute chinampas with only a few square meters of planting
area up to large chinampas 30 or 40 meters wide and several hundred
meters long.
The literature provides numerous estimates of the normal
96
dimensions of rectangular chinampas.
An excavation of fossil chinampas
shows them to have been about 3 or 4 meters wide by 40 meters long
(Parsons 1976:24).
The chinampas that Clavijero saw seemed to him to
be about 6 meters by 18 or 20 meters (Quoted in Leicht 1937:381).
Vanegas states that chinampas in Mixquic are traditionally about 8
meters wide and 20 or 30 meters long, while Wilkin gives dimensions of
the chinampas in the San Gregorio area as between 3 and 10 meters wide
and up to 200 meters long (Vanegas 1978:14; Wilken 1979a:2).
Air photographs show that the chinampas in the vicinity of San
Gregorio vary enormously in length and range from 25 meters to nearly
400 meters, with a median value of about 75 meters (Figure 23, page 69
above).
In other respects they are quite uniform. They are all long
narrow rectangles, and they are very rarely more than 15 meters wide,
with the vast majority being between 10 and 12 meters wide including the
strip of riparian vegetation growing, along the sides of the canals.
Chinampas in the vicinity of San Gregorio may not be typical of the
Aztec chinampas as a whole, but they were probably among the last of
the chinampas to be constructed.
If this is the case, they embody a
great deal of experience in chinampa agriculture, and their dimensions
may be close to the ideal.
It has been suggested in the literature that the height of the
surface of the chinampa platform above the level of the water in the
surrounding canals is one of the critical dimensions of chinampa agriculture, and. that if the difference between the two levels is appropriately adjusted., water is supplied to the plants by root suction and
capillary action, and the need for hand irrigation is eliminated, or at
least considerably reduced (Wilken 1979b:4).
This may be the case, but
97
we have few clues, either historical or contemporary, as to what this
distance might.be.
One colonial source gives the figure as one "vara",
or 0.84 meters, and another gives it as one "pied", or 0.28 meters
(Gibson 1964:256).
The modern chinampas are not a reliable guide. The
pumping of fossil water has caused subsidence, the profile of the
landscape has been distorted and the heights of chinampa platforms vary
widely.
Many modern platforms are flooded to a depth of a meter or
more, the lowest cultivated platforms are about 20 centimeters above
the water level and, at the other extreme, some of the platforms near
the shoreline of the antique lake are more than 2 meters above the level
of the surrounding canals.
Nor do the modern "chinamperos" offer
information that clarifies the issue.
They do not want their chinampas
to be waterlogged and they do not enjoy lifting water from the canals
to a great height, but if their chinampas are between 30 centimeters
and-100 centimeters above the water level, they are quite satisfied.
If the technique of passive irrigation is to work, then the
difference between the levels of platform surfaces and the water table
must be related to the depths to which crops extend their roots and
the difference should be such that the roots extend to the moist but
well aerated capillary fringe, but do not extend into the layer of
saturated soils below the water table.
Maize was the predominant crop and it probably rooted to a greater
depth than the vegetables and flowers that were cultivated in the winter
months.
Ideally, water levels would have fluctuated slightly in the
course of the year to accommodate variations in the depths to which the
crop plants rooted. This may actually have occurred in the chinampas.
Under uncontrolled conditions, water levels were higher in the winter
98
than is the summer.
If the hydraulic installations had the effect of
damping the fluctuations rather than eliminating them completely, then
the dampened seasonal, variation in stage would have corresponded with
the seasonal variations crop rooting depths.
If variations in stage
were entirely eliminated, then it is probable that the levels of
chinampa surfaces would have been adjusted to the requirements of
maize, which was the most important crop, and that bucket irrigation
would have been necessary in the winter months.
These considerations do not resolve the question of the ideal
elevation of chinampa platforms, but merely shift it to the problem of
how close to the water table crops could be cultivated.
We do not know
how far downwards maize extended its roots; modern hybrid varieties
root to a depth of about 80 centimeters but this may not have been the
case with the varieties grown in the chinampas (Epstein 1973:50).
There are a number of uncertainties about the behaviour of plant roots
in different conditions of soil moisture, and the maize grown in the
chinampas may have had some resistance to overly moist conditions.
The behaviour of moisture in soils is determined by minute characteristics of soil chemistry and structure, and the chinampa soils are not
sufficiently well described to allow estimates of how high above the
water table the capillary fringe might have extended.
The problem of
how high the chinampas were above the water table is a complex one, and
the most that can be said at present is that if there was a difference
in elevation of between 25 centimeters and 75 centimeters they appear
to function adequately.
99
Many of the canals in the modern chinampas are blocked with
weeds or entirely silted up, but they are not entirely obliterated,
and on the ground it is generally possible to make a rough estimate of
how wide the canals would have been when they were in better condition.
A count of 52 canals in the area of San Gregorio and San Luis
revealed the distribution shown in Table II.
The figures indicate a
tendency toward two quite different types of canal.
On the one hand,
there are narrow access canals that are just wide enough to permit the
passage of the small local "canoas".
80 and 100 centimeters wide.
The "canoas" are usually between
On the other hand, there are the larger
canals that allowed two boars, travelling in opposite directions to pass
each other, and that also served as the main drainage canals.
In the
San Luis area, the largest canal that was measured is about 8 meters
across, but in other areas of the extant chinampas, there are canals of
up to thirty meters.
TABLE II
WIDTHS OF CANALS IN THE CHINAMPAS OF SAN LUIS AND SAN GREGORIO
Width of canal in meters
% of canals
less than 0.75
15.4
0.76 - 1.75
53.8
1.76 - 2.75
17.3
2.76 - 3.75
5.8
more than 3.75
7.7
TOO
A few of the canals are so narrow that they would not have permitted the passage of boats; these invariably run along one side of a
chinampa only, so that access is available on the other side.
There
are a number of other canals that are uneconomically wide for use by a
single boat, but not quite wide enough for boats to pass each other.
However, the overall tendency is towards either narrow access canals or
large arterial canals.
The canals are generally quite deep, with values
ranging from about 1 meter in the smaller canals to 3 or 4 meters in
the longer ones.
Air photographs show that in the San Luis area there
are about 750 meters of canals per hectare.
This figure, combined with
the data on canal widths, indicates that when the canals were in good
condition, they occupied about 13% of the total area of the chinampas.
Ahuejote trees are planted along the perimeters of the chinampa
platforms and hold the banks of the canals in place.
In well maintained
chinampas, the banks of the canals are vertical and sometimes slightly
undercut, and the edge of the cultivable area can extend to within 50
centimeters in plan, of the edge of the water.
If one allows a figure
of 75 centimeters for this distance, then a further 11% of the total area
of the San Luis chinampas is. occupied by trees and riparian vegetation.
These measurements were made in an area where the chinampas were
particularly neatly aligned, where there is only one major canal, and
where there are no deep pockets of water that would have impeded colonization, and so they may overrepresent the proportion of land that
was cultivated in the entire Xochimilco and Chalco basins.
The calcu-
lations correspond almost exactly with Armillas estimates and it seems
1
clear that about 75% of the area of the chinampas was cultivable, and that
of the remaining area about half was occupied by canals and half by
101
vegetation on the borders of the platforms (Armillas 1971:660).
Flora and Fauna
We are accustomed to making a clear distinction between domesticated species of plants and wild ones, but in traditional agricultural
systems the distinction is often unclear and there are many plants
that fall between these two extremes.
Some of these are useful wild
plants which are tolerated by farmers even if they are not deliberately
propagated.
Others are plants that seed spontaneously but which are
encouraged by selective weeding and fertilization once they are established.
Others are plants that grow wild, but that are on occasions
deliberately planted by farmers to supplement the wild supplies.
In
the chinampas the distinction between wild and cultivated plants is
particularly difficult to make because the whole landscape is manmade
and because most.of the plants in the landscape are useful to the
"chinampers" in one way or another.
In the summary of the flora that
follows, groups are not distinguished so much by their status as wild
or cultivated, but by the niches in which they grow.
The plants of
the canals, the platform borders and the platform surfaces are treated
separately.
In the.modern chinampas the most evident water plant is the
water hyacinth known locally as "lirio" or "huachinango" and scientifically as Eichornia crassipes Kunth.
It is a widespread aquatic week
that is native to Brazil and that was introduced to Mexico during the
19th'Century.
It is a prolific plant, and it has contributed to the
decline of the chinampas by clogging canals and by displacing native
102
species that have a higher nutrient content and a greater value as green
fertilizers (C.H.C.V.M. 1971).
Table III gives a list of some of the more common native aquatics
that can s t i l l be found in the less polluted areas of the chinampas.
Many of the reed, reddmaces and rushes (Cyperaceae, Typhus spp„ Juncus
spp.)
were valuable for their structural properties.
Both the leaves
and stems of these species could be woven into mats, baskets, ropes and
other utensils, and the leaves of reedmace can be used to make cloth,
although not a particularly satisfactory one (_Bursche 1971:33; Dr. A.
Lot pers. com.; Sanders 1979:293).
Huts and protective coverings for
seedbeds were also made from these materials.
larly valuable.
Typhus spp.were particu-
The stems are quite buoyant and were probably the main
components, of the fleeting seedbeds discussed above.
Their dried leaves
expand when wetted and are widely used by North American Indians as a
caulking material (Erichsen-Brown 1979:213-14).
In addition, the rhizomes
are edible, raw or cooked, and they can be processed to provide a flour
that is as nutritious as that of wheat or rice.
They also give high
yields and modern experimental plots have produced 7,000 kilograms per
hectare or more (Brown et al. 1976:101).
This use is not confirmed
for the Aztecs, but the reedmaces might have been used as food.
The Umbellifers, and Potamogeion spp. and Polygonum spp. are a
highly variable group of plants that can adapt to a range of microhabitats.
Polygonum arnphium, the water smartweed, has two quite distinct
forms, and prospers as well in moist but exposed soils as it does in a
meter's depth of water, and other plants of this group were nearly as
adaptable.
These plants were the main constituents of the "cespedes" or
interwoven mats of floating vegetation which grew on the lakes.
They are
103
TABLE III
COMMON AQUATIC PLANTS OF THE CHINAMPAS
Mexican
Common Name
Scientific name
TYPHACEAE
Typhus
T.
CYPERACEAE
latifolia
angustifolia
Cyperus
C.
Bourgaei
hermaphroditus
Eeleodhavis
H.
S.
JUNCACEAE
GRAMINAE
laoushis
americanus
Junaus
J.
palustvis
acioulavis
Scivpus
Tule cuicho
Great reedmace
Tule
Lesser reedmace
Tule grande
Tule, Tulillo
esoulentis
C.
English
Common Name
bathcus
effusus
Tulillo
Cerbatcina
Tule, Tulillo
Common spike rush
Estapil
Bulrush
Zacatule
Tulillo
Rush
Tulillo
Rush
Hoja flecha
Arrowhead
Hoja flecha
Arrowhead
Berro
Narrow leaved
water parsnip
Pasp alum
humgoIdtianuum
Evagrostis
Glycera
ALISMACEAE
fluitans
Sagittaria
sagitti
S.
mexicana
folia
maovophylla
NAJADACEAE .
Najas
flexilis
UMBELLIFERAE
Berule
eveota
Hydvoootyle
vanunculoides
H. verticeI
lata
104
TABLE t i l (continued)
COMMON AQUATIC PLANTS OF THE CHINAMPAS
Mexican
Common name
Scientific name
UMBELLIFEME
Berro largo
L-Claeopsis
ocoidentalis
L.
sahaffneriana
POTAMOCETONACEAE Potamogeton
P.
P.
POLYGONACEAE
'
luaens
Achicorillo cambrai
foliosa
Achicorillo cambrai
Polygonum
Chi Lillo
Water smartweed
Chi Lillo
Pale smartweed
amphibium
lapathifolium
P. acre
Chi Lillo
P.
Chi Lillo
Ladysthumb
Attacuetzon
Water l i l y
persicarioides
Nymphea mexiaana •
N.
alba
Nympha
N. ampla
N.
LEMNACEAE
elegans
Chilecastle
Duckweed
Lemna minor
Chilecastle
Common duckweed
L.
Chilecastle
Gibbons duckweed
Chilecastle
Ivy duckweed
Chilecastle
Great duckweed
Wolff-ia
L.
spp.
gibba
tvisulea
Spyrodela
Source:
Pondweed
fluitans
P. pechumtus
P.
NIMPHAEACEAEA
English
Common name
polyrrhysa
Sanchez y Sanchez 1978.
105
generally good plants for use as green fertilizers and are s t i l l used
today by the "chinamperos" for this purpose.
Many of these plants are
floating leaved and contain pockets of air which are valuable for
aerating soils, and they are generally quite high in nutrients.
Potamogeton spp. have the property of accumulating crusts of calcium
which could allow selective fertilizing of crop plants.
and Berula erecta both have medicinal properties.
P. amphibium
The former is used
in treating stomach upsets and the latter is edible and has a marked
value (Bursche 1971:99; Dr. A. Lot pers. com.).
Several species of Lemnaceae grow on the canals and are known
locally as "chilecastle".
in the world.
The duckweeds are the most prolific plants
A single square inch of Lemna minor can expand to 1.2
acres in a mere 55 days and Lemna spp. can yield up to 60 kilograms
dry weight per hectare per day.
The Lemnaceae are excellent green
fertilizers, both because they have airpockets in the leaves and because
they are exceptionally high in nutrients.
Typically, nitrogen consti-
tutes 6% or 7% of the dry weight, phosphorus and potassium 1.4% to
3.0% each, and calcium about 1%. Many species of duckweed are edible.
Wolffia spp. have 20% protein, 44% carbohydrates and 5% fats, and some
species.are cultivated as a.vegetable in Southeast Asia. Spyrodela
polyrrhyza has 35% to 45% protein, compared to 37% for soybeans.
In
general, the edible Lemnaceae are more nutritious than the majority of
terrestrial crops (Brown et al. 1976:184ff).
There is no direct evi-
dence that the Aztecs exploited this resource, but they did gather a
substance called "tecuitlatl" from the surface of the lakes for consumption ,. and they may have harvested the Lemnaceae as well.
106
"Tecuitlatl" is a substance about which there has been a great
deal of confusion.
Sahagun listed it among the lower animals, and
described it as a green or purple scum which grew on the surfaces of
the lakes, and which was collected, dried and formed into edible cakes.
Gomara adds that the natives "hold the opinion that this skume or fatness of the water is ye cause that such great number of fowle cometh to
the lake" (Quoted in Deevey 1957:227).
The substance may possibly have
been Spirode!a polyrrhyza which has leaves that are reddish coloured
on the underside, but it is more likely that it was a combination of two
types of algae, Spirolina spp. and Oscillatoria spp.
Spiro!ina is a
filamentous blue-green algae that grows in large masses on the lakes
and is harvested as a food in the Valley today (Deevey 1957:227-28;
Sanders 1979:290; McDowell 1980:727):
It is extremely nutritious, with
70% protein by dry weight, and it contains all eight essential amino
acids.
Oscillatoria is also a filamentous algae and it commonly grows
intertwined with Spirolina.
At certain seasons of the year it turns
the water in which it grows to the colour of blood, and it is highly
motile.
A clump left on a saucer will climb the sides, and this would
account for Sahagun's classification (Prescott 1978:208).
Both species
are salt loving, and so they were probably restricted to the chinampas
of the Lake of Mexico, if they grew in the chinampas at a l l .
The narrow strips between the cultivated areas of the chinampa
platforms and the edges of the canals support the growth of a large
number of different species.
The most evident of these are the Ahuejote
trees, which are a species of willow, Salix bonplandiana Kunth. They
are narrow upright trees, with short erect branches, looking rather like
poplars.
They grow to a height of 12 meters, although they rarely reach
107
t h i s height
i n the chinampas.
They are g e n e r a l l y spaced about 2 meters
a p a r t a l o n g the margins o f the p l a t f o r m s
which amounts to some 700 o r
800 t r e e s per h e c t a r e , and they are propagated by hardwood
The r o o t s are e f f e c t i v e
platforms,
cuttings.
i n c o n s o l i d a t i n g the edges o f the chinampa
but they p r o b a b l y do not extend v e r y f a r below the
o f the w a t e r .
surface
In the modern chinampas the t r e e s are i n f e s t e d w i t h a
species of tent c a t e r p i l l a r
moth, Malacosoma a z t e c a , a n d ,
remedies are a v a i l a b l e , the Mexican a u t h o r i t i e s
of organizing t h e i r
a p p l i c a t i o n , and so i t
few y e a r s the chinampas w i l l
appear t o be i n c a p a b l e
i s probable t h a t w i t h i n a
be s t r i p p e d o f t h e i r
A h u e j o t e t r e e s ( S r . J . Cornejo p e r s .
although
n e c e s s a r y and g r a c e f u l
com.).
The v a s t m a j o r i t y o f the t r e e s i n the chinampas are S a l i x bonp l a n d i a n a , but o t h e r s p e c i e s are o c c a s i o n a l l y p r e s e n t .
S. l a s i o l e p i s
i s q u i t e common, and S . a c i c u l a r i s and S . acumulata may o c c u r as w e l l .
Sambucus m e x i c a n a , a n o t h e r s p e c i e s o f w i l l o w ,
as h o l d i n g c a n a l banks i n p l a c e i t
can be f o u n d , and as w e l l
has the a d d i t i o n a l
value that
its
l e a v e s a r e the b a s i s f o r an i n f u s i o n used i n t h e ' t r e a t m e n t o f coughs.
B u d d l e j a . c o r d a t a and A l n u s f i r m i f o l i a ,
a s p e c i e s o f a l d e r , are a l s o
found (West and A r m i l l a s 1950, 1 9 7 5 ; . S a n c h e z y Sanchez 1978; W i l k e n
1979b:13).
The borders o f the chinampa p l a t f o r m s
s u p p o r t the growth o f many
s p e c i e s o f g r a s s e s and h e r b a c i o u s p l a n t s , and some o f the more i m p o r t a n t
o f these are l i s t e d i n Table IV.
they a r e e d i b l e .
Some o f t h e s e p l a n t s
Chenopodium n u t t e l l i a
are u s e f u l
because
o r " q u e l i t e s " and P o r t u l a c a o l e r a -
c e a , " v e r d a l o g a " , are s e m i - d o m e s t i c a t e s , sometimes growing w i l d and
sometimes c u l t i v a t e d
in p l o t s .
These n a t i v e s are on t h e i r way t o
becoming d o m e s t i c a t e s , i n c o n t r a s t w i t h the European c e l e r y
which
108
TABLE IV
EDIBLE AND MEDICINAL PLANTS OF THE CHINAMPAS
Scientific Names
Notes
Lovegrass.
GRAMINEAE
Eragrostis
spp.
E.
diffusa
E.
mexicana
Setaria
Humid places
Foxtail grasses, related to millet.
spp.
Edible.
S.
Edible.
genioulata
Glycera
fluitans
1
"Zacate cerdoso".
Grows on waterlogged ground.
Grains and florets harvested from
7,11
water surface and used as food
Zea
"Tesoutle".
mexicana
"Teociatle".
AMERANTHACENE
Ameranthus
spp.
Alteranthera
achyrantha
Common weeds, with edible leaves.
"Vendaloga de Puerca".
Edible.
1
COMPOSITAE
Ambrosia
spp.
Bidens spp.
B.
laevis
Ragweeds; widespread weeds, some of
1
which are edible.
Spanish needles, some edible. 1
Burmarigold.
"Te de Milpa", grows
67
B. aurea
B.
pilosa
in humid places. '
"Acahual".^ Growing on ditchbanks,
"Acahual bianco".^
109
TABLE IV (continued)
EDIBLE AND MEDICINAL PLANTS OF THE CHINAMPAS
Scientific Names
Montanoa
tomentosa
Notes
"Cihuaptli" - woman's medicine.
Used by Aztecs and today as diuretic;
aid to childbirth, lactation, menstru5
ation and possibly effective.
Parthenium
Tagetes
hysterophorus
luaida
Used as an analgesic.
7
Infusion of roots used against colic.'
SOLONACEAE
Datura
stramonium
Jimsonweed.
"Toloache": narcotic with
a wide range of medicinal uses.
..
exotic.
Solanum
nigram
Possibly
8
Used in treating sores and wounds.
7
CHENOPODACEAE
Chenopodium spp.
Lambsquarters "Quelite".
Widely used
as leafy greens, growing both wild and
12
C
ambroslodes
3 7
cultivated in chinampas. ' ' '
Wormseed "Apazole"; medicinal uses,
C.
graveolens
cultivated at times.
"Epezatl"; used by Aztecs and today in
treatment of dysentery, intestinal
5 7
Atriplex
spp.
worms, asthma, and so effective.
Similar use as leafy greens.
'
no
TABLE IV (continued)
EDIBLE AND MEDICINAL PLANTS OF THE CHINAMPAS
Scientific Names
Notes
PAPAVERACEAE
Argemone
mexicana
"Chicalote"; "Ampola blanca" herb
tea.
A.
ochroleuca
7
"Ampola audi 11a".
Leaves and stems
contain substance similar to morphine,
used for eye and intestinal problems.
7
Other
Families:
Plantago
mexicana
Begonia
gracilis
"Acaxicote"; used by Aztecs as emetic,
57
and effective. '
Used as emetic.
Cuphea
augustifolia
Used in treating wounds and tumours.
Anacacia
atroplupura
Rumex spp.
Oxalis
spp.
7
7
Used in treating fever.
Pocks and Sorrels.
Used in treating
3 79
sores and diarrhea. ' '
Wood Sorrels. Edible. Diuretics and
Antiseptics.
1
Sanders 1974:287.
7
10
7
Sanchez y Sanchez 1978.
2 Erichsen-Brown, 1979:413.
8
USDA 1971.
3 Vanegas 1978:19.
9
Erichsen-Brown 1979:222.
4 Dressier 1953:128.
1 0
Erichsen-Brown 1979:334.
5 de Montellano 1975:219.
1 1
Pohl 1968:103.
6 Wilkinson & Jaques 1972.
Ill
now grows as a weed in the chinampas (Vanegas 1978:21).
Some of the
plants are used as herbs or in the preparation of herbal teas, and
others are valued for their medicinal properties.
Some of the latter
have been identified from Sahagun's account of Aztec medicine and
others are the basis of folk remedies which may well have been known
to the Aztecs (Montellano 1975).
"Teocintle", Zea mexicana, is a
weed that sometimes occurs in the chinampas, and where it grows near
maize it cross-pollinates quite freely to produce hybrids that are
similar to the early, pre-historic races of maize (Vanegas 1978:21).
The modern chinampas are not a good guide to the selection of
crops that were grown there by the Aztecs.
Soon after the Spanish
arrived they established gardens near Mexico City and started to raise
European vegetables.
These were amenable to cultivation by Indian
practices and, because the market for these exotic products increased
along with the growth of Mexico City while the demand for the.native
vegetables that satisfied Indian tastes declined along with the Indian
population, the new crops spread rapidly through the chinampas. The
extent to which the introductions displaced the indigenous crops
rather than supplementing them is unclear, but it is certain that the
selection of crops now grown in the chinampas is not representative
of the selection cultivated by the Aztecs.
Table V is a list of common Mesoamerican crops, omitting only
those that are cultivated exclusively in the humid lowlands.
Some of
these crops were undoubtedly cultivated by the Aztecs, and others may
have been.
The list includes some 48 different species and, while it
includes most of the major crops, it may omit some minor ones. A
casual examination of a single garden plot in highland Guatemala showed
112
TABLE V
COMMON HIGHLAND MESOAMERICAN CROPS
CEREALS
Maize
vars. "Chinampero" (probably several vars.)
3
Zea mays
"Chalqueno Conico". Most varieties are 6 month
1 2 28
hybrids with maize result. ' '
Amaranth
S. Watts, "HUATLI"' "alegria"
3
Amercinthus
leucoparpus
A.
L.; A. sanguineus;
cruentis
A.
leucocarpus
A.
tricolor.
is the most widespread; of consid-
able importance to Aztec economy.
Because of
religious associations, cultivation actively sup2 3 4
pressed by Spanish.
Panicum
sonorum
BEANS
Leaves also eaten. ' '
Beal panic grass, occasionally grown as a
cereal.
"AYECOTE" "frijol"
Phaseolus
coccineus
L . , common bean.
P. vulgaris
L . , Lima bean.
P. lunatus
A. Grey, Tepary bean.
P. acutifolius
Macfad.
P.. limensis
SQUASHES
Canivala
L . , scarlet runner, roots edible.
spp.
, Jack bean.^'^
CUCURBITACEAE
Cucurbita
ficifolia
Bouche, "chilacayote" oldest of normally
cultivated species.
C. mixta,
Pang., green striped, cushaw squash.
C. moschata
Duch., cushaw squash.
113
TABLE V (continued)
COMMON HIGHLAND MESOAMERICAN CROPS
C. pepo
L . , summer squash.
Sechium
edule
Langenavia
SW.
"chayote" "CHAyOTLI"
3
(Mol) "tecomate" bottle gourd valuable
sicevavia
for seeds, flowers, fruit and roots.
L.
sicevavia
is African in origin, but it was definitely cultivated prior to the Conquest, and probable that
fruits floated across the Atlantic. A further
4 7 89
21 spp. in Mexico, and most are edible. ' ' '
CHILE
CAPSICUM
annuum L.
Capsicum
L.
C. fvutescens
TOMATO
C. pubescens
R & p. ' '
Lycopevsicum
esculentum
Physatis
4
8
1 0
Mi 11, "xitomatl" "jitomate"
Brot, "tomatl".
ixocarpa
In Mixquic 2 varieties
recognized:
ORNAMENTALS
Polyanthus
tubevosa
Cav, dahlia, edible tuberous roots.
Dahlia
pinnata
Dahlia
coccinea
Tagetes
"Miltomatl", "Coztictomatl".
13
L . , tuberose.
14
evecta
Cav, "ACOCOXOCHITL" dahlia.
"CEMPOALXOCHITL", 'twenty flower' marigold.
4 12
0
\
• Day of the dead.
T. patula
Tigvidea
J
1
5
'
1
4
Taluana
'
(LE) Kerr, "OCELOXOCHITL" 'jaguar flower'
pavonia
acutifolia,
Chivodendvom
Medicinal properties. '
6
edible c o r n s . ' '
Plumevia
14
3
17
"CACALOXOCHITL" 'crowflower'. '
4
pentadactylon,
mexicana
15
15
"MACPALOXOCHITL" 'handflower'. '
"YOLOXOCHITL" 'heart f1ower'.
4
15
15
114
TABLE V (continued)
COMMON HIGHLAND MESOAMERICAN CROPS
Callandvia
"TLACOXILOXOCHITL" ghost f1ower'.
1
anomala,
15
"COATZONTECOXOCHITL" 'snake flower'.
Helianthus
"ITZQUIXOCHITL" sunflower; o i l ;
cinnus,
edible seeds. '
15
18
"COYOLXOCHITL" edible tuberous roots.
Bomarea edulis,
15
19
"COYOLXOCHITL".
19
B. acutifolia,
OTHER PLANTS
Chonopodium spp. \
widely used as leafy greens throughout
21
\ North & South America.
Growing both
12
Atriplex
spp.
J wild and cultivated in chinampas.
C. Nuttalliae
Safford,
"Quelite" seeds, leaves, unripe
20 28
fruit clusters eaten. '
C. ambrosiodes
"Apazote" wormseed with medicinal uses;
20 28
3
cultivated occasionally.
C. album,
'
"Quelite" maple leaved goosefoot.
Once thought
to be introduced, but now known as native. Most
21
widespread of "Quelites", and cultivated.
12
Vovtulaea
spp.
growing both wild and cultivated.
22
Salvia
hispanioa
L . , "Chia" seeds used to prepare drink.
3
I
Hyptis suaveolens
Port.,
Crotalaria
longirostrata
o
"Chia grande".
Hook & Arn., "Chi pi 1 in" leguminous
23
pot herb.
24
Euphorbia
pulcherrima
Mentha spp.
M. spicata
Wilid., "Noche Buena", Spurge.
spearmint, is commonly cultivated
now, is not native, and probably replaced
• 26
M.
avvensvs.
115
TABLE V (continued)
COMMON HIGHLAND MESOAMERICAN CROPS
Allium
oepa
L . , "XONOCATL", onions.
27
14
Gossypium
hirsutum
3
"IZCATL", cotton.
Notes:
Mexican common names, upper and lower case in double inverted commas.
a
k Nahuatl name, upper case in double inverted commas.
Literal translation of Nahuatl name, upper and lower case in single
c
inverted commas.
Sources:
1
Cornejo, J . , pers. com.
16
Dressler 1953:147.
2
Vanegas 1978:21.
17
Ibid.:149.
3
Dressier 1953:121.
18
Ibid.:136.
4
West & Armillas 1950:181.
19
lbid.:125.
Ibid.:128
Iy
5
Dressier 1953:141.
20
6
Ibid.:142, 143.
21
7
Ibid.:130, 131, 136, 146.
22
Dressler 1953:146.
8
Vanegas 1978:18.
23
Ibid.:130.
9
Cutler & Whitaker 1961:470.
24
Ibid.:236.
10
Dressler 1953:126.
1 1
Ibid.:132.
Vanegas 1978:19.
12
Erichsen-Brown 1979:413.
25
Sanchez y Sanchez 1978:330.
Erichsen-Brown 1979:285.
26
27
13
Dressler 1953:144.
28
14
Ibid.:132.
29
15
Schilling 1938:27.
Peterson 1962:167.
Sanders 1979:234, 236.
Vanegas 1978:24
116
that some 26 different species were being cultivated, and a survey of
garden plots in Tabasco showed that 143 different species were present
(Anderson 1952:136-37; Gliessman 1979b:7).
While the chinampas may
not have been quite as diverse as the gardens of the humid lowlands,
it is probable that a very large number of different species were cultivated.
level.
The list gives no indication of the diversity at the varietal
A single visit to the market in Xochimilco produced 14 different
varieties of Capsicum, al1 of which were reportedly grown locally. The
six commonly cultivate species of beans are liable to great variation, as
are the six commonly cultivated species of squash.
Cucurbita ficifolia
in particular is liable to hybridization with other species which results
in many novel forms, and the variety is increased yet further by the
presence of 21 other native.species of Cucurbita that are not normally
cultivated, but that are nevertheless edible (Cutler and Whitaker 1961:
470).
Maize was and is the single most important crop grown in the
chinampas.
The variety that is most commonly cultivated is a six month
maize, known as "Chalqueno conico", and it is possible that a number
of other local "criollo" varieties were grown as well.
crop that is not well known in temperate latitudes.
Amaranth is a
It is usually
cultivated for its seeds, but it is valuable as a leafy vegetable as
well.
The brilliantly coloured seeds were employed by the Aztecs in
ritual activities and for this reason the Spanish actively suppressed
the cultivation of Amaranth, but it was an important component of the
Aztec economy.
One of the lists of tribute due to Tenochtitlan includes
200,000 bushels of Amaranth, in addition to 280,000 bushels of maize and
230,000 bushels of beans, which is an indication of the relative importance
117
of these crops (Dressier 1953:121).
Chiles, squashes and tomatoes were
all cultivated'in the chinampas, but it is probable that the species of
tomato that is familiar to us, Lycopersicum esculentum, was less widely
cultivated than the Husk Tomato, Physalis ixocarpa:
Various species
of Portulaca, Chenopodium and Atrip!ex were probably cultivated as green
vegetables.
Xochimilco was a centre for floriculture, as it is today.
Many species were cultivated including dahlias which the Aztecs called
"Acocoxochitl" and marigolds or "cempoaloxochitl"; the latter had
important ceremonial functions and still adorn graves on the Mexican
"Day of the Dead".
In addition to its diverse flora the chinampa landscape was home
to a large number of animal species.
The following is a compilation of
those that are of particular interest or economic importance.
Several groups of insect were important.
valued because they were edible.
Some of these were
"Axayacatl" were the Coryxid water
bugs of which five species were of some economic importance: Ahuetlea
mexicana.Llave., Krisousacorixa azteca.Jacewski., K. femorata, Corisella
texcocana and C. mercanaria were all valuable for their eggs.
These
were called "ahuatli" or "water amaranth" and were cultivated by the
Aztecs.
Stems of reeds were placed in the water, and on these the bugs
laid their eggs.
The stems were then removed, and the eggs were stripped
off and'formed into small cakes.
"Aneneztli" were the eggs of dragon-
flies which were also collected and eaten.
"Cuculin" were the pupae
of salt flies, Ephydra hians, and in the late summer these came to the
surface of the lakes in great numbers and were collected.
"Izcuaatli"
were the larvae of the same species, which were "little grubs like
earthworms [and were] slender and pullilant in their multitude and
118
density."
These, too, were collected and eaten (Deevey 1957:224-28;
Gibson 1964:339-41).
Also included among the insects are a number of
species that we regard as crop pests.
The illustrations to Sahagun's
Historia show insects that are clearly identifiable as grasshoppers, the
larvae of June Beetles, Phyllophaga spp., leaf-eating catepiliars of
various types, corn ear worms, which are the larvae of the Noctid moth
Heliothis zea, and the larvae of the Geometer moths.
The Nahua term for
the latter-is "tetatamachluhqui" which translates literally to our
common name for the creature, which is "measuring worm" (Curran 1937).
Although not directly confirmed, it is undoubtedly the case that aphids,
leafhoppers, bean beetles, cucumber beetles, leaf beetles and other
common pest species were present in the chinampas.
The Crustaceans were represented by the "acocile" or freshwater
crayfish, Cambarellas moctezumae, and by a species of freshwater shrimp,
either Macrobrachium sp. or Palaemos sp.
Among the amphibians, frogs
were caught and eaten, together with the "axolotl" which is the larval
form of the. salamander Ambystoma sp.
These grew to about 8 inches in
length and were rolled in a maize dough and roasted.
Water snakes and
turtles were also hunted (Gibson 1964:339-41; Flannery 1968:83-84;
Niederberger 1979:133-34, 139).
Fish were an important resource.
A large proportion of these
were Athernidae, including Chirostoma humboldtianum, C. Jordani and
C. pegani, some of which were the freshwater whitefish and others the
saltwater "charales".
Fish of the minnow family, Cyprinidae, were
important' including Notropis azteca, Evarra spp. and Algansea tincella.
The Goodeid'Giradinichthys viviparus were also caught.
The Aztecs
distinguished species as "xohulin", "ajolote", "amilotl" and "iztacmichin",
119
but the identification of these is not clear.
An estimated one million
fish were caught annually during the 17th Century, and at least this
number may have been caught by the Aztec fishermen.
The older living
chinamperos report that in their youth the occasional fish, caught in
the chinampas, was one of the small advantages of their agricultural
systems (Deevey 1957:224-28; Gibson 1964:339-41; Vanegas 1978:41;
Niederberger 1979:133-34).
Migratory birds came to the Valley of Mexico in large numbers
during the winter, having passed the summer in prairie provinces of
Canada, and these waterfowl were a further resource for the Aztecs
(Deevey 1957:224-28; Gibson 1964:339-41; Vanegas 1978:41; Niederberger
1979:133-34).
Sahagun enumerated some 40 varieties of duck and geese
which were commonly hunted by the inhabitants of the Valley. A number
of species of birds other than these also inhabited the chinampas.
Although they are not mentioned in the literature and their economic
importance may have been slight, they were probably of some value in
controlling insect pests (Sr. J . Cornejo pers. com.).
The foregoing description of the chinampa platforms and of the
biota that they supported gives some indication of the structural
properties of the chinampas as an ecological system.
The chinampas
had some 700 to 800 Ahuejote trees per'hectare, which would have
amounted to a considerable standing biomass.
Waterweeds on tropical
lakes can accumulate to about 70 tons per hectare, dry weight and,
although the figure would not have been as high in the well-maintained
canals and in the cool conditions of the Valley of Mexico, aquatic
vegetation may have contributed significantly to the total biomass
(Brown et al. 1976).
In traditional systems in Tabasco, where maize
120
is produced by united cropping techniques, the standing biomass of
cultivated crops can amount to 19.5 tons per hectare, dry weight, and the
figures in the chinampas may have approached this value (Gliessman &
Amador 1979b:Tables 1 & 2).
In Mixquic, the annual yields of the sale-
able portions of vegetables average 3.9 kilograms per hectare, and as
vegetables are cultivated during the winter months, this suggests that
the biomass of the crop system may have been quite high throughout the
year.
The chinampas would appear to have rated quite high on most
indices of diversity.
The microtopography was varied, and they supported
plants of different life forms and different dimensions, which would
confer a high pattern diversity on the system.
The preceding tables
list 118 different species of plants that were likely to have grown
in the chinampas, and so the species diversity of the system was probably quite high.
Many of the plants had medicinal properties, or were
used as herbs or spices, which suggests that the biochemical diversity
of the system may also have been quite high.
The chinampas supported
some perennial plants and annuals were.present throughout the year,
with the result that the diversity of the system persisted through
every season.
In general, the information available on the chinampas
suggests that, for an agricultural system whose primary purpose was the
production of maize, they supported a substantial and varied biomass,
and did so throughout the year.
Seedbeds and transplanting
Agriculture can be considered as two linked endeavours, one being
to promote the growth of plants that are particularly inept at propagating
121
themselves, and the other being to suppress the growth of plants that
are much stronger competitors than the crops, and of populations of
animals against which the crops have few defences of their own.
In the chinampas seedbeds, known by the Spanish terms "camellone" or "almaciga", were at the heart of the set of techniques for
propagating crop plants.
Seeds were started in the beds while other
crops were maturing in the fields.
When the mature crops were harvested
the seedlings were planted in their place, and new seeds were started
in the beds.
In this fashion each plant was accorded only as much
space as it required at any particular stage of its development, and
the physical infrastructure of the chinampas was used to its maximum
advantage.
The Aztecs brought the use of seedbeds to a high art, and the
techniques that they developed are s t i l l used in the chinampas. A
small plot is selected for the construction of a seedbed; in this plot
the soil is carefully leveled, tilled and weeded.
A board is held
on its edge along the edge of the seedbed, and soil is gathered from
outside the bed and pressed into the angle formed by the board and the
ground.
The operation is repeated until the whole of the plot is
surrounded by a low earth wall 15 centimeters"to 20 centimeters high,
which is vertical on its inside face and sloping on its outside one.
Work then shifts to the canals where "agualodo" or bottom mud is
scooped up and ladled into boats.
A cloth scoop attached to the end of
a long pole, called a "zoquimaitl" is used for this job.
The bottom
mud is. then transported to the site of the seedbed and all the roots and
other debris'are removed.
Water is mixed in with the mud until it is
quite liquid, and then the mud is ladled into the seedbed.
In the past
122
a "zoquimaitl" or a bucket made out of the central part of the maguey
cactus was used, but today plastic buckets are the rule.
Because the
mud is quite liquid it flows over the area contained by the earth walls
and forms a perfectly level surface.
The seedbed is then left overnight,
during which time much of the water in the liquid mud drains into the
prepared soils below, and the level of the mud falls by about one third.
The mud that remains is quite firm and cohesive, but with a spongy and
resilient quality to it.
The layer of mud is then sliced at small and
regular intervals along both dimensions of the bed to form a number of
uniform blocks.
The cuts are made with a knife attached to the end of
a long handle or with a device called an "almagigero", which consists
of between 5 and 10 knives fixed to the end of a single handle. The
resulting blocks are called "chapines" or, in Mixquic, "tlacopehuales"
(Vanegas 1978:3).
Measurements are made throughout the operation using fractions
of the hand with fingers extended downwards to measure the depth of the
"agualodo" when it is poured into the seedbed, and multiples of fingerwidths to determine the distances between the cuts.
The resulting
"chapines" are closely matched to the sizes of the plants that are to
be grown in them and are usually about five fingers square by six or
seven fingers deep.
But occasionally they are minute, about one and a
half fingers square by two deep, or quite large, eight fingers or so
square and ten or more deep.
Once the "chapines" have been cut, a board is placed across the
seedbed to distribute the weight of the "chinampero", and a hole is
made in each "chapine" with a finger or a twig.
A single seed or
several seeds are placed in each hole, a pinch of fertilizer may be
123
added, and the hole is closed and smoothed over.
Most plants are seeded
individually, but for small herbs half a dozen seeds may be placed in a
single hole.
When the seedlings have grown to f i l l the capacity of the
"chapine", the "chapines" are broken apart.
They separate easily along
the lines of the cuts and from the friable soil beneath, and the blocks
keep their form so that the seedlings can be moved without disturbing
their roots.
Some crops are exported from the chinampas at this stage.
Others, particularly flowers, may be planted into larger "chapines"
and vegetables are planted out into larger plots.
sowing and transplanting varies.
The interval between
"Cilantro" or coriander is transplanted
after about 20 days, maize after 30 or 40 days, onions after 60 days, and
chiles after 90 or 100 days (West and Armillas 1950:77).
The fields are carefully prepared to receive the transplants.
A green fertilizer of aquatic vegetation is dug into the soil, either
over the whole area.of the plot, or only in the spots that are to receive
the transplants.
Sometimes furrows are formed so that irrigation water
will flow to the roots of the plants, and maize is planted in small
mounds of soil as is generally the practice in Middle America. When
the hole is excavated a handful of "agualodo" and compost may be added
before the seedlings and their "chapines" are deposited.
The success of the technique depends on the quality of the
"agualodo", and the modern "chinamperos" will travel a considerable
distance to secure supplies of mud that they consider to be of good
quality.
A few samples of chinampa muck have been analyzed, and one that
was considered to be of good quality was low in nitrogen, which is characteristic of submerged soils,, but was otherwise rich in nutrients (Wilken
1979a:Table 1).
The "agualodo" has a high cation exchange capacity
124
which facilitates the uptake of nutrients by plants (Vanegas 1978:11).
Very few samples of chinampa mucks have been analyzed, and the modern
soils may not be representative of pre-Hispanic conditions because they
are to some extent enriched by artificial fertilizers in the runoff from
the platform surfaces; but it seems probable that the "agualodo" was
generally well supplied with plant nutrients.
ture of the bottom mud is important.
The texture and struc-
Samples from the San Luis area
were smooth textured and very sticky and cohesive, which is the property
that allows the muck to.be formed into "chapines".
They are also very
dark and have a high organic matter content, over 60% by dry weight in
one sample.
The organic matter is important not only because i t is a
source of nutrients, but because it imparts to the mucks their light,
spongy character and allows the roots of the seedlings to develop freely.
The "chinamperos" of San Luis complain that the local "agualodo" has
an increasing tendency to stunt the growth of seedlings by constricting
their roots.
This may be related to the death.of a large number of the
Ahuejote trees, whose falling leaves are an important constituent of
the organic matter in the chinampa mucks (Gliessman pers. com.).
In addition to the seedbeds that can be seen today on the platform surfaces, the Aztecs probably made use of the floating seedbeds
mentioned.above.
These were the features that Clavijero confused with
the chinampas. themselves to create the myth of the "floating gardens".
They are mentioned by Padre Jose Acosta in his Historia Natural y Moral
de las Indias and they are described by Fray Hernando de Ojea, who wrote
in 1608 as follows:
125
In this lake (Lake Chalco) the Indians make use of a very
interesting thing which are movable gardens some 20 or 30
"pieds" (6.6 - 10.2 m) long, and as wide as they please, lying
on the surface of the water, on waterweeds, rushes and reeds,
on which they sow seedbeds with their vegetables, such as
pimiento, lettuce, cabbage etc., for transplanting elsewhere,
and these can be moved with cords, from one place to another.
(Quoted in Leicht 1957:377.
Trans. A.J.R.)
The description is perfectly clear, and both of these authors are quite
definite that the seeds were grown on these improvised rafts and not
simply transported on them.
The same authors provide descriptions of the
chinampa platforms themselves and, as these are accurate, there is no
good reason for supposing that their descriptions of floating seedbeds
are not similarly accurate.
Technically the device is quite feasible
and is in use in other parts of the world today, and so it seems probable
that the Aztecs did employ floating seedbeds but that the practice has
since been abandoned.
Seedbeds are s t i l l used for the cultivation of flowers in the
modern chinampas, but most other crops are sown directly into the fields.
The chinampas are an intensive agricultural system in decline and it is
characteristic.of such systems that they become less intensive as the
decline proceeds.
On these general grounds it is likely that seedbeds
were much more thoroughly exploited in the past than they are today.
This seems to be confirmed in the case of maize, which is now sown
directly into the fields from which it is to be harvested.
Many "chinam-
peros" report that they used to grow maize in seedbeds but that they
now no longer do so, and West and Armillas note that during the 1940 s
1
maize was grown in seedbeds in Mixquic which is rather more remote from
Mexico City than the other extant areas of chinampas and rather more
126
conservative in its practices, but that it was not so cultivated in
Xochimilco (West and Armillas 1950:1).
Fray Alonso Ponce who travelled
in New Spain between 1585 and 1587 notes that maize was cultivated in
seedbeds in the chinampas, and that the practice was peculiar to the
area (Leicht 1937:377).
Because it is so great a departure from normal
Middle American practice, maize is the least likely of all crops to be
started in seedbeds, and yet it seems clear that it was so planted.
If
this is the case it is likely that virtually all crops in the past were
started in seedbeds.
There is some linguistic evidence that the Nahua word "chinampa"
or "chinamitl" should be translated as "seedbed" and that it was not
used to refer to the agricultural platforms.
The latter were probably
called "tlateles", which is a derivation of the term for "a pile of
earth".
The fact that we now use the word "chinampa" to refer to the
platforms may arise from a misunderstanding on the part of the early
Spanish colonists (de Lameiras 1974:22-23).
Whether physically or
etymologically, seedbeds are very closely bound up with the practice of
chinampa agriculture.
Weeding and Pest Control
Seedbeds were elaborate and successful devices for propagating
populations of crop species that were not well equipped to propagate
themselves.
The devices used by the "chinamperos" for suppressing the
populations of unwanted species were very much less elaborate.
Weeds do not appear to have been a great problem in the seedbeds.
The ground upon which the beds were laid out was carefully weeded, and
once the "agualodo" was in place it suppressed the growth of weed
127
seedlings.
The mud which was dredged up from the bottom of the canals
may have contained few fertile weed seeds, and the layers of grass and
other materials that were laid over the surface of the seedbeds may
have helped prevent invasions by weeds during the few weeks of the life
of the seedbed.
In general the seedbeds in the modern chinampas seem to
be quite free of weeds.
In the modern chinampas, the crops are mulched when they are
planted out, which suppresses the growth of weeds to some extent.
nevertheless, weeding is necessary.
But,
The modern "chinamperos" weed their
maize about a month after it is sown when the plants are about 50 centimeters high, but most "chinamperos" do not undertake any subsequent cultivation.
It is probable that in the past the maize was more frequently
weeded than it is now.
The Aztecs may have weeded their crops at inter-
vals of about six weeks, which is the case with other intensive agricultural systems in Middle America today.
During the winter months
weed growth is to some extent held back by declining temperatures but,
even so, weeding is a chore that has to be carried out occasionally.
In the modern chinampas something like half of each year's crop
of maize is lost to insect pests and plant diseases (J. Cornejo pers.
com.).
We know from Sahagun's works that many of the pests that cause
this damage were present in the Aztec chinampas and yet it appears that,
although the potential for damage existed, it was not realized. There
are no historical references to damage by pests, and the older "chinamperos" today recall that in their youth pests were simply not a problem.
Nor was this because the "chinamperos" had some curious and efficient
means of eliminatingunwanted animals; as is generally the case with
128
traditional agricultural systems in Middle America, the repertoire of
agricultural techniques includes no methods or devices for combating
insect pests other than pinching the occasional caterpillar off a
plant if it happens to be noticed.
The means by which populations of insect and other pests are
regulated in traditional agriculture is a topic that is not well understood.
Even if such a practice is not imposed by the climate, most
traditional systems interrupt production during one season of the year.
One of the benefits of this practice is that populations of pests die
back somewhat during the period when the crops are not present in the
fields.
In the chinampas the seasonal alternation of maize with
horticultural products would have had this effect on populations that
were entirely dependent on maize.
The fact that the winters in the
Valley of Mexico are quite cold may have further regulated pest populations, but it is evident from the condition of the modern chinampas
that the climate alone is not capable of preventing unacceptable depradations.
There is mounting evidence that a diversified flora in the
vicinity of crops is an important element in the regulation of
populations of pests.
If there is a diversity of microhabitats, popu-
lations of predatory and parasitic species can survive during those
periods when crops are not in the fields, and when populations of the
pest species upon which they depend are at low levels.
When the crops
are reestablished, the pest species multiply but, because the predatory
and parasitic species are available in the vicinity, populations do not
increase to epidemic proportions.
129
There are difficulties with this theory, one of them being that
the diversified flora that allows the predatory and parasitic species
to survive may also contribute to the survival of the pest species.
However, workers in the field are becoming increasingly convinced that
it is mechanisms of this sort that account for the low levels of crop
damage by insect pests in traditional agroecosystems (Janzen 1973:1214;
Gliessman et al. 1978).
As indicated in the preceding sections, the
chinampas have a highly diversified flora and it seems probable that
these biological control mechanisms were at work on behalf of the Aztec
agriculturalists.
A similar set of mechanisms at a different scale may also act
to regulate populations of soil pathogens.
important role.
Birds, too, may play an
One of the reasons cited for the current problems of
the chinampas is that bird populations in the district have been hunted
almost to extinction (J. Cornejo pers. com.).
Also related to the control of pests is the attitudes of the
farmers themselves.
Farmers in the Yucatan accept that a certain pro-
portion of their crops will be lost to pests.
They anticipate the loss
by planting something additional "for the birds," and the attitude
appears to be quite general among Indian farmers.
Attitudes about the
pests themselves are also rather different from our own.
According to
Sahagun, the Aztecs considered harmless many of those species that we
regard as pests.
The corn ear worm, that we look upon with disgust, was
seen by the Aztecs as a tasty morsel that occasionally appeared in the
ears of corn (Curran 1937).
The Aztecs, along with other traditional
farmers, accepted a level of damage that we would not.
130
We hardly understand these mechanisms at a l l , but in the absence
of any plausible alternative it seems that the Aztecs relied chiefly on
the "balance of nature" to regulate populations of unwanted organisms.
The balance is a fragile one and in the modern chinampas it has been
disrupted, but in the past it appears to have maintained populations of
pests and pathogens at acceptable levels.
Material Cycles
Ecosystems are open with regard to materials.
They receive
materials from their environment, cycle them within the system for a
greater or lesser period, and then discharge them from the system.
Water entered the chinampas by the various routes discussed at considerable length in the preceding section of this work. Once within the
chinampas it was stored in the canals or in the underlying aquifers,
and it was cycled by capillary action within the chinampa soils and by
irrigation.
The most obvious need for irrigation is to satisfy the water
requirements of crop plants and to allow their cultivation in those
areas and at those times of year when rainfall alone is inadequate.
In climates when evaporation rates are high, irrigation is also required
to wash away the salts that accumulate on the soil surface.
In the Lake
of Mexico, where the water was brackish and evaporation rates were high,
this may have been a major problem.
Irrigation in the chinampas is now
carried out with plastic buckets, watering cans and mechanical pumps, but
in the past maguey buckets may have been used and the "chinamperos"
also employed a large wooden spoon attached to the end of a long pole,
which is called a "texpetlatl".
131
The Comision Hidraulica de la Cuenca Del Valle de Mexico
(C.H.C.V.M.) has made an estimate of the consumptive use of water for
different crops grown by modern methods in the vicinity of the chinampas, and the results are shown in Table VI. It is clear that rainfall
obviates the need for irrigation during the summer months, except
during August where there is a slight decline in precipitation values.
During the winter months when precipitation values are low, irrigation
is required. Maize is irrigated during April and May to insure that
the seeds' germinate whether or not the rains are on schedule.
The fact
that it is not irrigated during the "Veranillo", when other crops are, is
attributable to the fact that it transpires water at a relatively low
rate.
The figures estimated by the C.H.C.V.M. represent the needs of
crop plants grown at low elevations in the southern parts of the Basin
of Mexico, and they probably overrepresent the consumptive use of water
in the chinampas.
Mulching is an important element of chinampas agriculture. In
Mixquic, seed beds are sometimes covered with a layer of finely sifted
soil as soon as they are planted.
This protective layer is left in
place during the few.days that it takes the seeds to germinate, after
which it is carefully swept away and replaced with a layer of grass
(Vanegas 1978:16).
In other areas of the chinampas, the initial appli-
cation of soil is omitted, but seedbeds are covered with any materials
that will protect the seedlings, including plastic sheeting or old newspapers, as well as grass or leaves.
When plants in the "chapines"
have grown a few inches, a framework of branches or weeds is erected
over the seedbeds, and this is covered with grass, leaves or other
materials.
When the crops are planted out, a mulch of chopped up water
132
TABLE VI
CONSUMPTIVE USE OF WATER IN CENTIMETERS
FOR LOCALITIES AT LOW ELEVATIONS
IN SOUTH OF VALLEY OF MEXICO
Flowers
January
18
15
-
15
February
18
15
-
15
March
18
15
12
15
April
18
15
6
15
May
-
-
-
-
June
-
-
-
-
July
-
-
-
-
August
18
15
-
15
September
-
-
-
-
October
-
-
-
-
November
18
15
-
15
December
18
15
-
15
Source:
Maize
Clover
Vegetables
C.H.C.V.M. VI:135.
weeds is applied, either over the whole surface of the plot or in the
immediate vicinity of each plant.
The more delicate plants are pro-
tected with cones of staw, grass or newspaper, which are known as
"abrigos" or "overcoats".
These mulches and coverings are valuable because they provide
nutrients for plants, suppress weed growth to some extent, and provide
protection against low temperatures during the winter months, but one of
133
their chief effects is to reduce evaporation losses and to prevent
the soils and seedbeds from drying out.
The soils of the chinampas
have good tilth and a high organic matter content, and the Ahuejote
trees reduce windspeeds and provide shade, both of which features further
reduce the rate at which water is lost by evaporation.
With regard to
the latter point, it may be significant that the chinampas of the San
Luis area are laid out on an approximate north-south axis, which maximizes
the extent of the shaded area.
The initial fertility of the chinampas was high, and the Aztecs
maintained and enhanced this fertility with a variety of other nutrientrich materials.
Water weeds were used as a green fertilizer, and the
duckweeds in particular had a high nutrient value.
Both waterweeds and
crop residue are composted in heaps on the modern chinampas, and there is
some linguistic evidence that the practice is pre-Hispanic. The nutrient value of leaf mould and rotten wood was understood by the Aztecs.
They distinguished soil enriched with these materials as "Quauhtalli";
the materials are imported for use in the modern chinampas, and again
it is probable that this is a continuation of an old established practice.
The Aztecs had no large animals, but turkey manure, "totolcuit-
latl", was used as a fertilizer and bat dung was imported from caves at
Ixtapalapa, and from Guerrero and Morelos, for use in cultivating
chiles (West and Armillas 1950:167-68; Gibson 1964:306; de Lameiras
1974:25, 28; Sanders 1979:290).
The Nahua words for "fertilize" and
"defecate" are derived from the same root, and the Aztecs were undoubtedly aware of the value of human waste as a fertilizer.
Settlements
were more widely dispersed in the 16th Century chinampas than they are
today, and undoubtedly night soil was informally distributed around the
134
as is the case with the home maize fields in many parts of Middle
America today.
There is also some evidence that sewage was system-
atically collected from dwellings and public latrines in the larger
towns and delivered in boats to the chinampas for use as fertilizer
(Armillas 1971:654).
There are two difficulties with this practice,
one being that there is a danger of spreading pathogenic bacteria, and
the other being that fresh sewage has a high initial toxicity for
plants and can kill them completely if concentrations are too high.
Both these problems can be overcome quite simply by anaerobic digestion;
if the material is composted for one or two months in pits, it is quite
safe to use as a fertilizer, but we have no information on Aztec practices in this respect (Chaney 1973:69).
Quite a wide range of materials
were used as fertilizers by the Aztecs.
These are listed in Table VII,
together with an indication of their nutrient content.
TABLE VII
NUTRIENT CONTENT OF ORGANIC FERTILIZERS
Material
%
Bat Guano
1
P
1.5 - 2
3
1
0.85
0.45
1 - 3
0..5 - 4.,0
0.0 - 0.5
6 - 7
Trierweiler and Utzinger 1975:18.
Brown et al. 1976:150.
2
4
% P
2
Duckweed
% K0
6
1
1
° 2°5
4..5 - 8..5
1
Sewage Sludge (digested)
2
5 - 10
Poultry Manure
Poultry Manure (fresh)
N
1..4 - 3.,0
% K
1.5 - 3.0
135
Of all the macronutrients, nitrogen is the one that is most
frequently a limiting factor on crop production. Atmospheric nitrogen
is ubiquitous but it cannot be used in this form by plants, and the
materials such as bat dung, bloodmeal, fish meal and bone meal in which
there is a high concentration of nitrogen in a form suitable for applying to fields are quite scarce.
A common solution to this problem is
to incorporate legumes or one of the other species of plant that are
host to nitrogen fixing bacteria into the crop cycle, either by rotating them with the main crop or by planting them as companions of the
main crop.
The chinampas were used intensively and it is very unlikely
that maize was rotated with a leguminous crop on an annual basis.
Cer-
tainly, air photographs taken in 1947 and the photographs accompanying
Schilling's 1938 article on the chinampas show that virtually all of the
cultivable land was planted to maize during the summer months, which
precludes the possibility of a regular annual rotation.
It is possible
that a crop such as beans was rotated with maize on a seasonal basis,
and that at least part of the land was planted to beans during the winter
months when maize was not in the fields.
Alternatively, maize could
have been grown with squashes and beans as companions, which is a widespread Middle American practice.
This practice is not common in the
modern chinampas, but it is hard to say whether this is because the
chinampas are now being cultivated less thoroughly than in the past or
because the practice never occurred in the chinampas.
Measurements made
in Tabasco show that the yield of maize cultivated with beans and squash
is about 10% higher than when it is cultivated alone.
If the Aztecs
exploited every available means to increase the yields of their chinampas,
then it is probable that polyculture was part of their repertoire of
agricultural techniques (Gliessman and Amador 1979:Tables 1 and 2).
136
The structure of the chinampas and the techniques of chinampa
agriculture tended to promote a cyclical and conservative use of
nutrients (Figure 30).
The canals functioned as a repository for
nutrients, which entered by surface runoff from the surrounding h i l l sides or by leaching and runoff from the surface of the platforms.
Dissolved nutrients were incorporated in the tissue of aquatic plants,
and the water in the canals moved so slowly that even very fine suspended solids accumulated in the bottom mud. Nutrients were removed
from the canals in irrigation water, and by the collection of water
weeds for use as mulches and green fertilizers, and of bottom mud for
the construction of seedbeds.
Crop residues and water weeds were com-
posted and applied as fertilizer.
Any materials that decomposed to
such an extent that they could not be gathered up were finally returned
to the canals in runoff and accumulated there for future application to
the crops on the platform surface.
There was undoubtedly some loss of
nutrients to the system in slow drainage of water, but this loss was
made good by inputs of runoff from surrounding hillsides.
Nutrients
were also lost in the exportable components of crops, but if it really
was the case that sewage sludge was collected, then a proportion of
these would have been returned to the chinampas, and any shortfall
made up by the import of other fertilizer materials.
No precise data
are available, but it seems clear that the chinampas made a very conservative use of nutrients.
Agricultural Calendar
The modern chinampas are not cultivated as intensively as they
were in the past, but the general sequence of agricultural activities
137
Figure 30.
Schematic Representation of Nutrient
Transport in Chinampa Agriculture.
138
appears to be much the same now as it was during the 16th Century. This
is broadly confirmed by the sequence of Aztec ritual activities, about
which we do have some information.
Most of the major events of the
agricultural year were maked with ceremonies.
These are shown in
Table VIII, together with notes on the major activities in the modern
chinampas, supplemented by the few historical references that are available.
It is clear that the agricultural calendar was dominated by the
production of maize.
In the modern chinampas the preparations begin in
February, when the last of the winter crops are removed from the fields
and the ground is prepared for sowing.
In the 16th Century, when maize
was started in seedbeds, it is probable that the winter growing season
was extended into March.
The ritual activities in February were largely
concerned with assuring an adequate supply of rain for the spring planting.
In March and April, the maize was sown either directly into the
fields as it is now or, more probably, into seedbeds.
The sowing was
anticipated slightly by the festival of Xipe Totec, the god of spring
and regrowth, and various ceremonies involving the adoration of corn
deities and of the seed corn itself were performed.
The maize was
transplanted in May, 30 or 40 days after it had been sown, at about the
same time of year as the modern "chinamperos" weed and fertilize their
fields.
At the end of May children were sacrificed to the rain god
Tlaloc, presumably in an effort to insure the continuation of the rains.
June is a slack period for modern "chinamperos", but the Aztec farmers
may have weeded their fields at this time.
Young ears of corn were con-
sidered a delicacy, as they are today, and during July they became
available.
The Aztecs marked the season with a sacrifice, after which
TABLE VIII
AZTEC AGRICULTURAL AND RITUAL CALENDARS
Agricultural
Month
J
A
N
U
A
R
Y
Activities
7
No.
17
of
winter
crops
goddess.
18
1
squash
planted
p
6.
"Old Princess".
Sacrifice of woman impersonating
8. Women and children beaten to make them cry, stimulating rain.
1. IZCALLI.
2. "Resuscitation".
5. Firegod.
8.
3. XIUHTECUHTLI.
4. "Lord of the Year".
At the end of this month were five "useless days",
prepared
for
planting
1. ATCOUALCO. 2. "Lack of water".
3. CHALCHIUHTLICUE.
5. Goddess of rivers, lakes, and oceans.
ground
c
3. LLAMATECUHTLI. 4.
NEMONTEMI, required to correct the solar calendar.
Y
I
H
2. "Severe weather".
5. Creation and fertility goddess.
r
M
A
R
1. TITITL.
harvest
p
E
B
R
U
A
R
Aztec Solar Months and Ceremonial Activities
2
4. "Jewelled Robe.
7. TLALOC, "He who makes things
grow" in charge of rain, hail, thunder, floods.
8. Sacrifice of a child.
1. TLACAXIPEUALIZTLI.
3. XIPETOTEC.
2. "The flaying of men".
the Flayed One". 5. God of spring and regrowth.
4. "Our Lord,
6. Priests dressed in the
flayed skin of sacrificial victims, representing the germinating seed in the
dead husk.
TABLE VIII (continued)
AZTEC AGRICULTURAL AND RITUAL CALENDARS
Agricultural
Month
Activities
No.
3
maize
sown
4
3. COATLICUE. 4. "Serpent Skirt".
6. Offerings of flowers.
7. TLALOC.
1.
HUEL TOZOZTLI. 2. "long fast".
3. CENTEOTLI and CHICOMECOATL.
5. Maize deities.
6. Seed corn blessed
and worshipped.
maize
transplanted
or weeded and
fertilized
5
1. TOXCATL. 2. "Dug or slippery".
3. TEZCATLIPOCA and HUITZILIPOCHTLI.
4. & 5. See months 9 & 12.
2
6
1. EIZALQUALIZTLI.
2. "Bean porridge".
3. TLALOC.
4. & 5. See month 13.
6. Two children drowned by tying them to a boat loaded with sacrificial
J
hearts, and sinking the boat.
U
N
E
2. "Short fast".
4. "Corn God" and "Seven Snake".
L
Y
1. TOZOZIONTLI.
8. Child sacrifice.
P
R
I
A
2
5. Earth and fertility goddess.
A
M
Aztec Solar Months and Ceremonial Activities"
7
1. TECUHILHUITONTLI.
2. "Little feast of Lords".
4. "Salt Women". 5. Goddess of saltwaters.
3. HUIXTOCIHUATL.
TABLE VIII (continued)
AZTEC AGRICULTURAL AND RITUAL CALENDARS
Agricultural
Month
J
U
L
Y
Activities
selective
harvest of
young ears
No.
8
Aztec Solar Months and Ceremonial Activities
1. HUEITECUHILHUITL.
2. "Great Feast of Lands".
loose, simulating corn tassels; after sacrifice, "elotes" eaten.
of corn
1. TLAX0CHIMAC0.
2. "Birth of flowers".
4. "Hummingbird of the Left Hand".
A
maize
U
G
U
ripening
in
S
E
P
4. "Young
Corn Ear". 6. Sacrificed slave girl impersonating deity; women's hair worn
9
S
T
3. XILONEN.
3. HUITZILIPOCHTLI.
5. Tribal god of Mexica; god of
hunting and war.
10
fields
1. X0C0TLHUETZI.
God".
amaranth
harvested
2. "Fall of fruits".
5. Fire god.
3. HUEHUETEOTL. 4. "Venerable Old
6. Prisoners drugged, burned but not killed on fire,
dragged out with hooks and sacrificed.
2
11
1. OCHPANIZTLI.
2. "Month of brooms".
filth or sins".
5. Earth and corn goddess.
T
yellow as maize decapitated.
t
M
warriors.
3. TLAZ0LTE0TL. 4. "Eater of
6. Girl with face painted as
8. Efforts to avoid rain; ritual review of
TABLE VIII (continued)
AZTEC AGRICULTURAL AND RITUAL CALENDARS
Agricultural
Month
B
E
R
0
C
T
0
B
E
R
N
0
V
E
M
B
E
R
D
E
C
Activities
mai ze
harvested
No.
12
Aztec Solar Months and Ceremonial Activities
1. TEOTLECO. 2. "Return of Gods". 3. TEZCATLIPUCA.
5. Texcocan god of war and hunting.
chiles
planted
4. "Smoking Mirror".
8. Return of gods celebrated with
furnace sacrifice; ritual drunkenness.
tomatoes
planted
13
1. TEPEILHUITL.
2. "feast of the mountains".
3. TLALOC.
4. "He who
3
makes things grow".
5. God of rain, hail, thunder, floods, conceived
to live on top of mountains.
vegetables
Amaranth used.
6. Four women and one man sacrificed.
Ritual cannabalism.
and
flowers
planted
14
2. "bird month".
5. God of war and hunting.
15
start of
harvest of
1. QUECHOLLI.
1. PANQUETZALIZTLI.
above.
3. MIXCOATL. 4. "Cloud Serpent".
6. Ceremonial hunt; making of weapons.
2. "feast of flags".
3. HUITZILIPOCHTLI.
4. & 5. as
6. "Flowery Wars", ritual combats with neighbouring states in
which prisoners were taken for sacrifice.
TABLE VIII (continued)
AZTEC AGRICULTURAL AND RITUAL CALENDARS
Agricultural
Month
E
M
B
E
R
Note:
Activities
vegetables
and
flowers
chiles
transplanted
No.
16
Aztec Solar Months and
1. ATEMOZTLI.
2. "Fall of waters"
Name of the solar month
Meaning of the name of the month.
Name of principal deity associated with month.
Meaning of name of deity.
Function of deity.
Character of ceremonies.
Other deities associated with month.
Character of ceremonies associated with other deities.
Sources:
1
2
3
3. TLALOC. 4. & 5. as above.
The Aztec solar calendar comprised 18 months of 20 days each, together with five "useless days"
which were required to correct the calendar to the solar year.
Explanation of numbers:
1.
2.
3.
4.
5.
6.
7.
8.
7
Ceremonial Activities
Vaillant 1965:187-190, 200-202.
Peterson 1959:125-136, 167.
Gibson 1964:121.
144
the "elotes" were harvested.
During August the rains decline and at
the end of August amaranth, which matures slightly faster than maize,
was harvested, providing some insurance against a late failure of the
corn crop.
During the latter part of August and the early part of
September, ritual activity was directed at achieving hot, dry weather
so that the corn would mature and dry out; towards the end of September
it was harvested, as it is now.
After the harvest the feast of "the
return of the gods" was celebrated, with ritual drunkenness that was
perhaps an expression of relief that the harvest was safely secured.
As soon as the maize is harvested, the winter cycle begins with the
planting of chilies and other vegetables and flowers.
continue through the f a l l .
These activities
The harvest of these various winter crops
begins in January and continues through the early spring. A great deal
of work is involved in these horticultural activities and the crops are
important, but not as important as maize.
During the winter, Aztec
ritual attention shifted from agriculture to hunting and war.
The modern chinampas are cultivated quite extensively and there
is a tendency to avoid the use of seedbeds for crops other than flowers,
and to sow directly into the fields once the preceding crop has been
harvested.
In the past this was probably not the case, and the succes-
sion of crops overlapped with a second crop sown in the seedbeds before
the first was harvested from the fields.
By exploiting seedbeds in this
way the agricultural calendar may have been far more intricate than is
indicated by this generalized summary, and with irrigation, fertilization
and careful timing it would have been quite possible to maintain the
entire area of chinampas in continuous production.
145
Yields and Energy Ratios
Table IX gives the yields of shelled maize from seven plots in
modern Mixquic and shows values ranging from 1.5 tons per hectare to
5.1 tons per hectare, with a mean value of 3.9 tons per hectare. The
Xochimilco chinampas are extremely polluted and large areas have been
abandoned, but the C.H.C.V.M. estimates that the yields from the areas
that are cultivated are about 3.5 tons per hectare.
How close these
modern figures may be to pre-Hispanic yields is uncertain.
The higher
modern values reflect some use of chemical fertiliziers, which would
suggest' that Aztec production may not have reached these levels. However, the chinampas are now in very poor physical condition and they may
have been more productive in the past than they are now.
The modern
chinampas are not well looked after, and the low yields from some of
the Mixquic plots are entirely attributable to poor management.
During
the 16th Century the work of agricultural labourers was carefully supervised by state appointed officials, and it is probable that the whole
area of the chinampas was very carefully managed.
Both of these factors
could have resulted in 16th Century yields that were at least as high as
modern average yields, and quite possibly very much higher than that.
If
one assumes that the lack of chemical fertilizer was offset by the
better conditions and management, then a yield between three and four tons
per hectare of maize would seem to be a reasonable estimate of the productivity of the Aztec chinampas, and most modern writers support estimates in this range (Sanders and Price 1968:148; Cainek 1972:111;
Parsons 1976:242-45; Vanegas 1978:6).
The estimate is of the yields of
the cultivable areas of the chinampas only.
Given that about one quarter
146
TABLE IX
YIELDS OF SHELLED CORN, MIXQUIC
Source:
Plot No. 1
4612 Kg/Ha
Plot No. 2
1519 Kg/Ha
Plot No. 3
3923 Kg/Ha
Plot No. 4
4585 Kg/Ha
Plot No. 5
3037 Kg/Ha
Plot No. 6
5098 Kg/Ha
Plot No. 7
4543 Kg/Ha
Vanegas 1978:22-23.
of the total area of the chinampas.is occupied by uncultivable ditches
and canals, this gives a yield of between 2250 and 3000 kilograms per
hectare for the whole area of the chinampas.
The chinampas yielded a number of crops"in addition to maize.
Table X gives yields of the saleable portions of various vegetables
cultivated in modern Mixquic.
The figures are for year-round production
of vegetables and are not representative of the 16th Century chinampas,
but they suggest that the yield of horticultural products could have
been quite high.
If, as an approximation of 16th Century yields, one
takes the average of the yield shown in Table X and assumes that vegetables were cultivated only during the six winter months, that the
yield of indigenous varieties grown during the off season was only half
as great as the yields of modern varieties grown throughout the year,
and that in any one year only half the cultivable land was sown to
winter vegetables, then the chinampas yielded about 5 tons per hectare
147
TABLE X
YIELDS OF SALEABLE PORTIONS
OF SELECTED VEGETABLES, MIXQUIC
Celery
Acelga
Spinach
5.4 kg/m
p
2
6.6 kg/m
2.2 kg/m
2
Coriander
1.7 kg/m
2
Source:
Vanegas 1978:20.
of vegetables from cultivable land, or about 3.75 tons per hectare overall,
in addition to the yield of 2.25 to 3 tons per hectare of maize.
To put
these figures in perspective, the yield of maize from hand-tilled extensive systems in other parts of Mexico and Guatemala is generally in the
vicinity of one ton per hectare, or less than 0.25 tons per hectare i f
fallow land is included in the calculations.
Yields of commercial corn
production in the United States are rather more than 5 tons per hectare.
An alternative to expressing yields as output per unit area is
to express them as output per worker.
Sanders has estimated that in the
Aztec economy, a single "chinampero" could have worked between 0.5 and
0.75 hectares of chinampa land (Cited in Parsons 1976:242).
If the lower
figure is combined with the estimate of yields per hectare, then a single
"chinamperos" could in the course of a year have produced between 1.125
and 1.5 tons of maize, together with about 1.875 tons of vegetables.
The estimate for maize seems reasonable, and it conforms with data from
North West Guatemala and the Yucatan that indicates that farmers in
extensive traditional agricultural systems produce about 1.2 or 1.3 tons
148
of maize in the course of a growing season (Redfield and Villa 1934:52,
55; Stadelman 1940:103, 147, 149; Steggerda 1941:118; Villa 1945:60, 61).
Sanders' higher figure leads to estimates of 1.7 to 2.2 tons of maize
plus 2.8 tons of vegetables as the annual output of a single worker, but
although this may have been possible in the productive chinampa land,
the figure seems too high.
If energy values are assigned both to the output and the input
of an agricultural system, then the ratio of the output to input gives a
useful measure of the energy efficiency of the system.
In the present
calculations, inputs and outputs are expressed as the energy equivalent
of a kilogram of maize.
The outputs of the chinampas included 2250 to
3000 kilograms per hectare of maize, together with some 3750 kilograms
per hectare of assorted vegetables.
Expressed as a percentage of the
calorific content of maize, vegetables range in value from about 16% in
the case of leafy greens to about 120% in the case of beans.
Bearing
in mind that beans were an important secondary crop, it seems reasonable to employ a figure of 40% as an expression of the amount of vegetables that could be substituted or exchanged for a given volume of
maize.
Using this figure, the total output of each hectare of chinampa
land amounted to between 3750 and 4500 kilograms of maize or maize
equivalents.
Agroecosystems receive inputs of energy in the form of solar
radiation and of cultural energy.
The latter term includes all inputs
of energy in the construction of capital installations and equipment,
in the production and transport of'materials that are required in the
operation of the agricultural system, and in the modification of the
149
physical structure of the system by tilling, cultivating, harvesting or
any other agricultural activity.
In comparative studies of the efficiency
of agricultural systems solar energy is generally ignored and the calculations are restricted to the inputs of cultural energies.
It is clear
that a great deal of energy was invested in constructing the chinampas
and their associated hydraulic installations but, because this investment was amortized over many generations, the proportion of the initial
investment that should be debited to the energy budget of any particular
year of operation is so small as to be negligible.
The tools employed
in chinampa agriculture were generally quite simple and would not, for
the most part, have required more than a few hours' labour to construct.
Even if such items as "zoquimaitls" or "texpetlatls" had to be repaired
or replaced several times in the course of a year, the investment of
energy they represent is slight.
The only major piece of capital equip-
ment that the "chinamperos" required was a boat or a share in a boat.
If
modern practices are representative of Aztec times, the boats were constructed by specialists and were expensive to acquire, but with careful
maintenance their purchase price could be amortized over several years.
A good boat would last for 15 or 20 years before it had to be replaced.
Bat-guano, leaf mould and sewage sludge were imported to the chinampas,
but we have no indication of the quantities involved or of the amount of
labour employed in assembling and transporting these materials.
Apart
from these costs, the only major input of energy to the chinampas was in
the form of ongoing agricultural labour.
Using the diet of modern
Mexican "campesinos" as a guide, a single agriculturalist required something of the order ot 160 kilograms of maize for a year's subsistence,
together with other foodstuffs, bringing the total dietary input of each
150
worker to about 200 kilograms of maize or maize equivalent each year
(Parsons 1976:243).
If one assumes two workers per hectare and allows
a further 100 kilograms of maize or maize equivalent to cover the costs
of boat construction and material transport, then the energy equivalent
of 500 kilograms per hectare can be taken as an estimate of the input of
cultural energy to chinampa agriculture which, with an output of 3750 to
4000 kilograms, gives an energy ratio of between 7.5 and 8.0.
If one
assumes that a single worker could have managed 0.75 hectares of chinampa
land and allows 100 kilograms per hectare for additional costs, then the
input falls to 367 kilograms per hectare, and the efficiency increases to
between 10.2 and 12.3.
It is clear that even slight modifications to the assumptions
would make quite large differences to these estimates of yields and
energy efficiency.
However, the estimates do appear to be quite consis-
tent with data on other systems of traditional agriculture and they are
generally confirmed by other published accounts of the yields of chinampa
agriculture.
The Principles of Chinampa Agriculture
The chinampas can be thought of as comprising three separate but
complementary ecosystems, each with its distinctive characteristics, and
each contributing in a different fashion to the function of the system
as a whole.
One of these ecosystems consisted of the canals, which
occupied about 13% of the area of the chinampas, and which supported
the growth of many of the species of aquatic plants that had flourished
in the lake complex prior to its colonization for agriculture. Aquatic
plants are generally high yielding, and the fact that the canals of the
151
extant chinampas must be frequently cleaved of vegetation if they are
to remain navigable suggests that the productivity of this ecosystem
may have been quite high.
The platform borders and the community of plants that they
supported comprised a second ecosystem, which occupied about 11% of
the area of the chinampas.
This ecosystem included Ahuejote and other
trees, which, with a density of about one tree per one and a half square
meters, amounted to a considerable standing biomass.
It included species
of different dimensions, life-spans and growth habits, and it appears
to have had a high and persistent pattern diversity, at least in comparison to other ecological systems within the chinampas.
Taken together,
these characteristics suggest that the platform borders were quite generalized and had some of the characteristics of stable and mature ecosystems .
The remaining area of the chinampas was occupied by crop plant
communities of the platform surfaces.
A large number of different
species were grown and polyculture was practiced.
Different species
may have been grown together from seeds planted in the same holes, as
is the case in some extant Middle American agroecosystems.
More
probably, different species were interspersed in the same rows, in
adjacent rows, or in adjacent plots, as is the practice in the chinampas
today.
The crop communities may have been least diverse during the
summer when maize was the predominant crop but, even then, uninterrupted pure stands were limited to the area of a single chinampa platform, and it is possible that within the area of a single platform,
stands of maize were interspersed with stands of amaranth, flowers or
other crops.
During the winter, when a greater number of species were
152
cultivated, the plots would have been smaller, and the diversity of the
community correspondingly greater.
The practice of polyculture contrib-
uted a degree of complexity and structural diversity to the crop ecosystems of the chinampas, which would have enhanced their stability and
resilience.
However, the crop species were short lived, vulnerable to
the depredations of pests and the competition of uncultivated plants,
and, like crop ecosystems generally, those of the chinampas tended to
be immature and unstable.
The three ecological systems were closely interrelated. The
canals and the platform borders provided a number of biotic resources
that were of direct economic importance, but their major role in the
chinampas appears to have been to provide support and protection for the
crop ecosystems.
The canals acted as reservoirs for the materials that
were cycled within the chinampas.
Water was stored in them and nutrients
were stored in the water, in the bottom mud, and in the tissue of aquatic
plants.
These materials were available throughout the year.
When they
were required they were withdrawn from the canals and applied to the
platform surfaces.
The canal ecosystems not only stored water and nutri-
ents, but they also transformed them into materials that were valuable
for their structural properties.
Reeds and similar plants were used for
the construction of floating seedbeds and shelters, and they were woven
into a variety of useful artifacts.
Many of the plants that were used
as green fertilizers were valuable as well for the pockets of air trapped
in their leaves, which maintained the tilth of the chinampa soils, and
it was the peculiar spongy character of the bottom mud that made it
indispensable to the construction of seedbeds.
The canals and platform
borders acted to condition the microclimate of the chinampas.
The supply
153
of water in the canals may have been effective in maintaining a high
level of moisture in the vicinity of crops. The Ahnejote trees protected
the platform surfaces from wind and sun, and in so doing, smoothed
fluctuations in temperatures, evaporation rates and other climatic
variables.
Finally, the platform borders may have played an important
part in regulating populations of pests and pathogens.
The assertion
is not entirely substantiated, but it is probable that generalized
ecosystems in the vicinity of crop ecosystems provide a refuge for
birds and for predatory and parasitic insects and microorganisms,
with the result that these are available to suppress populations of
pests and pathogens when the crops are in the field.
It was suggested
that the canal border ecosystems were quite generalized, at least in
comparison to the crop ecosystems, and they may have performed this
function within the chinampas.
The relationship between the crop ecosystem, on the one hand,
and the canal and platform border ecosystems, on the other, was
intimate and intricate.
The crop ecosystems exploited the non-crop
ecosystems for their stock of water, nutrients and structure, and for
protection from climatic variables, but they did so in a manner that
allowed the latter to continue functioning unimpaired. The demand for
microclimate control and pest regulation was mitigated to the extent
that the heterogeneity of the crop ecosystems themselves permitted
the performance of these functions.
The exploitation of the canals for
materials was frugal, and most of the water and nutrients that were
removed were returned by the passive processes of decay and runoff,
where they again became available for future use.
Any exploitation
that represented a degradation of the structure of the canal ecosystems
154
was offset by the high productivity of the aquatic plant communities.
The relationship between the crop and non-crop ecosystems could be
described as a commensurate one in which the crop ecosystems benefitted
from the presence of the other two ecosystems, but the latter neither
benefitted nor suffered from the association.
The chinampas are here viewed as comprising three separate
ecosystems, and the hypothesis is put forward that the primary role of
the two non-crop ecosystems was to provide support and protection for
the fragile crop communities of the platform surfaces.
Much work
remains to be done before the hypothesis is substantiated and, in
particular, the role of the narrow strip of trees and herbs along the
borders of the canals in suppressing the growth of pest populations
requires investigation.
However, the hypothesis does account for the
facts that are available, and it is a useful one because it provides
a number of insights into the origins and functions of chinampa
agriculture.
First, it sets chinampa agriculture securely in its developmental context.
One of the earliest and most widespread forms of
agriculture in Middle America is Rosa cultivation, which essentially
consists of inserting a small crop ecosystem into a generalized natural
ecosystem.
The crop ecosystem is relatively unstable, but it is
supported by nutrients from the natural ecosystem, and to some extent
it is protected from, climatic flux and the explosive growth of pest
populations by the natural ecosystem.
The association is managed in
such a way that the crop ecosystem benefits and the function of the
natural ecosystem is not, in the long term at least, impaired. The
chinampas differed from Rosa cultivation in all particulars, not least
155
in the fact that the generalized, non-crop ecosystems were as artificial
as the crop ecosystems themselves, but the principle that underlies the
two forms is the same.
In both, a fragile crop ecosystem is supported
and protected by the existence of a more generalized ecosystem in its
close vicinity.
The chinampas were an elaboration of a pre-existing
form, and the multiple ecosystem construct emphasizes this.
The hypothesis also allows a fairly succinct expression of the
means by which the chinampas produced high yields without incurring a
commensurate cost in cultural energy inputs.
The high yields of the
chinampas, or of any other intensive agricultural system, were a function of the immaturity of the crop ecosystem.
The costs of maintaining
an ecosystem in an immature state are exacted in the measures that are
required to provide a suitable growing environment for the fragile
crop plants, and to protect them from diseases, pests and competing
plant species.
To some extent, both these functions were performed
in the chinampas by increasing the intensity of human labour, and this
was reflected in an energy ratio
that was lower than that of more
extensive traditional agroecosystems.
However, to a large extent,
these functions were performed in the chinampas by the canal ecosystems
that provided the crop ecosystems with readily assimilable nutrients
and by the platform border ecosystems that protected the crops from
climatic fluctuations and the growth of pest populations.
The cost of
these functions was not eliminated but, to the extent that it was
performed by the non crop ecosystems, it was exacted in the form of
the solar energy required to maintain these two ecosystems, rather
than in the form of human labour applied directly to the crop ecosystems.
156
Finally, the multiple ecosystem construct emphasizes the fundamental distinction between the principles of chinampa agriculture and
of modern western agriculture.
Both support crop ecosystems that are
fragile, specialized and unstable, and both include means for provisioning and protecting the crop ecosystems.
However, the chemical and
mechanical means that are employed by Western agriculturalists exact
a very high cultural energy cost, with the result that the energy
efficiency of Western agricultural systems tends to be very low.
In
the chinampas the same functions were performed by secondary ecological
systems, which although.they may not have been quite as effective as
the techniques of Western agriculture, exacted a very low cultural
energy cost with a result that the overall efficiency of chinampa
agriculture was considerably higher than the efficiency of commercial
Western agriculture, while the yields were at least as high, and
possibly very much higher.
157
CHAPTER V
THE MANAGEMENT OF CHINAMPA AGRICULTURE
Management is here understood as the process by which decisions
are made that have the effect of modifying the structure, and thence
the function, of managed physical systems.
The term is defined broadly
and in the present discussion the individual "chinampero" who makes
decisions with regard to a particular plant and the high order administrative institution that makes decisions with regard to the whole of
the lake complex are both understood as being managers.
It was sug-
gested that the development of the hydraulic works necessary for the
control of water levels was an essential element of the development of
chinampa agriculture and, this being the case, the most important function of management in the developmental phases of chinampa agriculture
was the organization of the labour and materials necessary for the
construction of hydraulic installations.
The management of these
installations in the Valley of Mexico, as elsewhere, was intimately
bound up with the government of the state.
To a very large extent the
development of institutions for the management of hydraulic resources
was identified with the development of the state itself.
Because
of this a brief account of the social organization and political history
of the Aztecs is a necessary preliminary to the discussion of the
management of the chinampas.
It is convenient to distinguish three phases in the development
of the Aztec state.
In the first phase, during the unsettled period
following the collapse of Toltec civilization in the late 12th Century,
158
the Valley was colonized by refugees from Tula and by immigrants from
beyond the borders of the Basin, and the political entity that is best
described as the city-state emerged.
In the second phase, city-states
were aggregated into progressively larger tribute states, culminating
in the formation of the Aztec state in 1433.
In the final phase, s t i l l
underway at the time of the Spanish Conquest, the basic institutions
of the large tribute state were modified and reworked, and the institutions of the centralized and bureaucratic state emerged.
Each of
these three phases exhibited distinctive characteristics in the relationship between the state and its hydraulic resources, and the character
of the management institutions that finally emerged was determined by
the processes that occurred during this period of evolution.
The City-State
Following the collapse of Teotihuacan in the late 9th Century,
and of Tula in the late 12th Century, the surviving inhabitants of
these great centres established themselves at settlements in other
parts of the Valley.
Among the more important settlements established
at this time were Atzcapotzalco, which was founded by refugees from
Teotihuacan, and Xico, Coatepec and Culhuacan, which were founded by
Toltec groups (Figure 31).
During the 13th and 14th Centuries this
population was augmented by the arrival of Otomi and Nahua tribes from
regions to the north of the Valley.
The new arrivals were known
collectively to the more civilized inhabitants of the Valley as
"chichimeca" or "dog-people" which is a term that carries all the
significance of "barbarian". The Otomi tribes arrived first and settled
towards the north of the Valley in such centres as Tenayuca, Zumpango
159
Figure 31.
Principal Settlements of the Valley of Mexico
in the 16th Century (Sanders, Parsons and
Santley 1979 Map 20; Palerm 1973).
160
and Xaltocan, and one group allied itself to the settlement of Coatepec
which became the nucleus of an important state whose centre subsequently
shifted to Texcoco.
The Nahua tribes arrived at a slightly later date
and established themselves at such centres as Xochimilco, Cuitlahuac,
Mixquic and Chalco, towards the south of the Valley.
The Mexico were
one of the last of the Nahua tribes to arrive, and they did not establish their settlement at Tenochtitlan until 1345 (Wolf 1959; Peterson
1962; Gibson 1964; Vaillant 1966; Davies 1977; Leon-Portilla (ed.)
1977).
The Chichimec tribes were attracted by the civilizations of the
Valley and soon acquired many of the traits of civilization, but they
also brought with them cultural traits and institutions of their own.
One of these was the "calpulli", and an understanding of this term is
essential to an understanding of the subsequent political and social
developments.
"Calpulli" or in the plural, "calpultin", translates
literally as "big house" or "big family", and it was originally an
endogamous lineage group, distinguished from other such groups by a
patron deity, a name and battle insignia, and by a common if putative
ancestry.
The institution may have originated in some migratory
prehistory of the Chichimec tribes, but in the settled context of the
Valley it underwent considerable modification and acquired territorial
connotations.
By .the time of the Conquest the term referred not
only to a lineage group, but also to the tract of land claimed by
that group and to the village or neighbourhood of a town occupied
by the group.
These various associations of the term caused con-
siderable confusion in early Colonial accounts, much of which has
persisted in the modern literature (Wolf 1959:135; Carrasco 1971; Munzon
1977).
The "calpulli" was administered by a council called the "huehuetque",
161
which was headed by a senior member called the "caipullec". Members
of the council may originally have been elected from the population at
large, but in the late period of Aztec history they were drawn only from
the upper classes.
The "calpullec" was elected by other members of the
council and held his office for life.
The "huehuetque" was responsible
for the organization of education, civil order, public works projects
and religious and military activities, and it was provided with officials
for the execution of these various undertakings.
The "calpulli" was
also the institution in which was vested the ownership of agricultural
land, and the "huehuetque" administered the use of this land.
The larger "calpultin" were divided into segments, called
"tlaxilacalli", which consisted of groups of closely related families,
and which in conformity with the territorial associations of the
"calpulli" also carried the significance of a street or a block within
a neighbourhood of a town.
The smaller "calpultin" and the "tlaxilacalli"
of the larger "calpultin" were made up of individual families. Membership of a "calpulli" gave an individual certain rights, including the
right to the use of a plot of "calpulli" land.
It also carried with
it certain obligations including the obligation of cultivating the plot
of land and of providing goods or services for the support of public
works projects, religious or military activities and any other undertakings that were endorsed by the "calpulli".
The "calpullecs" served on a higher order council called the
"tlatocan", which translates literally as "place of discourse", and
this council was headed by a "tlatoani" or "speaker".
The "tlatocan"
may have originated as an institution roughly corresponding to a tribe,
but as the Nahua tribes settled in the Valley of Mexico it came to be
162
better described as the administrative body of a small city-state.
A city-state comprised agroup of "calpultin", either genealogically
related or in a close political alliance, together with the villages
and towns that the "calpultin" occupied and the surrounding territories
to which they laid claim.
The smaller city-states consisted of a town
and a few outlying settlements, and they were administered by a single
"tlatoani".
The larger city-states were made up of several towns, each
with its territories and outlying settlements and each with its own
"tlatoani".
In this case the "tlatoani" of the largest and most influ-
ential town was in effect the ruler of the state.
The Confederation and the Tribute State
The city-states were quite stable institutions, and they became
the constituents of larger political aggregations.
Two rather different
types of supra-city-state polities developed in the Valley of Mexico,
the confederation and the tribute state.
Elements of the political
confederation were evident in a number of the larger states that
developed in the Valley, including the "Triple Alliance" that was the
core of the Aztec empire, but the form appears to have achieved its
fullest expression in the Chalco-Amecameca Confederation.
The Chalco
basin was.occupied by many different tribal groups, including two
Toltec groups settled at Xico and Tlapacoya, and several Nahua groups,
including the Mixquica, the Chalca and the Amecameca. Sometime during
the 13th Century these groups formed a confederation which at one time
included twenty-six separate city-states, each with its own "tlatoani"
(Davies 1977:46).
As far as one can tell from the limited historical
sources, this was a true confederation in which all the participants
163
acted together in affairs of common concern, but in which each citystate retained authority over its internal affairs.
The Confederation
that developed in the Chalco basin was powerful and able to withstand
attacks by other large states in the Valley; it was stable and persisted
as an independent entity until 1465, at which time it s t i l l had thirteen
separate "tlatoani" (Davies 1977:46).
A more common device for the formation of'polities at higher
levels than the city-state was the tribute state.
The Nahua and Otomi
tribes of the Valley of Mexico were nothing if not bellicose, and there
was constant warfare in the Valley.
Wars were quite formalized affairs,
and they were considered to be completed when one side succeeded in
sacking the chief temple of the city of the other.
The glyph that
means "victory" is a picture of a burning temple (Peterson 1962:160).
When the battle was completed the two sides sat down to negotiate the
value of the tribute due to the victors.
The payments were made both
in the form of goods, including the specialized products of the conquered
city.as well as common subsistence staples, and in the form of services,
including both labour on public works projects and military service as
the involuntary ally of the conquering state.
Wars were endemic and, as
the outcomes of successive campaigns, alliances and rebellions accumulated,
all of the cities of the Valley became enmeshed in an extremely complex
network of tribute payments.
As the result of past victories and
defeats, a single town might be the recipient of tribute from several
other towns, and be at the same time the donor to as many more (Gibson
1964).
The tribute states tended to be unstable and their borders
shifted as the result of rebellions by imperfectly subjugates tributaries
164
or the attacks of powerful neighbours.
For example, the city-state of
Cuitlahuac, which was unfortunate enough to be located midway between
the territories of three powerful neighbours, was at different times a
part of the territory of Culhuacan, Xochimilco and the Chalco-Amecameca
Confederation.
However, little by l i t t l e , the more powerful states
consolidated their hold on their territories and by the mid-14th Century
the earlier profusion of small city-states had been reduced to the
relative simplicity of.five large tribute states and the Chalco-Amecameca
Confederation, with only a few small city-states maintaining a semblance
of independence in the shadows of their more powerful neighbours. The
Chalco-Amecameca Confederation controlled most of the Chalco drainage
basin.
The Xochimilea state had at one time included a large part of the
Chalco basin and considerable areas in Morelos, outside the Valley, but
by the mid-14th Century it had contracted to include only areas within
the Valley, to the south and southwest of Lake Xochimilco. The Culhuaque
controlled the Ixtapalapa peninsula and, although its territory was
quite small, the state was pretigious because of its associations with
the ancient city of Tula.
Atzcapotzalco was the centre of the Tepanec
state that extended over most of the western parts of the Valley and
that included the recently established Mexica city of Tenochtitlan as
one of its tributaries.
Xaltocan appears to have been the dominant
centre in the north of the Valley, but not a great deal is known about
this region.
The Acolhua domain extended over the eastern parts of
the Valley.
It was administered from the city of Texcoco, which was
unrivalled as a centre of the arts and the sciences of highland Mesoamerican civilization.
165
It was inevitable that one of these states should attempt to
overpower the other five and gain control of the entire Valley. The
Tepaneca came very close to doing this.
In 1353 Tezozomoc became the
"tlatoani" of Atzcapotzalco and embarked on an unprecedented career
of empire building.
In 1357 Culhuacan was defeated by the Tepaneca
and their allies, who included the Mexica. A few years later Cuitlahuac
and Mixquic were conquered by the Mexica, acting on behalf of the
Tepaneca as well as in their own self interest.
Campaigns were initi-
ated against the Xochimilca in the 1360's, largely by the Mexica, but
with Tepanec support, but the Xochimilca were not finally defeated
until 1391.
In 1365 the Mexica embarked on their protracted war with
the Chalco-Amecameca Confederation, which lasted for about ninety years.
In 1395 the Tepaneca successfully attacked Xaltocan, and by the end of
the 14th Century all of the Valley except for Texcoco and the ChalcoAmecameca Confederation was a part of the Tepanec empire.
Tezozomoc
initiated campaigns against Texcoco in the early part of the 15th Century
which resulted in the defeat of Texcoco in 1416 or 1418.
Itzcoatl,
the "tlatoani" of Texcoco was killed and his son, Nezahualcoyotl, was
forced into exile.
The Tepaneca installed a puppet ruler in Texcoco,
and only the Chalco-Amecameca Confederation resisted assimilation into
the Tepanec empire.
Tezozomoc came very close to uniting the whole of
the Valley, but he did not quite succeed.
In 1426, at the reported
age of 106, he died and the Tepanec empire began to fragment.
1428 Nezahualcoyotl regained control of Texcoco.
In
Shortly after, the
Tepanecs of Tlacopan and the Mexico of Tenochtitlan rebelled against
Atzcapotzalco and joined the Texcocans.
In 1433 this "Triple Alliance"
166
defeated Atzcapotzalco, and the Aztec empire was established. The
"Triple Alliance" acquired all the tributaries of Tepanec empire
and effectively controlled the entire Valley, excepting only the ChalcoAmecameca Confederation, which resisted incorporation into the Aztec
state until 1468.
The Lake Aztec State
The concentration of political power continued after the victory
of the "Triple Alliance" in 1433.
Tlacopan was at the outset the least
powerful of the allies and it was quickly relegated to a subordinate
status,.leaving Texcoco and Tenochtitlan in control of the empire.
The Mexica had transformed themselves from a minor tribe to a major
power in the Valley in less than a hundred years, and they were an
efficient and assertive people.
Nezahualcoyotl was an able and res-
pected ruler and, while he remained alive, the Texcocans were able to
resist the assertive tendencies of the Mexica and to maintain a balance
in the power of the two states.
However, the power of Texcoco began to
decline after the death of Nezahualcoyotl.
By 1500 the decline had pro-
ceeded to such an extent that the Mexica were able to impose a puppet
ruler on Texcoco and Tenochtitlan became the single most powerful city,
not only in the Valley but in the whole of Middle America.
The process by which political power was concentrated, and
progressively larger polities were formed, was accompanied by a modification of social and political institutions.
The "calpulli" had been
an efficient device for marshalling the resources of the city-state
and the resources of several city states could be organized by means
of the tribute system, but finally both institutions were inadequate for
167
the administration of a large empire.
They were cumbersome and ineffi-
cient when operating over a large population or through many hierarchic
levels and, by maintaining local rulers in place, they tended to foster
the identity of the constituent parts of the empire rather than the
identity of the empire itself.
These institutions were s t i l l in opera-
tion at the time.of the Conquest which is why we have a fairly detailed
knowledge of how they functioned, but they were in the process of being
replaced by a very much more centralized administrative apparatus and
by a rigidly stratified social system.
Many of the institutions of the late Aztec state had been transmitted to the Nahua and Otomi tribes by the survivors of Teotihuacom
and Tula, and subsequently modified and integrated with more purely
Nahua institutions.
Like the earlier societies of the Valley, the Aztec
state was divided into a number of ranked social classes.
The major distinction was between the ruling class and those they
ruled.
The former were "tecuhtzin" (sing, "tecuhtli").
The class
included numerous ranked groups or individuals ranging from the "hueytlatoaui" or "great tlatoani" of Tenochtitlan, down to the lesser
nobility, or "pipi 1tin".
The status of "tecuhtli" carried with it
certain rights, including exception from tribute payments, and certain
duties, including submission to a set of laws that were somewhat more
harsh than those that governed the bulk of the population (Peterson
1962:123).
At the time of the Conquest the lower class was divided into a
number of ranked groups.
The "macehualtin" (sing, "macehual1i") are
probably,best understood as a class of free commoners, and included
skilled craftsmen and agriculturalists who had rights under the "calpulli"
168
system.
"Tlalmaitl" were also agriculturalists, but they had fewer
rights and were obliged to live and work on the large tracts of land
that were set aside for the support of high ranking individuals or
institutions.
class.
"Tlacotin" (sing. "Tlacotli") were of the lowest social
The term is often translated as "slave", but although individ-
uals in this condition were obliged to work for their masters and could
be bought and sold, they had a number of civil rights and were not
considered as property or chattels.
The social distinctions were marked by sumptuary regulations
that, for instance, prohibited the wearing of lip-plugs by any but
"tecuhtzin", and of jade or golden lip-plugs by any but high ranking
"tecuhtzin".
The distinctions were perpetuated by an educational system
in which the children of lords and those of commoners were educated in
different types of schools.
At the time of the Conquest, a certain
amount of social mobility persisted and, by exceptional military
accomplishments, it was possible for an individual of low status to
become a member of the ruling class, but the indications are that
status was to an increasing extent becoming hereditary.
At the same
time the distinctions between "macehualtin" and "Halmaitl" were becoming blurred, and a single undifferentiated lower class was beginning
to develop.
These horizontal social strata were crosscut by three major
vertical divisions, each of which was represented at all social levels
and all of which were united at the apex by the overlapping offices of
the "tlatoani" of Tenochtitlan. There was a well developed military
hierarchy.
All males owed military service and the members of each
"calpulli" fought as a unit, with military specialists on the "calpulli"
169
council functioning as officers.
With a social status bordering on
that of the nobility were the specialized military orders of "eagle"
and "jaguar" warriors.
High level officers were drawn from the ranks
of the nobility, and two senior commanders, the "tlacatecatl" and
"tlacochcalatl", the commander of warriors" and the "man of the javelin
house", were two of the four senior advisors to the "tlatoani" of
Tenochtitlan.. The latter carried the title of "tlacatecuhtli" or
"chief of warriors", and was in one of his functions the commander in
chief of the army.
There was an equally well developed religious hierarchy made
up"of various types of "tlamacazqui" or full-time religious functionaries.
At the lower levels were junior priests and priestesses,
including the "quacu.illi" who were.associated with the "calpultin",
and the "tlamacaztequiuaque" who were combination priests and warriors.
High priests of the many deities had the status of nobles and were
1
known collectively as "tlenamacac", and the high priests of Huitzilipochtli and Tlaloc, two of the most important Aztec deities, functioned
with thetwo high ranking military officials as the chief advisors to
the "tlatoani" of Tenochtitlan.
The latter included among his many
functions that of the high priest of Quetzalcoatl.
The third major hierarchy was administrative.
Ranking immedi-
ately below the "tlatoani" of Tenochtitlan were the two military and
two religious functionaries, one of whom carried the additional title of
"ciuacoatl" or "woman serpent", and who functioned as the vice-ruler
of the state.
Ranking below these was a council of thirteen high
making dignitaries called the "tecuhtlatoque".
This council included
high ranking military and religious officials, nobles with special
responsibilities for the administration of justice, and the "tlatoani"
170
of the more prominent city-states.
The administration was served by a
large number of officials of lower rank, including scribes or "tlacuilos"
and "calpixque" or "house attendants".
These latter functioned in many
capacities, but most particularly as tax collectors and tribute gatherers.
"Centecpanpixque" had the responsibility of collecting dues from
twenty families, and each group of five of these officials were under
the authority of a "macuiltecpanpixque".
served by an elaborate court system.
of relatively slight importance.
messengers'or policemen
The administration was also
Low level judges dealt with matters
They were assisted by scribes, by
called "topolli",;and by cryers or "tecpoyotl".
These judges were subject to strict rules of conduct and were responsible
to higher order courts which, in turn, were responsible to a supreme
court, the "tlacxitlan", which was presided over by the "hueytlatoani"
himself.
The authority of the central government was extended to the
constituent city-states of the empire through the agency of local
"tlatoani".
These offices were hereditary and generally filled by
local nobility, but it came to-be the case that the local "tlatoani"
had to be confirmed in their office by the central government, and they
came to function rather as state appointed provincial governors rather
than as independent local rulers.
The transition to a centralized and bureaucratic government and
to a stratified society was accompanied by a change in the institutions
of land tenure.
By military means, land was withdrawn from the "cal-
pulli" system and passed into various categories of state ownership
with designated tracts allocated to.the support of specified individuals
or institutions.
Numerous classes of land were distinguished, of which
the most important were lands for the support of religious activities
171
with particular, tracts set aside for the support of the priests of
particular gods; I'milchimalli" or "lands of the shield" and "yaotlalli"
or "war fields" which were for the support of military ventures;
"tecpantlalli" were government lands set aside for the support of
judges, scribes, and other officials as well as for singers and entertainers; and "tecuhtlalli" and "pillalli" were for the support of
individual nobles.
Particularly large areas were for the satisfaction
of the needs of the "hueytlatoani".
These lands were ascribed to
individuals, but they are probably better understood as being for the
support of the offices held by those individuals (Moreno 1977:418ff).
At the same time that land passed from "calpulli" to state ownership,
the status of the agriculturalists passed from that of "macehualtin"
to "tlalmaitl". The former paid a fixed share of whatever tribute
their "calpulli" owed, while the latter were obliged to give up all
their surplus production to the "calpixque", and were allowed to
retain only that portion that was considered necessary for their
subsistence.
The city-states had their origins in tribal groups and they
were quite stable and enduring polities.
They were aggregated into
larger polities by means of the tribute system, but the tribute empires
were unstable and tended to fragment into their constituent city-states.
The institutions of the late Aztec empire acted to reduce these divisive
tendencies.
Local rulers came to function as regional governors; land
was withdrawn from the control of the city-states and incorporated in
large estates that were under the direct control of the central government, and a rigid class system was imposed that undermined local, ethnic
allegiances and defined the role of the individual in terms of his
172
place in the state as a whole.
The transformation was not complete at
the time of the Conquest and many of the institutions of the "calpulli"
and the city-states remained in place, but it is clear that the older
institutions were being replaced by the apparatus of the centralized
bureaucratic state and that all the resources of the Valley were being
brought under the direct control of the government of Tenochtitlan.
The Construction of Hydraulic Installations
It was suggested that the first phase in the development of
chinampa agriculture was the excavation of ditches and, possibly, the
construction of platforms around the periphery of the lake complex.
The work could have been carried out in increments over long periods
by small family groups, or it could have been carried out quite rapidly
by working parties organized under the "calpulli" system.
Technically,
the devices are amenable to construction in either fashion, but in the
context of the social organization of the Nahua settlers it seems likely
that the "calpulli" was involved in the work.
Small dykes across the mouths of streams could have been undertaken by individual families, but with the dyking of larger areas,
technical considerations arise that suggest that the work was organized
at a higher hierarchic level.
The early dykes were simple constructions
of earth and stone and they would have been liable to erosion.
If they
were constructed in small increments, then effort would have been
wasted when the partially completed structures were eroded away or
when labour was expended on the maintenance of structures that in their
partially completed state served no useful purpose.
This would seem to
suggest that'the larger dykes would have been completed quite rapidly,
and this would have required the organization of large labour forces.
173
A second characteristic of dykes is that although they do not themselves
occupy a large area, they may control water levels in large areas.
It
is unlikely that the inhabitants of sites suitable for dyking would
have behaved in an altruistic fashion, and would have invested a considerable effort in constructing a dyke that was of benefit to others
beside themselves.
Under these circumstances it seems more likely that
the work was organized by some institution capable of marshalling
labour from the entire area to be serviced by the dyke, and of applying
it at the site of the dyke.
The "calpulli" and "tlatocan" institutions were clearly well
suited for the performance of these functions.
That the "calpulli"
could command the labour and material resources of its members was a
well established tradition, and the "tlatocan" could similarly call
on the resources of its constituent "calpultin".
Furthermore, both
institutions were quite stable, endorsed by history and bound by
kinship, and so they were appropriate for the organization of maintenance work on the installations once they were completed.
Small installations could be constructed and managed with the
resources available to the "calpulli" or "tlatocan", but with larger
installations the participation of institutions capable of commanding
greater sources of labour and materials was required. When hydraulic
installations became so large that they required a significant part of
the resources of the city-state as a whole for their construction and
operation, the state itself became the institution that was necessary
for the management of the undertaking. At this point the government
of the state became identified with the management of hydraulic
174
agriculture, and the subsequent history of the development of institutions for the management of hydraulic agriculture becomes identified
with the development of the state itself.
The period during which the tribute states were the dominant
political institutions of the Valley of Mexico is historically remote,
and the hydraulic installations that were constructed during this
period were obscured by later constructions, with the result that we
know virtually nothing of the works that were organized by these
polities.
The importance of the Tepanec empire suggests that it might
have enlarged its. resource base by constructing large areas of chinampas
along the western shores of what was to become the Lake of Mexico.
Because of its high salinity, Lake Texcoco was unsuitable for colonization for agriculture, but the Texcocans are associated with an expertise in hydraulic engineering and they may have been able to exploit
the discharge of fresh water from the slopes of Sierra Nevada to construct some areas of chinampas along the eastern shores of Lake Texcoco.
There is some evidence of early chinampas in the vicinity of Xochimilco
and Culhuacan, and both of these states may have been successful in
colonizing quite large areas of Lake Xochimilco. Lake Chalco was
shallow and amply provided with fresh water which made it the most
suitable of the lakes for colonization.
There are ethno-historical
indications'that hydraulic agriculture was practiced at an early date and
it is probable that this lake was the first to be fully controlled and
adapted to chinampa agriculture.
The tribute states had the capacity
to organize labour and materials for large hydraulic works and, although
the assertion cannot be substantiated in any detail, it is probable that
the governments of the tribute states organized the colonization of quite
large areas of the lake complex for chinampa agriculture.
175
Virtually all of the large hydraulic installations that the
Spanish encountered when they arrived in the Valley of Mexico were
constructed after the victory of the "Triple Alliance" in 1433, and
the ethnohistorical sources provide descriptions of the construction
of two of these large installations.
was constructed in 1466.
The Albarradon de Nezahualcoyotl
The work was initiated by Moctezuma the
Elder, the "tlatoani" of Tenochtitlan, who
. . . sent messengers to the kind of Texcoco . . . asking that
he prepare a plan to prevent the flooding of the city (of
Tenochtitlan) because many buildings were ruined and had already
collapsed. Nezahualcoytl came quickly to Mexico (Tenochtitlan)
and told Moctezuma that the best and most efficient remedy
would be to construct a barrier of wood and stone which would
hold back the water so that it would not reach the city
(Palerm 1973:83).
Torquemada's account then proceeds to details of the materials employed
in the construction, but he adds that both of the rulers initiated the
work with their own hands (Davies 1977:92).
We also have some detailed information on the construction of the
aqueduct from Coyoacan to Tenochtitlan in the last years of the 15th
Century and of the disastrous flooding that was caused by the aqueduct.
The Mexica required the water for the operation of their chinampas
because, without it, their plants were "drying out and withering."
With the approval of all his advisors, Ahuitzotl, the "huetlatoani"
of Tenochtitlan, sent two high ranking courtiers to the ruler of
Coyoacan with his request for the use of the water, together with a.
military detachment to back up the request.
Tzutzumatzin, the ruler of
Coyoacan, acceded to the request because "he and his republic were the
vassals of (Ahuitzotl) and were obliged to comply," but he added a
176
warning to the effect that a great deal of water was involved, and that
i f the project was carried through it might result in flooding in
Tenochtitlan (Davies 1977:92).
In spite of this warning and against the
advice of his advisors who were persuaded of the danger, Ahuitzotl
proceeded with the construction, having first found it necessary to
arrange the assassination of Tzutzumatzin.
Labour was provided by Tenochtitlan's tributaries, as were the
materials required for the construction.
In addition to labour,
Texcoco provided light and heavy stone; Xochimilco provided tools and
canoes loaded with earth; Calco provided timber; and the Tepaneca
provided heavy stone.
"Grand masters" and "architects" were employed,
as were the best craftsmen that could be found (Davies 1977:92). The
aqueduct was completed and, after the appropriate and elaborate rituals
and ceremonies had been performed, the flow of water from the springs
at Coyoacan was directed through.the aqueduct.
Nothing untoward
occurred for a few days but after a while the waters of the Lake of
Mexico began to rise dangerously.
Ahuitzotl attempted a number of
entirely supernatural solutions to the problem, but these were ineffectual and he was finally obliged to ask the advice of Nezahualpil1i, the
son of Nezahualcoyotl and the ruler of Texoco.
Nezahualpilli "assembled
all the architects who were in his realm" and visited the scene of the
disaster (Ixtlixochitl, quoted in Palerm 1973:111).
He then criticized
Ahuitzotl for having killed the respected ruler of Coyoacan, for having
ignored the warnings that he had been given, and for having offended
the gods.
He then gave the advice that the aqueduct should be dis-
mantled, that the flow of water should be allowed to return to its
original course, and that a girl should be sacrificed to propitiate the
177
relevant deities.
These measures were duly carried out, and the flood
subsided.
These accounts of the construction of the Albarradon de
Nezahualcoyotl and of the Coyoacan aqueduct include virtually all of the
information that we have on Aztec management practices.
The most
notable feature of the accounts is that the rulers of the two most
powerful cities in the Valley are described as being personally involved
in the projects, and the rulers of Texcoco in particular are described
as having a special expertise in hydraulic matters.
While it seems
clear that these individuals did participate personally, it may be the
case that their participation was entirely in a ceremonial capacity
and that the decisions that are attributed to them may in fact only have
been made in their names.
However, it may be that the texts are liter-
ally true and that Nezahualcoyotl and Nezahualpilli were, as individuals,
highly qualified hydraulic engineers.
There are in addition the refer-
ences to "councillors", "architects" and "grand masters" which would
seem to imply the existence of a group of individuals who had particular
competence in hydraulic matters.
That the rulers should even aspire to
be considered as a part of this group indicates that the hydraulic
managers were of high social standing.
The texts also contain refer-
ences to skilled craftsmen and to divers "who had learned to go under
water."
The early Nahua-Spanish dictionaries contain several terms
that describe jobs associated with hydraulic matters:
"atzaqui", for
instance, means "one who stops the flow of water" and "tlapechoani"
means "one who blocks up a hole with something heavy" and carries with
it the implication that the hole is one through which water flows
(de Lameiras 1974:28).
This suggests that the specialists commanded
178
the services of skilled workers who specialized in jobs relating to the
manipulation of water.
The only other concrete information that we
have with reference to the management of water resources are statements
to the effect that the canals were cleaned out on specified.days, and
that the lower courts.were charged with the responsibility for seeing
that the work was carried out (Palerm 1973:239).
The regularity of the layout of some of the existing areas of
chinampas, and the experience of Colonial administrators who found that
the various lakes of the complex could not be managed individually, give
the impression that the hydraulic installations of the Valley of Mexico
were' the product of centralized planning, and that the planners had
administrative control over the whole of the lake complex (Armillas 1971:
660; Calnek 1972:111; Palerm 1973:22).
The few historical details
that are available on the planning, construction and management of the
hydraulic engineering works confirm this impression and indicate that
it was the governing institution of the Aztec state itself that was
responsible for the management of hydraulic installations in the lake
complex.
The State and its Hydraulic Installation
There are a number of instances where general political or social
characteristics of the states that developed in the Valley of Mexico
appear to reflect or to correspond with specific characteristics of
the lake basins in which the states developed.
These correspondences
betweeen the physicaland social systems are most clearly evident in
the contrasts between the Mexica state and the Chalco-Amecameca Confederation.
179
Of all the lakes in the Valley, Lake Chalco was the most suitable for conversion to chinampa agriculture.
It was shallow, well
provided with fresh water from springs along its southern shoreline,
favoured with a well defined mouth, and elevated sufficiently high
above Lake Texcoco that discharge could occur in all but exceptionally
wet periods.
It was also the first in the linked series of southern
lakes, with the consequence that it suffered none of the complications
of fluctuations passed down from hydraulic installations upstream.
The Lake of Mexico, in contrast, was the least suitable of the lakes
for colonization.
It was brackish and it had no well defined mouth
that could be easily dyked.
It was initially at the same level as
Lake Texcoco, with the consequence that drainage could only occur by
the artificial elevation of water levels, and even then it could be
prevented by quite slight increases in the level of Texcoco.
Because
it was the penultimate lake in the series, its regime was liable to
disruption by hydraulic activities in the lakes upstream.
There are several ways in which the contrasts between the two
states that developed in these basins can be related to contrasts in
the physical characteristics of the basins.
The first of these con-
cerns the times at which the two states emerged.
The origins of the
Chalco-Amecameca confederation are obscure, but it probably emerged
towards the end of the 12th Century and it was a significant power in
the Valley by the mid 13th Century.
The Mexica state, in contrast,
was not even established until 1345, when Tenochtitlan was founded,
and it was not a significant power in the Valley until the last quarter
of the 14th Century (Davies 1977:47).
The differences in the times at
which the two states developed may be related to the fact that Lake
180
Chalco was suitable for conversion to chinampa agriculture, while the
Lake of Mexico was not.
A second contrast is between the internal structures of the two
states.
The Chalco-Amecameca confederation appears to have been a true
confederation in which some twenty-six independent city-states formed
a voluntary, stable and enduring alliance in order to further projects
that were in their common interest.
The fact that it was a coopera-
tive institution that developed in the Chalco basin may at least in
part be related to the physical characteristics of the basin itself.
It functioned as an open system from which any excess volumes of water
could be freely discharged.
A dyke constructed at Cuitlahuac would
have been of benefit to all the agricultural communities on or around
the lake, and it was therefore in the interests of each of these
communities to cooperate with the others in the construction of the dyke
and'its associated canals and secondary dykes.
The Mexica state was
unambiguously a tribute state, assembled by force and maintained against
the will of its constituent cities and city-states.
This characteristic
and its association with the hydrology of the basin is illustrated by
the events surrounding the construction of the Coyoacan aqueduct, when
the Mexica found it necessary to assassinate the ruler of Coyoacan,
and to threaten the use of their army in order to obtain the supplies
of water that they required.
Coercion appears to have been an integral
part of Mexica water management practices, and this appears to reflect
the fact that the Lake of Mexico was not well suited for colonization
and that fresh water was in short supply.
The contrasting characteristics of the basins of Lakes Chalco
and Mexico seem also to have had some effect on the manner in which
181
the states that developed in those basins conducted their relationships
with polities outside the areas of the drainage basins in which they
were located.
date.
The Chalco-Amecameca Confederation emerged at an early
It was a.powerful polity that was capable of fending off the
Tepanec and Mexica armies in the late 14th Century and of resisting
the Aztec armies until long after all the other states in the Valley
had succumbed.
However, although it was powerful, it never extended
its power beyond the boundaries of the Chalco basin.
Because of its
upstream location, water level could be brought under control as
effectively as possible with the prevailing techniques by the construction of. the dyke at Cuitlahuac, and the fact that the polity that
developed in the basin never extended its power beyond the basin may
be related to the fact that there was no technical advantage in doing so.
Again, the situation in the Lake of Mexico was quite different.
With its downstream location it was the recipient of excess volumes of
water passed on from Lakes Chalco and Xochimilco, and even modifications
to the regimes of Lakes Zumpango and Xaltocan in the north of the Valley
could have had quite immediate consequences.on the levels and behaviour
of the Lake of Mexico. At the same time, there was no natural difference in the level of Lakes Mexico and Texcoco, with the result that
levels in the Lake of Mexico had to be artificially maintained if discharge was to occur at all and, even then, high water levels in Texcoco
frequently prevented discharge.
The only technically feasible mechanism
available to the Mexica for alleviating their problems was to exploit
the upstream capacity of the lake complex.
Because such an expedient
was not in the interests of the polities located upstream, the only
means by which the Mexica could make use of this storage capacity was
182
to gain political control of the upstream polities and coerce them into
accepting the floodwaters that would otherwise cause damage in Tenochtitlan.
This appears to be precisely what the Mexica did. Within a
few years of founding Tenochtitlan, they embarked on aggressive campaigns against Culhuacan, Cuitlahuac, Mixquic, Xochimilco and the ChalcoAmecameca Confederation and, although the campaigns were related to a
number of purely political factors, they were in precisely the areas
that the Mexica needed to control in order to avert flooding in the
Lake of Mexico.
Subsequent campaigns as a member of the "Triple
Alliance" gave the Mexica control of the entire lake complex.
The Chalco-Amecameca Confederation developed at an early date,
its constituent city states were linked by cooperative institutions and,
although powerful, it did not extend its power beyond the boundaries of
the Chalco basin.
These general characteristics of the state appear
to bear some relationship to the fact that Lake Chalco was suitable for
colonization, that fresh water was in sufficient supply to satisfy the
needs of all the constituent city states, and that there was no technical
advantage to extending power beyond the basin.
The Mexica state devel-
oped late in the history of the Valley, its constituent city states
were bound together by force, and it finally expanded its power over the
whole of the watershed, and beyond.
These general characteristics of
the state appear to be associated with the difficulty of colonizing the
Lake of Mexico, with the necessity of acquiring supplies of fresh water
at the expense of those city states already using i t , and with the need
to control the upstream storage capacity of the lake complex.
183
The Management of Agriculture
The management of hydraulic installations was only one aspect
of the management of the chinampas': equally important was the management of the more purely agricultural aspects of the system.
Direct
information on the management of the chinampas is again quite limited
but it is not as tightly limited as is the case with respect to
hydraulic management.
Agriculture is closely associated with social
and political patterns, which were matters in which the Spanish
Chroniclers had some interest, and so there is a larger basis for
inference about the management of agriculture than there is about the
management of hydraulic installations.
At the same time, management
of agriculture at low hierarchic levels is an aspect of a peasant
society, of a "little tradition" of practices which have persisted
through the intervening five hundred years in a way that the "great
traditions" of the Aztec state have not, with the result that many
of the techniques and activities of contemporary Indian farmers bear
at least a general resemblance to the practices of their pre-Hispanic
forbears.
For both these reasons our understanding of agricultural
management in Aztec society is better than our understanding of
hydraulic management.
Decisions that had a consequence in the physical structure of
the chinampa agroecosystems were made at various hierarchic levels;
the decisions that involved the least expenditure of energy and that
had consequences in the physical system for the shortest periods of
time were made at the lowest hierarchic levels.
The basic organizational
unit of Middle American society is the farming family.
This was as true
in Aztec society as it is in the more remote parts of Middle America
184
today.
Early records from Cortez estate in Morelos show that the
1
family typically consisted of a single married couple and their
children, but that this unit was occasionally extended to include the
spouses of the children and to include the children of the next generation.
In Tepoztlan, in the 16th Century, the average household included
5.4 individuals and 1.4 married couples (Carrasco 1971:363ff).
There
are some indications that "chinampero" families were slightly larger,
but it is unlikely that the average family comprised more than about
half a dozen individuals.
Full time agricultural activity was restricted to the male
members of the family, and a father and his sons made up the typical
unit of agricultural labour; women assisted in the performance of light
work and at exceptionally busy seasons.
Under the "calpulli" system,
it was the head of the farming family who held usufruct rights to land.
Generally the head of the family was the father, but widows could hold
rights to the use of the land so long as they had male relations to
perform the work.
The head of the family was in effect the manager
of the land, and it was he who made most of the short-term decisions
about which of the available selection of crops should be planted,
where within his small domain they should be planted, and when to
carry out such commonplace tasks as sowing, weeding, fertilizing and
irrigating.
The situation of the "tlalmaitl" families who were tied to the
large estates of the late Aztec period is a little less clear.
We know
that the agricultural activities in these estates were closely supervised by "calpixque" who lived in the vicinity, and who directed the
activities of tenant farmers (Parsons 1976:236).
It is possible that
these officials functioned as labour bosses, organizing every detail
185
of the work of groups of agricultural labourers.
Certainly, officials
known as "tequitl" performed this function in the execution of public
work projects, and they may have performed a similar function in agricultural labour.
However, such an arrangement runs counter to all
contemporary Middle American practice, and it seems more likely that
the heads of the "tlalmaitl" families acted as the managers of their
small plots in the same way that the heads of "macehualtin" families
did, albeit under the supervision of resident "calpixque".
Resident "calpixque" supervised the work of the "tlalmaitl"
families, and the "calpulli" councils performed the same function with
respect to the "macehual" families.
Although individuals in this
latter group had rights to. the use of land, the rights were conditional
on the land being cultivated.
If it was not cultivated in two consecu-
tive years the family could be dispossessed and replaced by a more
industrious one (Carrasco 1971:363ff).
The "calpulli" council was pro-
vided with officers who could enforce the decision if necessary. The
use of the land appears to have been very closely supervised in the
late Aztec state; maps were kept by the supervisors, which showed
individual plots, the name of the owner, the number of people in the
family, and which included a description of the soil in the plot
(Gibson 1964:300).
Whether these records were kept by the "calpixque"
or the "calpulli" is unclear, but their existence suggests that the
supervision was very careful indeed.
The effect of this supervision
was to insure that the agricultural land was kept in production, and
that it produced as much as possible.
Even if the powers of the
supervisors were limited to the removal of the farmers who performed
poorly, their decisions resulted in modifications to the physical
186
structure of the land, and in the sense in which the term is used here,
both "calpulli" councils and "calpixque" functioned as agricultural
managers.
The "calpulli" councils and the "calpixque" were responsible to
higher order institutions, but it appears unlikely that the governors
of cities or states were actively involved in the day-to-day management
of agriculture.
Once the goal of maximizing yields had been established
and the responsibility of realizing the goal had been delegated to lower
order authorities, the function of the government in the day-to-day
management of agriculture was discharged.
The overt concern of the higher
order institutions would have been to enlarge the resource base, and in
the chinampas the decisions that were made with a view to accomplishing
this goal would have related to the construction of hydraulic works
and impinged on hydraulic management.
However, the higher order institutions did manage agricultural
resources, but they did so in a manner very much more diffuse but
fundamentally more important than the manner in which the lower order
institutions did. There is an interesting text that relates to the war
between the Mexica and the Tepaneca, following the death of Tezozomoc.
At issue was the question of whether or not the Mexica should rebel
against their former masters.
On one side of the issue were the
Mexica nobility, who favoured the war, and on the other side, "the
people", who did not.
After some discussion, Tlacaelel, the "woman
serpent" or vice-ruler of the Mexica, spoke on behalf of the nobles,
saying "If we are unsuccessful in our undertaking, we will place
ourselves in your hands, that our bodies may sustain you, and you may
thus take your vengeance and devour us in dirty and broken pots." The
187
spokesman of the people responded, "and thus we pledge ourselves, i f
you should succeed in your undertaking, to serve you and pay tribute,
and be your labourers and build your houses, and to serve you as our
true lords." The outcome of this strange bargain was, of course,
that the nobles won the war and the service of the people (Davies
1977:65).
The story may be apocryphal or it may have been deliberately
fabricated at a later date to justify the privileged position of the
nobility.
But whether true, apocryphal or a fabrication, it indicates
that the development of the centralized and stratified state was an
issue that was overtly discussed, and the consequence of deliberate
decisions rather than the unconscious expression of underlying developmental trends.
If this is the case, then it is also the case that the
creation of estates and the apparatus necessary for their administration
was the product of deliberate decisions, and to the extent that these
novel institutions modified the physical properties of the agroecosystems
by insuring that they were well managed, the governing councils of
the emerging Aztec state were acting deliberately and overtly as agricultural managers.
In summary, management decisions were made at a variety of different hierarchic levels, by a variety of individuals and institutions
within the Aztec state.
The government itself was directly involved
in the construction of hydraulic installations and in the creation of
the administrative apparatus for the maintenance and agricultural
exploitation of those installations.
Decisions made at these high
hierarchic levels involved the expenditures of large amounts of cultural
energy and made an impact on the physical structure of the managed
systems that endured for long periods; in the case of the decisions made
188
with regard to the layout of the chinampas, the effects are s t i l l
evident half a millennium later.
Decisions were also made at lower
hierarchic levels, by the "calpixque" and the "calpulli" councils
who oversaw the production of individual "chinampero" families.
At the lowest hierarchic level were the decisions made by the head of
the farming family with regard to the use of his small agricultural
plot and to the plants that were cultivated there.
These decisions
regulated the use of only a small amount of energy and had consequences
in the structure of the physical systems for a correspondingly short
period of time.
The Evaluation of Aztec Management
Management involves the making of decisions that have the effect
of modifying the physical structure and thence the function of the
managed systems.
Essentially this process is to do with the transmis-
sion and.modification of information.
Management may be conceived
of as a circuit in which information about the state of a physical
environment is collected by the managers, processed and supplemented
in various ways, and returned to the physical system in the form of
directives for the modification of the system.
stages are involved.
Four quite distinct
First, the physical system is perceived, either
directly by the managers or indirectly by agents who report to the
managers.
Second, the perceived state of the system is compared to
some model held by the managers of a desired state of the system.
The
model of the desired state of the system is derived from the goals of
the society in which the managers are operating, modified by the
manager's knowledge of what is or is not feasible with regard to the
189
nature of the system and the nature of the technology available.
Third,
a strategy is selected by the managers that will, in the event of a
discrepancy between the perceived and desired states of the system,
bring the perceived state into conformity with the desired state or
that, in the event of a conformity between the two states, maintain
that conformity.
The set of strategies available to the managers are
derived again from general characteristics of the society in which the
managers operate, in particular, from the technical and administrative
capabilities of the society.
Fourth, and finally, the selected strategy
is implemented on the physical system, either by the managers themselves
or by their agents.
When the modification has been implemented on the
system, the whole process is then repeated.
This is a highly generalized conception of management and it can
be applied to the management of any physical system in any society, at
any scale.
The model is useful because it provides some means of
evaluating the performance of the management of a physical resource
system.
As the model indicates, management involves the transmission of
information, and a potential source of inefficiency in resource management is poor communication. There are a number of reasons for suggesting that the Aztecs were quite efficient at communicating information
about the state of their resource systems.
The management systems com-
prised only a few individuals, and these were people who through shared
culture and experience were to a large degree mutually intelligible.
This is evident in the case of management at the level of the farming
family, where communication was between members of the same family. A
few more individuals may have been involved when management was carried
190
out at higher hierarchic levels, by "calpulli" councils or by "calpixque".
However, the fact that agricultural overseers were resident in the areas
that they administered suggests that tours of inspection were among
their duties, and that they were appraised of the states of the systems
that they managed by personal observation, and any instructions that they
issued were extremely unlikely to have been misunderstood by the heads
of the farming families who implemented the decisions of the "calpixque"
on the systems.
Even at high hierarchic levels, there was little scope
for confusion in communications.
The history of the Valley of Mexico
is so.complicated that one tends to forget how small the Valley is.
Managers who were resident in Tenochtitlan or Texcoco could easily
have visited all the localities over which their domain extended.
Certainly, the records state explicitly that Nezahualpilli visited the
site of the ill-fated aqueduct at Coyoacan before he suggested methods
of dealing with the crisis.
Even if the hydraulic managers did not
base their decisions on personal observations, it is unlikely that
more than one individual was interposed between the managers and the
managed system.
As such individuals would have been known personally
to the manager, and would have been specialists in hydraulic matters,
the possibility of confusion would have been slight.
Rather larger
numbers of individuals were involved in the execution of management
decisions, but the labour force was cooperative and worked willingly.
It was supervised efficiently by "tequilatos" and the work itself,
although onerous, was quite straightforward.
Managers undoubtedly
visited the sites of construction to check on progress, and all of
this would have reduced to a minimum the possibilities for confusion
and misunderstanding in the execution of management decisions.
191
It is a property of hierarchic systems that as messages are
passed up through the system information is lost by aggregation; data
that at low levels in the system may be discrete are aggregated as
they ascend to the apex of the system (Simon 1962).
Thus a management
system with many hierarchic levels responds to very much less specific
information.about the state of the system than does one with only a
few hierarchic levels, or, one might say that management with few
hierarchic levels is capable of a very much more detailed and precise
response than management with many levels.
There appears to have been a tendencywithin the Aztec state
to manage physical systems at the lowest hierarchic level that was
feasible.
Each farmer was the manager of his own small plot and he
could respond to quite particular conditions of the system, whether by
rectifying problems or exploiting variations.
The scale of management
was so small that the Aztec chinamperos could have treated each plant
individually, as their descendants do today.
"Tender loving care"
is a property of agricultural management with few hierarchic levels.
It is true that the work of the farmers was supervised, but the supervisors did not substitute for the function of the plot managers, they
merely insured that the managers fulfilled their functions.
At the
other extreme of scale, the construction of "hydraulic installations
was managed at a high level in the administrative hierarchy simply
because high level administrative institutions were necessary for the
organization of labour on the appropriate scale, but this is not to
say that the management system itself had many hierarchic levels.
If
the managers were supplied with information by specialists who functioned as surveyors and if they issued instructions to labour supervisors,
192
the number of hierarchic levels involved would have been quite small
even if a large number of individuals participated in the actual execution of the work.
Considering the fairly simple technologies involved,
there was no need for many levels of management and, if this was the
case, then the hydraulic managers as well as the individual farmers
could have responded to quite specific characteristics of the systems
that they managed.
Beyond fulfilling basic subsistence needs resource systems
are intended to satisfy social goals, and when a society attempts to
satisfy more than one set of goals from the same resource system there
is a potential for inefficient management.
Either compromise decisions
must be made that satisfy neither set of goals entirely, or one set of
goals must be forcibly imposed on the others.
conflicts of interest within Aztec society.
There were undoubtedly
The Mexica's requirements
for a plentiful supply of fresh water in the Lake of Mexico conflicted
with the interests of those cities such as Coyoacan where the water was
already in use, and the fact that the Mexica were forced to exploit
upstream storage capacity for the control of floods in Tenochtitlan
conflicted with the interests of the inhabitants of the Chalco and
Xochimilco basins.
At the same time the interests of the ruling classes
conflicted with those of the ruled.
The conflicts were resolved by
force and, to the extent that the maintenance of an army and a police
force reduced the agricultural labour force, consumed agricultural surplus: and disrupted agricultural production, the need for enforcement
impaired the efficiency of Aztec management.
However, although the
use of force was itself an inefficiency, the coercive apparatus of the
Aztecs was well organized and was probably as efficient as it was
193
possible for such a set of institutions to be.
At the same time, any
tendency that the state may have had.to dissever along social or
political boundaries was held to a minimum by an ethic that emphasized
duty to the state above all other values, and by an awesome and sanguinary state-sponsored religion that communicated the idea that the existence of the universe was dependent on submission to the authority of
the state (Wolf. 1959:144; Peterson 1962:110-11 ).
There were conflicting
goals in Aztec society, but the goals of the ruling class were efficiently propagandized and enforced and the potential for inefficient
management was. held to a minimum.
Management is based on the knowledge that the managers have of
the systems that they manage.
All the indications are that the Aztec
managers had an extremely thorough knowledge not only of their resource
systems, but of all aspects of their physical environment.
One chronicler
notes that "almost a l l , including the boys, know the names of all the
birds., animals, trees and herbs., knowing as many as a thousand varieties
of the latter, and what they are good for (Zorita 1963:163).
This is
what Polyani calls "personal knowledge", learned by individual observation and long apprenticeship (Polanyi 1944).
It is knowledge that is
restricted to the small localities in which most individuals lived out
their lives in pre-industrial societies, but within these small areas
the knowledge is extraordinarily detailed—the accumulation not just of
a single lifetime's observation, but of the observations of many generations transmitted by precept and example.
It is clear that the Aztec
agricultural managers based their decisions on this type of knowledge,
learned by apprenticeship to their fathers.
The purview of the
hydraulic managers was somewhat larger and may not have been quite as
194
intimate as the knowledge of the farmers but, even so, the areas
involved were small, the specialists probably served long apprenticeships, and the knowledge upon which they based their decisions would
have been extremely detailed.
A final quality of Aztec resource management was that all aspects
of the process had been quite thoroughly tested by experience.
Pre-
industrial societies generally tend to be quite conservative, and when
development and change do occur, they occur over long periods of time
by incremental modifications to existing practices rather than by the
rapid adoption of new ones.
This was evidently the case in such basic
aspects of agricultural management as the selection of crops and the
repertoire of techniques for weeding, fertilizing and harvesting which
the chinampas shared with other, older Middle American agricultural
systems.
Even the practices that were peculiar to the chinampas such
as seedbedding and transplanting with employed bottom mud are unlikely
to have been the consequence of specific inventions, but were developed
slowly over long periods.
It was suggested above that even the basic
strategy underlying chinampa agriculture was a development of the basic
strategy of Rosa cultivation.
Similar considerations apply to the
development of hydraulic installations which, it was argued, although
they developed quite rapidly, developed by the elaboration of existing
facilities and practices—practices that had already been quite thoroughly tested by experience.
What is true of the physical installa-
tions is also true of the institutions by which the installations were
managed and operated.
The elaborate apparatus of the 16th Century was
not suddenly imposed on the society, but developed from the earlier
"calpulli" institutions and from the residual institutions of the
195
earlier cultures of Tula and Teotihuacan.
Virtually all of the physical
installations of the Valley and of the administrative institutions had
been validated by generations of experience.
It appears to have been
the case that only very late in the 15th Century did the Aztecs begin
to employ procedures that had not been validated by experience, and
that caused serious technical problems.
There is a yet more fundamental aspect to this characteristic
of experiential validation of techniques and procedures.
Any culture
orders its experience of the objective world in terms of a set of
religious beliefs and an epistemology that provide models of the manner
in which the physical environment functions; some of those models
provide a more adequate basis for interaction with the objective world
than others.
We do not know in what terms the Aztec farmers understood
the processes that they manipulated.
The goddess Tlazolteotl, or
"filth eater", thrived on the confession of sins, but there also
indications that she was a representation of the earth and that she
thrived on filth in the quite literal sense of the term, and this may
have been an expression of the role of organic fertilizers in regenerating the soil and promoting plant growth (de Lameiras 1974:39).
Modern Mayan farmers believe that their maize plants are endowed with
spirits so that if the plot in which the plant is growing is not
weeded the spirit of the maize will be offended and move to a cleaner
field, and the Aztec farmers may have held similar beliefs (Vogt 1969:
46).
If the construct of the earth as a deity who thrives on filth
results in fertilization, and if the construct of sentient maize taking
umbrage results in the weeding of plots, the plants will do well, and
the constructs are validated.
We do not know how the Aztec farmers
196
understood the processes that they manipulated, but whatever models or
epistemologies they did employ, they were models that had been tested
by generations of farmers and that had been validated by the accumulated
experience of those generations.
Management is the control mechanism of resource systems.
Its
essential function is to maintain or induce conformity between the
physical state of the system and some state of the system that is
desired by the managers.
The argument is made here that Aztec manage-
ment was efficient in communicating information, whether information
about the current state of the system or information that resulted in
modifications to the system; that the information which was communicated
to the managers and upon which they based their decisions amounted to a
thorough and detailed description of the systems that they managed;
that the managers were not confronted with the need to make compromise
decisions that satisfied conflicting goals, and that the techniques
and procedures employed by the managers were for the most part quite
thoroughly tested by experience.
The desired state of the chinampas was the one that resulted
in high yields, and Aztec management was effective in bringing this
about.
Management decisions were based on an intimate and tested
knowledge of plant cultivation, on the conditions of the immediate
environment of agriculture, and of the hydrological system in which
the chinampas were embedded.
The management subsystems operated at
a scale very close to the scale of the managed systems with the
result that it was capable of a rapid response to changes in the condition of the systems and of a specific response to minute spatial
197
variations within the systems.
Aztec management was, in short, effi-
cient at fulfilling the goal of maximizing food production.
198
CHAPTER VI
SUMMARY AND CONCLUSIONS
At the outset the question of how the chinampas worked was
posed and it was suggested that a useful method of answering the
question was to provide a description of the structure of the system
and an account of its functions.
The chinampas were discussed in
terms of their three major subsystems, namely, the hydrological system
in which the platforms were constructed, the ecological systems in which
crop plants were raised, and the management system that regulated the
function of the physical subsystems.
These three subsystems have been
treated at some length, and a number of conclusions have been presented
as to their structure, function and interrelationship.
It was argued that the lake complex of the Basin of Mexico had
two properties in particular that were of potential advantage to agriculturalists:
it had water available throughout the year and it was
particularly rich in nutrients.
In addition to these potential advan-
tages, however, it had the major disadvantage that, as the consequence
of large climatic variations, water levels in the lakes fluctuated so
greatly both from season to season and from year to year that installations constructed in the complex were either rendered unusable in many
years, or else had to be built to impractically large tolerances.
The
only means of exploiting the lake complex for its water and nutrients
was to reduce the values of these fluctuations.
The only method of
doing this was to construct a complex of dykes and canals that removed
199
excess volumes of water during times of surplus and that held water
back during times of shortage.
The costs of such constructions were
high and they were not constructed in any significant numbers until a
point had been reached when the population of the Valley pressed so
hard on its carrying capacity at existing technological levels that
there was no alternative but to invest the necessary effort.
From
the mid 14th Century onwards, hydraulic installations were constructed
and water levels in increasingly large areas of the lake complex were
brought under control.
With the levels controlled the complex could
be colonized and its stocks of water and nutrients exploited for agriculture.
The immediate function of the hydraulic works was to control
water levels, but their function in the chinampa agriculture system
was to make available material inputs to the crop systems at values
which could be exploited.
The second major system of chinampa agriculture comprised the
multitude of chinampa platforms together with the small and essentially
similar ecological systems that each"platform supported.
It was suggested
that the ecosystems of the chinampas are best understood as comprising
three quite separate systems, each one of which had different structural
properties, and each of which performed different but complementary
functions in the chinampa plant community as a whole.
One of the sub-
systems was an agroecological system which performed the function common
to all such systems of combining nutrients with solar energy to produce
biological materials that were of use to man. Like any other agroecological systems, the crop plant communities of the chinampas were
valued for their high growth rates, but for reasons relating to the
nature of ecological systems high growth rates could only be had at
200
the expense of stability.
The chinampas were high-yielding and, with
yields of three or four tons per hectare, they compared favourably with
modern agricultural systems.
To the extent that they were high yielding,
however, they were also vulnerable to pests and the competition of
plants more.robust than crop plants are.
High yields could only be had
at the cost of large investments of energy in producing appropriate
growing conditions for the crop plants and in protecting them from
depredatory animals and competing plants.
It was in this capacity that
the two other sub-ecological systems of the chinampas functioned. The
canals and the platform borders, together with the plant communities
they supported, performed the function of supplying the crop systems
with the materials they required and with protection from insect pests,
soil pathogens and variations in microclimatic values.
To the extent
that the non-crop ecosystems performed they reduced the amount of labour
that had to.be expended directly in maintaining the fragile crop plants
and increased the yields of chinampa agriculture.
In the majority of
agricultural systems these functions are performed at the cost of
cultural energy, either in the form of human or animal work or in the
form of commercial energy products.
The genius of chinampa agriculture
lay in the fact that ecological systems were employed to perform much
of this work and by this means to provide the necessary inputs of
energy, not in the form of scarce cultural energy, but in the form
of solar energy.
The third major subsystem in the chinampas was the management
system by means of which the physical structures of the other subsystems were modified and their functions regulated.
The limited
evidence on the management of chinampa agriculture suggests that Aztec
201
managers operated at levels in their society that were roughly commensurate with the size of the labour force that was required for the execution of their decisions.
Most of the day-to-day managerial decisions
made with regard to the modifications of the agroecosystems and their
supporting non-crop ecosystems were made at low levels in the social
hierarchy by the heads of the farming families and the necessary labour
was provided by the small familial work force.
The construction of
hydrological installations required the expenditure of rather greater
amounts of energy and these undertakings were managed by institutions
or individuals who were situated sufficiently high in the social
hierarchy that they could command the necessary resources.
For the larger
undertakings, at least, such large resources of labour and materials
were required that the management of the undertakings was carried out
at the level of the government of the state itself and, quite possibly,
by the rulers of the state.
It was further argued that in spite of differences in dimensions
and other attributes, the management of chinampa. platforms and of
hydrological installations shared some basic characteristics.
In both
cases only a few individuals were involved in the management of the
system.
There was' a tendency for the management subsystems to comprise
only a few hierarchic levels, the consequences being that information
necessary for management was communicated with a minimum of confusion
and aggregation, and that the managers could respond rapidly and accurately to small variations in both the temporal and physical characteristics of the systems which they managed.
In agricultural management
the scale was so fine that plants could be treated individually.
Because the managers lived out their lives in small localities and
202
acquired their knowledge by personal observation and long apprenticeship, the knowledge upon which they based their decisions was extremely
detailed and quite particular to the systems that they managed.
Because
the development of agricultural techniques and practices and, to a lesser
extent, the techniques of water control had been developed over long
periods and in small increments, most of the procedures and strategies
adopted by the managers had the quality of being experientially validated and thoroughly tested.
Finally, Aztec management was charac-
terized by a certain clarity of purpose and uniformity of goals.
Management was intended to produce agricultural surplus for the support
of the state, or, more specifically, for the support of the ruling
classes of the state.
Managers of high social standing worked willingly
towards this goal because it was in their interests to do so.
Managers
of low social standing also worked towards this goal, not because it was
in their interests to do so, but because the entire coercive apparatus
of the state obliged them to do so.
The Aztec managers had efficient
communications, a thorough knowledge of the systems that they managed and
a well tested set of strategies.
For these reasons they were efficient
in the construction of'installations for the regulation of water levels
in the lake complex, in operating these once they had been constructed,
and in exploiting the agricultural potential of the lake complex that
the hydraulic installations made available.
The chinampas, like any other material resource system, were
a technical device for transforming the materials of the physical
environment into goods for thei satisfaction of human needs and cultural desires.
The chinampas converted nutrients and water into food-
stuffs and by some criteria they appear to have performed this function
203
quite efficiently.
If efficiency is understood in terms of the ratio of
output to input, then the efficiency of a resource system must be
evaluated in terms of a specific input; the inputs that are most generally of interest with respect to agricultural systems are land, labour
and energy.
The chinampas appear to have been quite efficient with
regard to outputs per unit input of land; exact figures are not available, but the impression is that the chinampas yielded between three
and four tons of shelled maize per hectare and possibly a quantity of
horticultural products in addition to this.
These yields are respect-
ably high, which is to say, they were very much higher than the yields
of virtually all other Middle American agricultural systems and not very
far short of the yields of modern corn production. Like other preindustrial systems, the chinampas were labour intensive and their output
per agricultural worker was probably in the vicinity of one and a half
tons of maize per year, which is a small fraction of the output of
agricultural workers in the developed economies.
The returns per unit
investment of energy were again respectably high and with an Energy
Ratio of between 10 and 15 they were considerably more efficient than
modern commercial agriculture, but only about half as efficient as more
extensive traditional systems in Middle America.
A second aspect of how efficiently a resource.system performs
its function relates to the question of how long it could have continued
to perform its function without in some way impairing its ability to do
so in the future.
By this criterion, too, the chinampas appear to have
been quite efficient.' So long as the canals and platforms were maintained and new trees planted to replace those that died, the internal
204
structure of the system could have persisted indefinitely.
With regard
to their external relations, the chinampas exploited an exceptionally
fertile locality and they did so extremely frugally.
A large part of
the nutrients that were withdrawn from the canals and applied to the
platform surfaces were returned to the canals by the processes of decay
and surface runoff, where they again became available for future use.
The export of nutrients from the platform surfaces was held to a minimum by exporting only the usable portions of crops.
Much of the crop
that was exported from the platforms was consumed locally and the
nutrients that they contained were returned to the canals in the form of
human waste and household dejecta.
Sewage may have been systematically
collected and returned to the chinampas for use as fertilizer.
Nutrients
were used so conservatively in the chinampas that it is unlikely that
they were consumed or exported from the system at a greater rate than
the one at which they became available to the chinampas in the course
of their larger natural cycles.
If this was the case, then the chinam-
pas did not reduce the capacity of their environment to continue to
support them and the system could have continued to function indefinitely.
Nor is it likely that i f the 16th Century hydrological conditions
had been maintained water would ever have become a limiting factor on
chinampa production. The chinampas were also quite self-sufficient in
terms of energy use.
The only major input of cultural energy was in
the form of the human labour expended in agricultural activities and, to
some extent, in the activities associated with procuring lime, bat-dung
and.leaf mould. But because these expenditures could have been met from
the production of chinampa agriculture they did not represent a subsidy
205
of any kind.
Otherwise, the only input of energy was in the form of
solar radiation and, so long as the sun shone, the chinampas could
have continued to function.
Chinampa agriculture functioned quite
independently of any significant source of non-renewable materials or
of cultural energy; its inputs were entirely limited to those nutrients
and supplies of water that became available in the course of their
natural cycles and to solar radiation.
Under these circumstances they
could have persisted indefinitely, and the fact that large areas of
chinampas were s t i l l in production at the beginning of this century
is direct proof of the stability of the form.
There may have been no limits on the amount of time that the
chinampas could have persisted, but there were limits to the extent to
which the area of chinampa agriculture could be increased.
In a closed
drainage basin such as the Valley of Mexico, the volume of water present
at any particular time was determined by the difference between evaporation and precipitation.
The running discrepancy between the values of
these two variables caused changes in the volumes of water stored in
the lake complex; these, in turn, caused fluctuations in water levels.
Because the Aztecs had no control over the volumes of water present,
the most that.their dykes and canals could accomplish was the redistribution of the volumes of water that were already present in the
system.
They could prevent water levels from rising or falling in one
part of the lake complex, but only at the cost of increasing fluctuations
in some other part of the complex.
Calculations with the hydrological
model suggest that Lake Texcoco had sufficient capacity to absorb the
fluctuations of Lakes Xochimilco and Chalco and some small areas in
other parts of the system, and that if chinampa agriculture had remained
206
restricted to these areas it would have been quite stable.
The hydraulic
managers of the Aztec state, however, extended water level control
into progressively larger areas of the Lake of Mexico and, in so doing,
reduced the volume of storage capacity available in Lake Texcoco. The
fact that the city of Tenochtitlan stood in the waters of the Lake
of Mexico at the time of the Spanish Conquest indicates that the Aztecs
had not quite reached the limits to which they could extend chinampa
agriculture.
The fact that floods were common in the Lake of Mexico
in the years prior to the Conquest and that it was occasionally necessary to impose floods on the two southern lakes in order to reduce
flooding in Tenochtitlan suggests that the Aztecs were very close to
the limits to which the chinampas could have been extended.
There
were limits on the development of the chinampas, but up to those limits
the system appears to have been remarkably stable and productive.
At the beginning of this essay it was suggested that the
chinampa system.of cultivation might have a relevance in the larger
context of the development of agriculture in the 20th Century and
that it might provide an indication of how the energy efficiency of
modern agriculture might be improved without sacrificing high yields.
Some aspects of chinampa cultivation do seem to provide such indications.
The chinampas made a very conservative use of materials:
exports
from the system were minimized and materials were as far as possible
recycled within the system.
The chinampas were particularly well endowed
with nutrients and water but, because exploitation was frugal, the rate
at which materials in their environment were consumed was very lowprobably lower than the rates at which the materials became available
in the course of their large natural cycles.
Because of this the
207
availability of material inputs was unlikely ever to have become a
limiting factor on chinampa production. Had the chinampas been less
frugal in their use of materials they would have required the input
of greater volumes which would, in turn, have increased the energy
investment in the system and reduced its efficiency.
This suggests
that the techniques and devices by means of which materials were
conserved in the chinampas could serve as models for similar devices
in other agroecosystems.
Such practices as the use of canals as
repositories for nutrients, the exploitation of productive aquatic
ecosystems for fixing atmospheric nitrogen and for converting nutrients
into forms that are readily accessible to plants, the careful cycling
of crop residues and field litter, the selection of crops that are
closely adapted to local climatic conditions and the use of large
perennial plants and mixed cropping techniques for the regulation of
microclimatic conditions could be usefully replicated.
To the extent
that such practices reduce the use of water and nutrients, they also
reduce the volumes of materials that must be imported to the agroecosystem and they reduce the cultural energy costs implicit in the
provision.of these materials.
A second feature of the chinampas was a tendency for certain
necessary agricultural functions to be performed, not actively by the
agriculturalists, but passively as a consequence of the structure of
the system itself.
The canals collected and stored nutrients and so
reduced the amount of work involved in procuring these materials; the
relative elevations of the canals and platform surfaces reduced the
amount of work required in irrigation; and, to the extent that the noncrop ecosystems supported and protected the crops, they reduced the
208
amount of effort that the agriculturalists had to expend in these
activities.
In each of these instances, the function was performed,
but the onus for the performance was shifted from the agriculturalist
to some aspect of the structure of the system.
Work was involved in
the construction and maintenance of the inorganic and.non-crop biotic
components of the system.
But in the case of the inorganic installa-
tions the great stability of the chinampas allowed the investment to
be amortized over extremely long periods, and in the case of the biotic
infrastructure the work performed by the "chinamperos" was slight
compared to the amount of solar energy that was harnessed to the benefit
of the crop plant communities.
The tendency of chinampa agriculture
to exploit passive processes that were vested in the structure of the
system and that did not require high cultural energy investments,
rather than to expend considerable effort in the direct performance of
these functions contributed to the energy efficiency of the system and
is another feature of the chinampas that deserves emulation.
A third quite basic property of chinampa agriculture was that
the scale of the various constituent artificial systems was very close
to the scale of the natural•systems upon which they impinged. With
regard to the ecological systems, the areas that were treated and
managed as a unit were very small and, because they were small,
certain types of functions occurred that could not have occurred had
the units been larger.
Passive irrigation depends on the availability
of water in close proximity to crops; trees can regulate microclimate
only within an area of a few square meters; the range over which a
generalized ecosystem can extend its beneficial influences is limited
by the range over which it influences microclimate, or by such factors
209
as the range of insect predators.
The range over which ecological
interactions of this type can operate is not well known, but the example
of the chinampas suggests that it is quite small and that, if the interactions are to occur, any agricultural system that seeks to exploit
them must be quite small.
itself a desirable quality.
This is not to suggest that small size is of
The hydrological elements of chinampa
agriculture extended over the whole area of the lake complex and, because
the different parts of the lake complex were interrelated, it was necessary to manage the lake complex as a single unit in order to modify its
function successfully.
What seems to be suggested by the example of
the chinampas is not any absolute criteria for the dimensions of
energy-efficient agricultural systems, but the need for a concern with
matching the scale of the managed units with the scale of the natural
processes that are manipulated in these units, and that at a point
where a discontinuity between the scale of the managed system and the
scale of the managed processes occurs the system should be extended
not by increasing the area of the units but by replicating them.
The chinampas, then, do seem to offer some indications of
strategies for the increase of the energy efficiency of agricultural
systems without incurring high energy costs.
This is not to suggest
that it would be desirable to recreate the 16th Century chinampas in
every detail.
Much of the work in the chinampas was drudgery and the
productivity of each worker was below the level at which even basic
needs could be satisfied in a modern economy.
What is suggested is
that underlying and informing the practice of chinampa agriculture are
certain principles and strategies that have the effect of reducing
energy inputs to the system, and that by the study of the chinampas
210
and similar intensive pre-industrial agricultural systems, these principles can be understood and used as the basis for the development of
biotic resource systems that are efficient in transforming soil and
water and sunlight into useful goods.
211
APPENDIX
RECONSTRUCTION OF THE 16th CENTURY HYDROLOGY OF THE
BASIN OF MEXICO
212
Part I
The Use of Modern Data in Reconstructing 16th Century Conditions
In the calculations that follow modern data are used as the
basis of a description of the hydrology of the Basin of Mexico in the
late 15th and early 16th Centuries. The use of data that are 500
years out of date requires some discussion.
The impression given in
the literature is that the climate of the Valley of Mexico has not
changed very greatly during the past 500 years (C.H.C.V.M. 1964 V:
132).
Temperatures are conditioned by latitude and elevation; so
modern figures are probably quite representative of pre-Hispanic
conditions except in the immediate locality of larger built-up areas
where urban heat islands have developed (Jaurequi 1973-74).
Precipi-
tation values have changed slightly during the past half millennium.
Lorenzo has analyzed the elevations of lakeshore settlements at different archaeological periods and from them has produced estimates of
long term changes in mean lake levels and in precipitation (Lorenzo
1956).
The results of these analyses are shown in Figure 32 and it
can be seen that over the entire period of occupance there have been
some fairly large changes in precipitation values.
However, in the
relatively short period that has elapsed since the Conquest the changes
in values have been quite small, with mean values declining from 600
millimeters to just above 500 millimeters.
As the lake complex was
largely supplied with water from areas where mean annual precipitation
is in excess of 1000 millimeters the decline is proportionately less.
213
-\
1600
1
1
1
1200
Figure 32.
1
1
800
1 1—~i
1
400
BC/AD
1 1
400
1
1
1
1
1
1 r
800
1200
1600
2000
Changing Lake Levels and Precipitation in the
Basin of Mexico, 1600 B.C. to Present (Lorenzo
1956, cited in Sanders 1970:88).
214
Insofar as evaporation rates are tied to temperature and precipitation, they too may have changed somewhat during the past 500 years,
and modern values may differ from pre-Hispanic values.
Whether or not these differences in precipitation and atmospherically conditioned evaporation are significant depends very largely
on the degree of accuracy required.
The present work aspires only to
a very rough approximation of pre-Hispanic conditions, and the differences are assumed to be insignificant.. Modern precipitation data are
used without modification, except that some of the isohyets in the
vicinity of Mexico City have been smoothed to obscure the effects of
the urban heat island in the area.
Modern Potential Evaporation data,
adjusted in the manner recommended by the Comision Hidraulica de la
Cuenca del Valle de Mexico (C.H.C.V.M.) as being most appropriate to
the estimation of evaporation losses from open water and saturated
soils have been applied to the areas of the antique lake complex to
provide estimates of evaporation losses from the complex.
This may
result in some inaccuracies in the reconstruction of 16th Century
conditions, but within the present frame of reference they are considered
acceptable.
A much greater source of error is the use of modern data on
evaporation rates from groundsurfaces on hillsides, and on streamflow
and infiltrated flows in the same areas.
Although not as extensively
modified as the Valley floor, the hillsides have been greatly altered
since the 16th Century. Prior to the Conquest the lower slopes of
the southern and central parts of the Valley were thoroughly colonized
for agriculture.
Indian agricultural techniques involved considerable
use of green manures which, amongst their other benefits, improved the
215
water retention capacities of soils.
Polyculture and the practice of
growing perennials along the boundaries of agricultural plots tended
to insure plant cover through much of the year.
Virtually all of the
lower and steeper slopes of the Basin were terraced and irrigation
techniques involved the use of thousands of small dams and distribution ditches.
In addition, the upper slopes of the Valley were more
thoroughly forested than they are today.
The combined effect of these
factors was to retain a considerable volume of precipitation on the
hillsides.
Soon after the Conquest the Indian population suffered a
savage decline as the result of the introduction of European diseases
to which it had no immunity, and land that had been under careful
Indian management came under Spanish control.
The Spanish substituted
the monoculture of wheat for the polyculture of maize, beans and
squash, which would have reduced the effectiveness of crops as persistent ground cover.
They substituted the plough for the digging
stick which may have reduced the soil moisture retention capacities, and
which resulted in the removal of much of the perennial growth that had
bounded the small Indian plots.
Most of the terraces and many of the
small irrigation systems were abandoned.
Cattle and sheep were intro-
duced soon after the Conquest, and their numbers grew rapidly.
It is
estimated that by 1600 some 5,000 head of cattle and 76,000 sheep were
roaming more or less without restraint over the Valley (Simpson 1955:55).
Cattle's hooves compact the soil and the close cropping teeth of sheep
discourage the regrowth of vegetation.
The cumulative effect of these
changes on surface runoff would have been to increase the ratio of
runoff to storage on the hillsides.
Streamflow would have increased,
and the interval between precipitation and stream discharge would have
216
decreased.
With the removal of vegetation cover and the accelerated
movement of water, the hillsides would have been more liable to erosion
and suspended loads would have contributed to a more rapid silting of
the lake complex.
The effect of these changes on evaporation and trans-
piration rates are not as clear.
On one hand, actual evaporation rates
would have been reduced by the more rapid removal of water from the
vicinity of precipitation but, on the other hand, the reduction of
ground cover and the exposure of soil surfaces would have allowed
evaporation mechanisms to operate more efficiently, and potential
evaporation rates may have increased.
The effects of these changes
on infiltrated flows are entirely obscure, but it is very likely that
they were affected the same way.
These changes are particularly
critical in the present work where the focus is on the function of the
lake complex.
The hillsides were the catchment areas for the lakes,
and changes in values per unit area that may have been quite slight in
the large area of the hillsides would be magnified when concentrated
in the relatively small area of the lake complex.
It would, in theory at least, be possible to make some systematic
adjustment to modern data so that they more accurately represent preHispanic conditions.
However, this work has not been carried out by
other writers and it is beyond the capacities of the present one. The
figures used for evaporation and discharge from the hillsides are modern
and are used without adjustment.
Because the ratio of runoff to evapora-
tion may have changed since the Conquest, there may be inaccuracies in
the calculation of mean annual volumes; because the lag time between
precipitation and discharge may have altered, there may be inaccuracies
in the calculation of the monthly distributions of these volumes.
217
Unfortunately, there are no means presently available for correcting
these inaccuracies, nor for estimating how great they may be.
Modern data are also used in the calculation of evaporation
losses from the lakeshore plains.
This too may lead to erroneous
results, not only because land use patterns have changed, but also
because the areas were very much moister during the 16th Century than
they are now and this would have modified evaporation rates.
The
overall effect of these errors on the reconstruction of 16th Century
conditions is probably not great, simply because the areas involved are
not great.
The hydrology of the Basin floor has been so thoroughly modified
by the drainage of the lake complex, by the pumping of subsurface water
and by imports of water from outside the Basin, that modern data on
drainage and storage characteristics clearly bear little relationship
to past conditions and thus these data are not used.
In their place
some assumptions about the behaviour of water in the Valley floor have
been made on the basis of general characteristics of the geology of
the region.
Basin.
Figure 33 shows two sections of the southern parts of the
It can be seen that the Valley floor overlies a more or less
level layer of Lower and Mid Tertiary volcanic rocks and that it is
bounded by the products of Upper Tertiary and Quaternary vulcanism.
At various, points in the region the underlying rocks project towards
the surface and the surrounding rocks approach each other laterally to
form a series of constrictions that divide the whole area of the Valley
floor into a number of distinct compartments, each corresponding with
the basin of one of the antique lakes.
These compartments are filled
with recent alluvial materials and with water.
The alluvial materials
218
Quaternary Volcanic Rocks
Quaternary Alluvial Fill
Tarango Formations
m
Upper Tertiary Volcanic Rocks
Middle and Lower Tertiary
Volcanic Rocks
Possible contact with
Sedimentary Marine Rocks
Elevations in m x 1000 above M.S.L.
Figure 33.
Locations of Sections
Geological Sections of the Basin of Mexico
(C.H.C.V.M. 1961:69).
219
are largely Bentonitic clays and they contain as much as 83% water by
volume (Fox 1965:533).
The clays do not so much contain the waters
of the lakes as sink to the bottom of them, and the enveloping volcanic
rocks contain both water and clay.
When water levels rose above the
level of the alluvial f i l l , lakes occurred, and when they rose above
the levels of the igneous constrictions, lake discharge occurred. The
effect is of a single body of water in each of the compartments, some
801 meters deep near Xochimilco, but with 20% of the lower 800 meters
filled with alluvial materials (C.H.C.V.M. 1961:22).
This description
is of course much simplified and the stratigraphy of the alluvial
deposits is extremely complex, but it is evident that in the upper few
meters of the Valley floor there is no very clear distinction between
groundwater storage and lake storage.
Water storage in the floor of the Basin is largely subterranean
and so too is the discharge of water into these reservoirs.
The geology
of the surrounding mountains and hills is tortuous in the extreme with
many kinds of igneous rocks overlaying and interpenetrating each other.
In the Xochimilco and Chalco watersheds, some 57.2% of the surface of
the lake catchment area is composed of Quaternary fractured basalts,
which are extremely porous, and a further 14.6% of the area is composed
of older basalts, which are only slightly less porous.
The remaining
areas of the mountains and hills are composed of andesites and volcanic
ashes, into which water also percolates quite freely.
With these large
areas of permeable rocks it is estimated that for the entire area of
the Basin about 29% of precipitation is infiltrated, compared with only
3.6% which is discharged in surface streamflow (C.H.C.V.M. 1964 V:133,
141; C.H.C.V.M. 1967:86-87).
The Chalco and Xochimilco regions are
220
exceptionally permeable, but even in the other watersheds of the Basin
a very high proportion of precipitation is infiltrated.
In the light of this information the calculations that follow
make the assumption that there was a relatively free exchange of water
between surface and groundwater reservoirs.
It is also assumed that
the lakes were charged directly with surface runoff from their catchment areas and that groundwater reservoirs were supplied by flows of
indirect and infiltrated runoff.
Finally, it is assumed that over long
periods stocks of groundwater remained approximately constant, and that
inputs to these stocks were balanced by outputs either in the form of
evaporation from the lakeshore plains or by seepage and indirect flow
to the lakes.
The lakes in turn lost water either by evaporation from
their surfaces or by discharge to lakes downstream.
These assumptions
are perhaps over-generalized, but with respect to mean annual volumes
they may not result in great inaccuracies.
However, there is a major
problem associated with the lag times, and this probably results in
substantial errors in calculating the monthly distributions of inputs
to the lakes.
This problem is discussed more fully in Part III of
this Appendix.
In sum, modern climatic and hydrological data are used extensively in this reconstruction of the conditions of the 16th Century.
The use of modern data on precipitation and evaporation insofar as
it is conditioned by atmospheric variables may distort this reconstruction to some extent, but in general they are probably quite representative of pre-Hispanic conditions.
Modern data on drainage and drainage
patterns within the area of the Valley floor are not used because it is
clear that they bear little or no resemblance to pre-Hispanic conditions.
221
The simplified model of these patterns that is substituted in their place
may result in some inaccuracies, particularly with respect to monthly
distributions.
Modern data on evaporation from ground surfaces and on
streamflow, infiltration and indirect flows in the catchment areas of
the.lakes are used--and this is liable to be a source of considerable
error in the reconstruction.
Unfortunately, there is no convenient
means of making systematic adjustments to these data so that they
provide a better representation of pre-Hispanic conditions.
However,
the reconstruction does appear to be worthwhile, even if inaccurate,
partly because many of the arguments in the main body of this work are
based on relative rather than on absolute values and are not invalidated
even i f many of the absolute values are in error, and partly because
it does increase our understanding of the problems that Aztec engineers
confronted and surmounted and of the means by which they contrived a
successful system of hydraulic agriculture.
222
Part II
Derivation of Mean Annual Values for the 16th Century Hydrological System
1.
Drainage Basins
The seven separate drainage basins are slightly modified from the
Hydrological Zones used in C.H.C.V.M. Evaporation studies.
The boundar-
ies of the watersheds used in these calculations are shown in Figure 3.
2.
Physiographic Divisions
The boundaries of the mountains (m) are those used in the
C.H.C.V.M. Evaporation studies.
The lakeshore plains (p) are here taken as existing only in the
five central and southern basins, and are taken as the areas between
the 2,250 meters and boundaries of the mountain.
The valleys (v) comprise the Teotihuacan Valley and the lowlying
regions of Pachuca and the Northeast, and their boundaries are those
used in the C.H.C.V.M. evaporation studies.
The lakes (1) comprise the area of the lake and marsh complex
that originally occupied the central portions of the Valley, together
with the few small lakes that persist in the Northeast, and they are
taken as the area enclosed by the modern 2,250 meter contour.
3.
Precipitation (P)
Precipitation values are derived without modification from the
mean annual isohyets (1920 - 1959), published by the C.H.C.V.M. and
reproduced in Figure 4.
Maps of the Isohyets and of the Drainage
223
Basins were prepared at the same scale and overlaid, and volumes were
calculated by measuring the area in each precipitation class, multiplying the resulting value by the precipitation class value, and summing
the volumes so calculated for each physiographic region of each basin.
The same procedure was used.for calculating evaporation losses. The
total volume for precipitation calculated in this manner for the Basin
of Mexico was 6,836 x 10 m , compared with 6,933 x 10 m calculated
by the C.H.C.V.M. (C.H.C.V.M. 1964 V:333).
4.
Evapotranspiration (E)
a)
Mountains (m), Valleys (v) and the Teotihuacan Valley (t).
Values for total evaporation losses have been published by
the C.H.C.V.M. and are available by Study Regions and fractions of
those regions (C.H.C.V.M. 1964 V:333).
Because the regions employed
in the C.H.C.V.M. evaporation studies do not exactly correspond with
those used in the present study, it was in some instances necessary to
proportion a single given value between two regions, and this was done
on the basis of measures of areas.
Apart from this, the values given
by the C.H.C.V.M. are used without modification.
b)
Lakeshore Plains (p) and Lakes (1)
The model assumes a free exchange of water between the antique
lakes and the groundwater reservoirs beneath both lakes and lakeshore
plains.
Because of this both these regions were treated in the same
manner.
Modern values for evaporation losses in these areas were not
employed.
Instead, an adjusted potential evaporation rate (Epo. adj)
equal to 70% of the potential evaporation rate was substituted.
This
measure is used by the C.H.C.V.M. as an estimate of losses from open
224
water and saturated soils, and is plotted in Figure 13 (C.H.C.V.M. 1964
V:44).
Values for the present reconstruction were derived by the same
method used for estimates of precipitation, for the area of the antique
lakes and the lakeshore plains.
5.
Agricultural Evaporation Supplement (A)
Evaporation rates, as calculated by or derived from C.H.C.V.M.
publications, include values for water transpired by wild plants, but
do not account for the consumptive use of water in agriculture. To
account for these losses, the C.H.C.V.M. employs an agricultural supfi 3
plement (A), which is equal to 680 x 10 m for the entire area of the
Basin.
This value has been applied without modification in the present
reconstruction, on the assumption that Aztec agriculture, although less
profligate in its use of water than modern agriculture, was more widespread; i.e., spray irrigation was not used in the past, but many areas
that were cultivated in the 16th Century are now abandoned (C.H.C.V.M.
1964:VII).
Unfortunately, this single datum is the only information
presently available on the agricultural supplement, and so it has been
necessary to distribute the value by the following formula:
B. x F. x 680
A.
=
where
A. is the volume of supplementary evaporation in a particular
region.
225
b\ is either 34%, as it is in the mountains of Xochimilco,
Chalco, Mexico or Texcoco, or 100% of the lakeshore plains
of those basins, or 25% of the lakeshore plains of Cuautitlan.
These fractions are intended as a representation of the areas
of cultivable land in the Basin, and the northern parts of
the Basin are assumed to have supported only a little rainfed
agriculture.
F^ is a factor to include variations in the evaporation climate,
is equal to the primary evaporation divided by the area of the
section for which values are being calculated.
For an explana-
tion of primary evaporation, see Appendix Part III, 3, below.
6.
Surface Discharge (Q)
The figures used in the diagram are the measured volumes of
runoff for each of the Zones of the Valley.
In the Xochimilco and
Chalco basins, data are presently available for all of the major streams,
and as the drainage areas of these streams correspond with the areas of
the mountains of these basins, all streamflow has been allocated to
the mountains (C.H.C.V.M. 1964 111:91-94; 141-148).
In the remaining
areas of the Basin of Mexico, only summary data are available at
present, which give the total discharge for each of the hydrological
zones employed by the C.H.C.V.M., and the total catchment area for that
discharge.
In the basins for which complete data are not available,
all discharge has been allocated to the mountains, unless the given
catchment area exceeds the measured area of the mountains, in which
case the surplus area was allotted to the valleys of lakeshore plains
and the total volume of discharge allocated proportionately. ;
226
7.
Undifferentiated Runoff (R)
Measured surface discharge accounts for only a small portion of
the drainage in the Basin. Water transfer also occurs by flow in surface channels that are too small to merit metering, by seepage from
streams between metering stations and by subsurface flows through
fractured Basalts and Andesites. In the absence of any useful estimates
of these various flows, they are aggregated as Undifferentiated Runoff
(R) which is defined as:
R = P -(E - A - Q)
These values were calculated for each physiographic region and each
basin.
8.
Lake Discharge (QI)
Discharge from lakes is simply calculated as the sum of all the
inputs, less the outputs of evaporation, for each of the basins. The
assumption is made that over the long term lake storage remained constant.
9.
Mean Annual Totals
The values calculated by these various means are shown in
Table XI , and they are incorporated in the Flow Diagram, Figure 2.
227
TABLE XI
SUMMARY OF ESTIMATES OF 16TH CENTURY MEAN ANNUAL WATER BALANCE,
BASIN OF MEXICO
0)
(3)
E
m xl0
Area
km
(2)
P
m xl0
Chalco
Mountains
Plains
Lakes
Total
880.2
107.7
140.9
1128.8
904.3
85.6
112.1
1102.0
336.4
134.0
183.2
653.6
Xochimilco
Mountains
Plains
Lakes
Total
407.4
28.1
99.7
535.2
370.6
14.5
70.7
455.8
Mexico
Mountains
Plains
Lakes
Total
741.9
142.5
178.8
1063.2
Texcoco
Mountains
Teotihuacan
Plains
Lakes
Total
Cuautitlan
Mountains
Plains
Lakes
Total
2
3
6
3
6
(4)
(5)
A
Q
m xlO m xl0
3
6
(6)
R
3
6
m xlO
1n
88.0
49.0
40.9
439.0
137.0
40.9
439.0
132.5
31.5
94.1
258.1
41.0
13.0
5.0
683.5
88.7
111.3
883.5
276.4
123.8
194.6
594.8
780.4
172.2
153.1
483.7
1589.4
575.0
108.1
96.6
305.4
1085.1
301.5
205.3
179.8
777.7
1464.3
853.2
232.8
317.2
1403.2
558.1
158.9
216.4
933.4
323.0
264.4
469.6
1057.0
-
-
54.0
-
-
-
6
+311.4
-
•-
192.0
76.0
64.0
151.6
22.9
179.5
65.8
140.0
174.5
245.3
79.0
155.0
39.8
154.7
234.0
39.8
154.7,
115
124.0
5.9
111.0
153.0
115
129.9
264.0
-
3
192.0
5.0
-
(8)
QI
m xl0
-
+143.7
+148.7
-613.2
-238.6
228
TABLE XI (continued)
SUMMARY OF ESTIMATES OF 16TH CENTURY MEAN ANNUAL WATER BALANCE,
BASIN OF MEXICO
(1)
Area
(2)
P
km
m xl0
Pachuca
Mountains
Valleys
Lakes
Total
1421..9
631..3
7,.8
2061,.0
778.,4
326.,2
4..0
1108..6
516..0
486..0
9.,9
1011..9
12.,3
3.,2
250.,1
323.,0
15..5
399..1
Northeast
Mountains
Valleys
Lakes
Total
1232,.6
561,.8
45,.8
1840,.2
868,,0
369.,8
30,.2
1268..0
417..0
641..7
52,.3
1111,.9
36.,0
16.,6
415..0
353.,0
52..6
768..0
+157.0
Total Watershed
Mountains
Plains
Valleys
Lakes
Total
6317.6
664.2
1365.3
1273.9
9621.0
4737.9
444.3
804.1
850.1
6836.4
2302.8
733.5
1333.0
1781.4
6150.7
458.2
2462.1
+5.7
2
3
(3)
E
6
(4)
A
m x10
3
6
m xl0
3
(5)
Q
6
m xl0
3
-
-
680.0
(6)
R
6
m xl0
3
(8)
QI
6
-
m xl0
3
6
+96.7
229
Part III
Estimation of Mean Monthly Distributions
The specific intent of these investigations into the hydrology
of the Basin of Mexico in the 16th Century is to come to an understanding of the behaviour of the lake complex in which the chinampas
were located.
Figure 2 shows estimates of the volumes that in the
course of a normal year passed through the lake complex, but it gives
no indication of the monthly distribution of these volumes.
The cal-
culations described in the following pages are intended to produce
estimates, however approximate, of all inputs to the various lakes
and of all evaporation losses.
1.
Monthly Distribution of Precipitation
The mean annual, figures calculated above for precipitation in
the various watersheds or fractions of watersheds have been distributed
in proportion to the measured monthly values of precipitation at a
station or a group of stations in, or in the vicinity of, each of the
watersheds, by use of the following formula:
PI.
PI. x P2.
t
-\
where
PI. is the value of precipitation in month i for the area.
Pl
t
is the calculated mean annual volume of precipitation for
the area for which values are being calculated.
230
P2.. is the monthly value of precipitation at the station or
stations in the vicinity.
P2 is the total annual precipitation for the station in the
t
vicinity.
Values for individual watersheds were estimated with reference to
particular stations, as shown in Table XII. Figure 34 shows locations
of the stations.
The results of these calculations are shown in
Table XV, Rows 1, 6 and 9.
2.
Monthly Distribution of Surface Discharge
a)
Xochimilco and Chalco
The C.H.C.V.M. has kept records or made estimates of the larger
river basins in the Valley of Mexico.
Of these data, only those for
the Xochimilco and Chalco basins are presently available, and these
are used without modification (C.H.C.V.M. 1964 111:91-94; 141-146).
b)
Mexico, Texcoco, Cuautitlan, Pachuca and the Northeast
In the remaining basins only figures for mean annual discharge
are presently available (C.H.C.V.M. 1964 111:6-9).
These have been
distributed between individual months on the assumption that the proportion of discharge to precipitation in any individual month is the
same in the whole of the Basin as it is in the Xochimilco and Chalco
watersheds.
The following formula has been used to generate figures
for the remaining basins:
0 m
g m
= Pm Qt _ fm Qt
Pt
100
231
TABLE XII
STATIONS USED IN CALCULATING MONTHLY VALUES OF CLIMATIC VARIABLES
(Figure 34 shows location of stations)
Basin
Subdivision
Chalco
Mountains
San Rafael
Mil pa Alta
Juchitepee
Amecameca
Repetadora TV
Valley
Chalco
Tlahuac
Los Reys
S. Luis Ameca
Mountains
S. Francisco
Ajusco
El Guarda
Valley
Moyoguarda
San Gregoria
Mountains
Pena Pobre
Huixcuilacan
Tlalnepantla
Xochimilco
Mexico
Station
Data
1961-1975
mean
mean
Source
*
#
#
*
*
Valley
Tacubaya
Xoco
1961-1975
1961-1975
Mountains
Texcoco
S. Rafael
Chapingo
Texcoco
Teotihuacan
mean
1961-1975
mean
mean
mean
Cuautitlan
Tizayuca
Tlalnepantla
mean
mean
#
Pachuca
Pachuca
Tizayuca
mean
mean
#
#
Northeast
Pachuca
Teotihuacan
mean
mean
#
#
Texcoco
Valley
Teotihuacan
*
#
*
#
#
#
C.H.C.V.,M. Boletfn Hidrologico Nos. 81-90; 128; 136; 138; 161; 162.
# Thornthwaite 1964.
232
Amecameca
San R a f a e l
3- R e p e t a d o r a T.V.
Juchltepec
4.
San L u i s Ameca
56.
Chalco
San F r a n c i s c o
78.
Milpa A l t a
Tlahuac
910. San G r e g o r i o
11. A j u s c o
12. Moyoguarda
1.
2.
F i g u r e 34.
13- E l Guarda
14. Los Reyes
15- Xoco
16. Pena Pobre
17- Tacubaya
18. H u i x c u i l a c a n
19- T l a l n e p a n t l a
20. Texcoco
21. Chapingo
22. T e o t i h u a c a n
23. T i z a y u e a
24. Pachuca
L o c a t i o n o f m e t e o r o l o g i c a l s t a t i o n s i n the
B a s i n o f Mexico ( C . H . C . V . M . 1964 V : 2 4 9 ) .
233
where
3
fi
the proportion of total annual discharge in m x 10
in any particular basin and month.
Qm is
Pm is the precipitation for the same basin and month.
Qt is the mean annual discharge for the same basin.
Pt is the mean annual precipitation for the same basin.
fm is a factor equal to the difference between precipitation
in any particular month in the Xochi-Chalco basin, expressed
as a % of mean annual precipitation, and the discharge in
any particular month in the Xochi-Chalco basin expressed as
a % of total discharge.
Monthly discharge as % of total discharge and monthly precipitation
as a % of total precipitation in the Xochi-Chalco basin are graphed
in Figure 35.
Fm is the vertical distance between the two plots,
with positive values when precipitation is greater than discharge,
and negative values otherwise.
The results of these calculations are given in Table XV,
Rows 13 and 15.
3.
Primary and Secondary evapotranspiration
In its calculations of evapotranspiration values the C.H.C.V.M.
distinguishes three different types of evapotranspiration, namely,
total evaporation (Eto), primary evaporation (Epr) and secondary
evaporation (Esc), which are simply related as:
Eto = Epr + Esc
Figure 35.
Precipitation and Discharge for Xochimilco
and Chalco basins (C.H.C.V.M. 1964 111:91-93;
141-146; Thornthwaite 1964:383, 386).
235
Primary evaporation is defined as:
The inevitable evaporation from the soil surface directly
from local precipitation. It excludes all evaporation
supplied by drainage, either surface or subsurface, from
neighbouring zones, as well as evaporation produced by
other imports of water, i.e., by pumping or diversion of
rivers by dams (C.H.C.V.M. 1964 V:243)
and the context makes it clear that the term "evaporation" should be
understood as "evapotranspiration".
Values for primary evaporation
in the Valley of Mexico are calculated by the formula:
Epr
= Z Epo
1
+ J] P - 2jEpo
2
+K
where
Epo.j is potential evaporation measured in evaporating pans,
in mm, on days when P > Epo.
P
is precipitation in mm during months when P < 40 mm.
Epo is Epo-| during months when P < 40 mm.
2
K
is an empirically derived adjustment which is equal to
50 mm where P is greater than 800 mm, and equal to zero
where P is less than 800 mm (C.H.C.V.M. 1964 V:333).
Secondary evaporation is understood as the evaporation of volumes of
water that are imported to a region by drainage or any other means,
and it is calculated by a variety of different procedures, depending
on the nature of the evaporating surface.
The agricultural supplement
is an addition to the total evaporation values, and is not included
in secondary evaporation.
There is no physical basis for the distinction between primary
and secondary evaporation.
Water is lost up to limits imposed by the
236
condition of the evaporating surface and of the atmosphere, and the
process is the same whether water became available by precipitation or
some form of drainage.
The distinction is employed by the C.H.C.V.M.
as a computational convenience which yields useful results in the
Valley of Mexico, and because much of the data is expressed in these
terms, it is retained in the following calculations.
The annual
values for primary and secondary evaporation were calculated using
the following terms:
Epr
= Primary evaporation.
Esc
= Secondary evaporation.
Etot
= Total evaporation losses calculated by the C.H.C.V.M.
for the Valley of Mexico today (C.H.C.V.M. 1964 V:333).
Epo.adj
= Adjusted potential evaporation, equal to 0.7 Epo
used by the C.H.C.V.M. as an estimate of losses from
from large areas of open water and saturated soils.
A plot of these values is shown in Figure 13.
E16
= Estimated total evaporation losses for the Valley
of Mexico in the 16th Century,
f
= Factor published by the C.H.C.V.M., reproduced
here as Table XIII, expressing primary evaporation
as a proportion of total evaporation per hydrological
zone.
The values for evaporation losses were calculated as follows:
a)
In the mountains (m)
E16
= Epr = Etot (C.H.C.V.M. 1964 V:333)
237
b)
In Northeast, lakes and valleys; Pachuca, valleys
El 6 = Etot
c)
Epr
=
f(Etot)
Esc
= Etot - Epr
In all remaining areas
E16 = Epo. adj
Epr =
f(Etot)
or, in Xochimilco 1, p; Texcoco 1, p, t, where (f)«Etot > P
(C.H.C.V.M. 1964 V:333)
Epr
= 0.95 P
Esc
= El6 - Epr.
TABLE XIII
PRIMARY EVAPORATION AS A PROPORTION OF TOTAL EVAPORATION
Hydrological Zone
Source:
Primary Evaporation (Epr) as
T of Total Evaporation (Etot)
Xochi-Chalco
61.0
S.O. Cd. de Mexico
60.5
N.O. Cuautitlan
60.0
N. Pachuca
62.0
N.E. Apcun
55.0
Texcoco
73.0
C.H.C.V.M. 1964:145.
238
4.
Monthly Distribution of Evaporation
The monthly distributions of primary evaporation are subsumed
in the calculations of monthly distributions of surplus (see 5-. below).
The mean annual values estimated for secondary evaporation
have been distributed in proportion to the distribution of potential
evapotranspiration derived from the Thornthwaite equations, for a
station or group of stations in or in the vicinity of individual basins.
The Thornthwaite equations are used because they are available for more
stations than are figures on the distribution of evaporation published
by the C.H.C.V.M.
For stations where both Thornthwaite and C.H.C.V.M.
data are available, the distributions correspond quite closely, so the
use of the Thornthwaite figures would not appear to result in too
great an inaccuracy in spite of the fact that the Thornthwaite equations do not provide a very adequate description of the Basin of
Mexico in other respects.
Values for secondary evaporation losses from lakes were estimated
with reference to potential evapotranspiration data.
Values for second-
ary evaporation losses from lakeshore plains and from valleys, and for
losses by the agricultural supplementary evaporation were estimated by
the same procedure except that figures for actual rather than potential
evapotranspiration were used.
Values for individual basins were esti-
mated with reference to stations shown in Table XIV. The results of
these calculations are shown in Table XV, rows 4, 17, 18 and 19.
5.
Monthly Distribution of Surplus
In order to make our estimate of the monthly distribution of
undifferentiated runoff (R), it was necessary to estimate the monthly
239
TABLE XIV
STATIONS USED IN ESTIMATION OF DISTRIBUTION OF SECONDARY EVAPORATION
Basin
Station
Reference
Xochimilco and Chalco
Moyoguarda
*
Mexico
Mexico City
#
Texcoco
#
Teotihuacan
Tizayuca
Tlalnepantla
#
.Texcoco
Teotihuacan
Cuautitlan
Pachuca
Pachuca
Northeast
Ti zayuca
Pachuca
Teotihuacan
*
#
#
#
C.H.C.V.M. 1964.
# Thornthwaite 1964
occurrence of surplus.
Surplus is intended as an expression of the
volumes of water that become available for surface or subsurface runoff after evaporation has occurred and after any depletions of soil
moisture are replenished.
There are a variety of methods for estimat-
ing surplus and one of the most widely used is the set of Thornthwaite
equations, whereby surplus is estimated from daily precipitation and
evapotranspiration data.
Thornthwaite water budgets have been calcu-
lated for several stations in the Basin of Mexico, and the results
are shown in Figure 36.
The Thornthwaite calculations have been carried out for stations
that are for the most part at low elevations in the Valley, and they do
240
mm
250
2001
150
100
50
J
F
M
A
M
J
J
A
S
O
N
J
D
F
M
A
M
J
Huixquilacan
Mexico City
1 9 ' 2 2 ' N . 99'12'W. elev. 2700m
1 9 ' 2 6 ' N . 99'08'W.
J
A
S
O
N
D
J
J
F
M
A
M
J
J
A
S
0
N
D
Texcoco
elev. 2259m.
19"31*N. 98"52'W.
mm
150
elev. 2216m
mm
100H
100'
/
:f \
50H /
^
J
—
i —
i —
i —
i —
i —
i —
i —
i —
i —
i
F
M
A
M
J
Teotihuacan
19'42'N. 9 8 ° 5 2 ' W .
J
A
S
O
elev. 2294m
N
J
D
J
F
ILi
M
A
M
\>
\\\
\\
50
•7'/
N
J
J
A
S
O
N
D
J
J
20"06'N 98'44'W
F
M
A
M
elev.
1 9 ° 5 0 ' N . 98"59'W.
2447m
Precipitation
—
Potential Evapotranspiration
Actual Evapotranspiration
F i g u r e 36.
J
J
A
S
O N
Tizayuca
Pachuca
Water Balance Diagrams f o r S e l e c t e d S t a t i o n s
i n the B a s i n o f Mexico ( T h o r n t h w a i t e 1964).
elev. 2270m
D
J
241
not provide a very adequate description of the evaporation climate at
high elevations, where most of the surplus is generated.
This is
evident in Figures 37 and 38 which show surplus and surface discharge
values for watersheds in the south of the Valley.
In both examples
discharge increases in April, well before the time of year when surplus becomes available.
Because surplus is defined as including dis-
charge, this should not be possible.
The Thornthwaite equations make
the assumption that all soil moisture deficits are recharged before
surplus becomes available, but even i f this assumption is relaxed
and surplus is assumed to occur as soon as precipitation exceeds
actual evapotranspiration, it s t i l l does not occur sufficiently early
in the year to account for the increase of discharge in April and May.
For the purposes of this discussion an estimate of the distribution of surplus has been made on the basis of the following assumptions:
a)
that the mean annual volume of surplus in any basin or part
of a basin is equal to the mean annual volume of precipitation minus
the mean annual volume of primary evaporation for that basin.
b)
that the relationship between precipitation, discharge
and surplus in the whole of the Basin of Mexico is the same as the
relationship between these variables in the Xochi-Chalco basin, and .
in the drainage basin of Rios Magdalena and Esclava.
c)
that surplus begins to occur at a time t-j when discharge
begins to occur.
d)
that in the conditions of rapid runoff that occur in the
Basin, surplus ceases to occur at a time
to a minimum at the end of the rainy season.
when precipitation falls
242
500
4 00
Figure 37.
Surplus and Discharge for Basin of Rios Magdalena
and Esclava, Basin of Mexico (C.H.C.V.M. Boletin
Hidrologico n.d.; Thornthwaite 1964:383).
Figure 38.
Surplus and Discharge for Xochimilco and Chalco
basins,- Basin of Mexico (C.H.C.V.M. Boletin
Hidrologico n.d.).
244
e)
that the volume of surplus and soil moisture recharge at
any time t .when t^ < t
n
<t
2
is directly proportional to the volume
of precipitation at time t .
n
f)
that at any time t
the proportion of surplus to soil
moisture recharge is the same as the proportion of t
- t-j to t,, - t-j.
On the basis of these assumptions some rather inelegant calculations
were carried out, yielding the estimates for the distribution of
surplus that are shown in Table XV, Rows 3, 8, 11 and 16.
6.
Monthly Distribution of Undifferentiated Runoff (R)
There are no good, direct indicators of when these flows
became available in the lake complex.
One possibility was that hydro-
graphic separations performed on mean annual streamflow records might
provide some indication of the properties of base and indirect flows.
However, most of the streams are intermittent and cease discharge
during the winter months.
Rios de la Compana, San Francisco, Amecameca,
Magdalena and Esclava are normally perennial, but their annual discharge
profiles do not appear to be suitable for this type of analysis (Figures
39 and 40).
All of these streams show a slight decrease in the rate
of decline during the winter months, but as this corresponds with, and
is almost certainly caused by a slight peak in precipitation during the
winter, it provides no information on baseflows.
When this peak is
smoothed out, the recession limbs of these rivers show a smooth and
steep decline, and the results of any attempts to separate the flows
depend entirely upon the selection of points for analysis and not at
all on the objective characteristics of the streamflow profiles.
Figure 39.
Estimated Mean Monthly Discharge of Rios de la
Compana and San Francisco, Valley of Mexico
(C.H.C.V.M. 1964 111:141, 142)
Figure 40.
Estimated Mean Monthly Discharge of Rio Amecameca,
Chalco Basin (C.H.C.V.M. 1964 111:141)
247
A second possibility was that data on the yield of springs in
the Xochimilco and Chalco regions would provide some indication of
subsurface flows.
Some data are available for the years of 1903 and
1904, before mechanical pumps were installed (Marroquin y Rivera
1914:151-154).
However, these are fragmentary, and the fact that
different government agencies making measurements of the same springs
at the same time recorded values that differed by as much as 100%
suggests that they may also be unreliable.
The measurements made by
one of these agencies are shown in Figure 41, but because they are so
fragmentary it is difficult to make any evaluation of the data.
A priori, it would seem that the yield of springs should in some way
be tied to variations in precipitation, but the available data show
very little evidence of this.
It is true that in the summer of 1903
most of the larger springs show peak yields between one and three
months after peaks in precipitation, but any hypothesis of a consistent
lag is frustrated by data on the winter of 1903-04 which shows an
increase in the output of Quetzalapa and some of the other springs
at a time of year when precipitation values are low.
This group of
springs is not even a representative sample because there were many
smaller and submerged springs about whose yields we have no information,
and so it is not even possible to make an evaluation of the extent to
which'the springs had the effect of smoothing inputs to the lake
complex by counter-variation.
In the absence of any useful indicators of the seasonal distribution of the flow of undifferentiated runoff into the lakeshore plains
and the lakes, the following procedure was adopted.
First, undifferen-
tiated runoff (R) was divided into two parts, infiltrated flow (I) and
248
Figure 41.
Observed Yield of Springs in the Xochimilco and
and Chalco basins (Marroquin y Rivera 1914:151-54)
1903-04.
249
and indirect flow (J), such that
PI
= I + J.
The infiltrated flow (I) is taken directly from estimates made by the
C.H.C.V.M. of the annual rates at which deep aquifers, depleted by
pumping, are recharged by the infiltration of precipitation.
It is
assumed that the Valley of Mexico in the 16th Century was an entirely
closed drainage basin, from which it follows that volumes infiltrated
at high elevations sooner or later made their way to the aquifers
underlying the Valley floor, and thence to the lake complex where they
became available for lake discharge, storage or evaporation. The
deep aquifer recharge volumes are here used as an approximation of the
volumes that became a part of this deep subsurface flow.
The indirect
flow (J) is intended to represent volumes draining in channels too
small to merit metering, or that were lost from streams by seepage
before they were metered.
This was calculated as the difference between
undifferentiated runoff (R) and infiltrated flows (I).
The monthly distributions of these flows were then estimated on
the basis of the following assumptions:
a)
that the flows in any one year begin at the times estimated
for the start of surplus.
b)
that indirect flows peak one month after the peak of surplus
and that the flows of infiltrated water peak two months after the peak
of surplus.
c)
that the rise side of the curve of indirect and infiltrated
flows is approximated by an S-shaped curve, drawn by eye, and shown
in Figure 42.
250
I
i
i
A
Figure 42.
i
i
i _
B
Graph used for estimates of monthly distributions
of infiltrated flows (I) and indirect flows (J).
Note: A and B indicate the start and peak of
surplus, respectively.
150
Figure 43.
Estimated Monthly Distribution of surplus and
runoff, Chalco Basin.
S - surplus; Q - surface discharge;
I - infiltrated flow; J - indirect flow.
251
d)
that the decay rate of the regression limb is approximated
by a constant K = 0.65 for indirect flow, and K = 0.75 for infiltrated
flow, which figures are based on the characteristically steep recession
limbs of the various perennial rivers in the southern parts of the
Basin.
Values for particular basins were estimated by adjusting the
time axis of the graph, Figure 42, so that it corresponded with the
period during which surplus was estimated to occur in that basin,
and the total volumes of infiltrated and indirect flows were proportioned against values taken from the vertical axis of the graph.
Figure 43 shows the values that resulted for the Chalco basin, together
with estimated data for discharge and surplus.
The values calculated
for the monthly distributions of infiltrated and indirect flows were
then added, to provide an estimate of the monthly inputs of water by
subsurface flows into the lake complex.
These values are shown in
Table XV, Rows 12 and 14.
7.
Lake Tlahuac and Lake Chalco
Some modern data are available for the stage of Lake Tlahuac,
which is a small residue of the antique Lake Chalco.
These data
exist only for the short period from 1950 to 1955, before which time
there are no records of stage and after which time the residual lake
was supplied with seasonal additions of treated water from Mexico
City, with the result that fluctuations in stage were considerably
reduced.
The observations were made well after pumping of groundwater
on a massive scale had commenced, and so the extent to which.the regime
of the modern Lake Tlahuac resembles that of antique Lake Chalco is
TABLE XV
ESTIMATED 16TH CENTURY MEAN MONTHLY WATER BALANCE, BASIN OF MEXICO
Abbreviation
Variable
Row Numbers
Derivation or Source
in Appendix Part III
Physiographic Regions
P
Precipitation
1,6,9
Section 1
m
mountains
Epr
Primary Evaporation
2,7,10
Epr = P - S
v
valleys
S
Surplus
3,8,11,16
Section 5
p
plains
A
Agricultural Supplement
4,17
Section 4
1
lakes
Om
Output from mountains
5
Om = Sm - Am
R
Undifferentiated Runoff
12,14
Section 6
Q
Surface Discharge
13,15
Section 2
Esc
Secondary Evaporation
18,19
Section 4
I
Total Input to Lake
Complex
20
Sum of rows
12-19
TABLE XV (continued)
Row
No. Variable
1
2
3
4
5
6
7
8
9
10
11
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
Total
+P.m
-Ept.m
=S.m
-A.m
14.5
14.5
7.2
7.2
13.6
13.6
38.0
33.0
164.5
89.5
168.1
66.1
160.0
39.2
153.7
14.8
67.8
48.8
19.0
4.1
7.2
7.2
904
336
0.0
4.1
0.0
0.0
3.3
5.0
6.8
90.4
63.9
26.5
102.0
10.4
120.8
138.9
84.9
14.9
0.0
568
10.3
75.0
11.2
10.1
3.3
-1.8
16.2
63.8
91.6
1.6
1.6
-4.6
0.8
0.8
1.4
8.0
6.5
14.4
10.2
17.8
10.9
13.7
5.9
6.3
0.0
2.1
0.0
1.0
1.4
0.0
1.8
3.8
3.5
110.7
16.8
5.5
9.4
88
-4.1
7.7
77.2
5.0-
=0.m
+P.p
9.0
129.9
0.3
4.9
1.5
10.4
4.2
6.9
23.2
7.8
17.9
4.0
8.2
0.9
0.7
0.2
2.1
1.8
0.0
4.6
0.3
8.6
1.8
8.6
9.3
-0.3
8.5
0.8
0.3
21.3
0.5
15.2
13.3
2.6
55.6
7.7
5.6
59.0
3.9
58.3
1.8
5.2
3.8
0.7
2.5
0.0
0.0
2.8
4.6
6.4
41.5
-Epr.p
=S.p
+P.1
-Epr.l
=S.l
4.6
13
+R.m
+Q.m
39.9
0.5
1.0
0.0
29.7
0.3
14
15
+R.p
+Q.p
1.7
1.1
0.7
16
17
18
+S.1
-A.p
0.0
0.0
2.3
3.8
0.0
0.0
2.5
4.2
0.0
0.0
1.9
19
-Esc.l
6.8
20
= 1.1
6.0
30.0
3.1
9.5
17.6
8.0
12
-Esc.p
0.0
0.9
0.5
0.7
0.0
0.3
3.8
6.2
0.0
1.8
11.2
-4.3
13.0
-9.8
5.7
9.5
18.8
13.8
5.0
18.1
6.3
1.8
0.0
5.0
6.2
10.3
12.3
2.4
8.7
8.1
2.3
15.0
8.2
21.9
12.3
9.6
30.1
8.1
46.4
7.7
4.1
5.3
0.0
8.2
0.0
9.3
0.0
8.5
5.8
0.0
9.6
5.6
5.0
4.3
0.0
0.3
3.1
9.6
11.7
23.4
9.3
11.3
42.8
8.3
10.0
54.9
7.0
5.1
8.8
56.5
7.0
49.0
1) Estimated Water Balance of the Chalco Basin
1.1
-5.0
0.5
0.5
480
86
53
0.0
0.7
33
112
0.7
0.0
52.1
69
43
439
41
33
0
43
49
81
114
312
TABLE XV (continued)
Row
No. Variable
1
2
3
4
+P.m
-Ept.m
=S.m
-A.m
5
6
=0.m
+P.p
7
8
9
-Epr.p
=S.p
+P.1
10
-Epr.l
11
12
=S.l
+R.m
+Q.m
+R.p
13
14
15
+Q.p
16
17
+S.1
18
-Esc.p
19
20
-Esc.l
-A.p
= 1.1
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
Total
7.1
7.1
2.6
2.6
5.2
5.2
0.0
2.1
0.0
2.3
0.0
2.9
12.2
10.7
1.5
3.4
28.2
19.9
8.3
63.4
33.7
29.7
3.0
2.8
0.2
371
133
238
3.2
41
-2.3
54.9
2.7
0.6
1.0
0.9
0.1
0.1
0.1
0.1
0.0
0.0
2.6
1.3
0.1
6.5
2.5
2.2
197
15
0.2
43.9
3.4
3.2
0.2
-2.3
0.1
0.1
0.0
-1.9
0.6
24.0
0.3
-2.9
0.2
2.6
-0.4
2.5
-2.1
66.0
4.4
61.6
3.8
57.8
4.5
2.3
2.2
4.7
75.3
16.3
59.0
4.1
26.7
-0.5
27.2
5.0
3.3
76.8
28.5
48.3
4.4
0.3
0.1
16.0
15.1
0.9
13.0
12.1
0.9
12.0
0.0
0.6
71
11.0
1.0
4.7
4.3
0.4
0.0
0.6
0.6
0.0
0.6
0.0
67
4
19.4
1.1
23.3
24.9
24.8
22.8
0.4
0.2
0.0
0.2
0.0
0.1
0.9
0.1
192
5
0.1
1
0.0
0.0
0.0
0.0
0.0
0
0.9
1.5
1.0
1.4
1.7
0.4
1.1
1.5
0.0
0.8
0.0
0.7
1.0
2.4
19.8
2.1
21.2
1.1
1.6
21.5
4
13
17
1.5
19.7
27
145
0.3
0.0
1.4
1.4
0.0
18.2
0.0
0.1
0.0
0.0
0.6
018
1.4
15.5
1.4
25.0
2.6
2.5
0.1
12.1
0.6
1.0
0.6
0.0
1.0
0.0
10.2
2.6
0.0
6.3
0.2
11.5
0.6
7.5
6.3
12.5
0.0
0.0
0.0
0.0
0.0
0.5
8.1
0.9
0.0
0.0
0.0
0.2
0.0
0.6
1.5
1.7
2.1
2.9
0.1
0.0
0.9
1.5
14.0
0.0
0.1
0.0
0.0
0.7
0.0
0.0
0.5
0.9
1.6
0.6
2.2
10.9
6.9
2)
0.0
1.0
1.3
2.7
2.5
2.0
3.1
0.4
2.9
1.2
2.0
2.8
8.4
2.5
0.2
2.0
2.7
15.3
Estimated Water Balance of the Xochimilco Basin
14
1
TABLE XV (continued)
Row
No. Variable
1
2
3
+P.m
-Ept.m
=S.m
4
5
6
-A.m
=0.m
+P.p
7
8
-Epr.p
9
10
11
12
13
14
15
16
17
18
19
20
=S.p
+P.1
-Epr.l
=S.l
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
Total
8.2
8.2
6.2
6.2
47.2
34.5
12.5
121.7
69.4
52.3
146.4
62.0
84.4
129.3
18.5
5.5
5.5
110.8
8.7
43.6
8.1
76.3
11.9
4.8
0.0
9.3
3.2
5.9
89.3
7.7
81.6
48.6
3.8
44.8
15.7
3.8
0.0
4.3
-4.3
0.5
19.1
17.1
2.0
6.2
-4.2
125.2
35.9
0.0
3.9
-3.9
10.9
10.9
0.0
5.4
684
276
408
76
332
17.9
10.5
18.6
8.4
10.2
19.1
6.0
13.1
12.4
22.3
13.1
9.2
23.2
10.5
23.7
7.5
16.2
19.1
3.7
15.4
7.3
12.7
17.9
2.7
19.5
26.5
23.3
2.7
12.7
4.0
16.2
8.1
10.7
7.9
1.3
1.3
0
1.5
1.5
-5.4
1.2
2.9
2.5
0.5
1.2
0
0.6
0.6
0
1.5
0
0
3.7
3.3
0.4
1.5
0.4
4.4
1.5
7.3
5.4
1.9
5.2
0
15.9
3.3
1.9
11.7
1.8
1.4
8,3
5.9
1.2
0.9
0.4
0.3
0.2
1.6
0.7
0.2
0.7
0.4
-A.p
0.0
2.5
-Esc.p
-Esc.l
= 1.1
3.3
7.7
8.0
0.0
2.7
3.6
8.8
0.1
0.0
3.5
4.7
0.4
4.2
5.5
1.9
6.6
8.8
12.1
-9.7
14.3
-15.2
16.6
-21.2
+R.m
+Q.m
+R.p
+Q.p
+S.1
3)
2.6
7.4
7.0
1.3
1.1
9.2
8.5
11.3
15.8
-9.7
12.7
15.0
14.9
3.6
10.5
14.5
36.9
7.0
103.8
15.2
2.8
47.0
4.0
7,1
15.4
7.2
9.5
12.9
67.2
Estimated Water Balance of the Mexico Basin
6.0
38.8
5.3
0.5
4.8
6.7
7.1
0.6
0.4
0.2
0.8
0.7
6.0
0.6
0.2
24.6
29.5
24.2
3.9
4.5
6.0
5.7
7.6
11.2
44.0
9.0
3.4
1.4
0.2
3.9
5.2
8.9
20.2
4.6
-4.6
0.5
0.5
0
0.6
0.6
0
21.4
4.6
2.5
0.7
0.0
3.2
4.3
8.2
13.5
89
39
50
111
49
62
180
152
27
23
62
64
85
146
149
TABLE XV (continued)
Row
No. Variable
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
+P.m
-Ept.m
=S.m
-A.m
=0.m
+P.p
-Epr.p
=S.p
+P.1
-Epr.l
=S.l
+R.m
+Q.m
+R.p
+Q.p
+S.1
-A.p
-Esc.p
-Esc.l
= 1.1
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
Total
7.2
5.2
5.2
10.4
10.4
0
5.1
19.8
18.0
1.8
47.7
37.1
10.6
103.5
66.8
119.1
62.8
115.0
47
87.7
26.4
42.0
10.2
10.9
5.5
5.7
5.4
575
302
36.7
57.1
61.3
31.8
9.4
1.2
9.6
9.1
7.7
0.3
4.1
273
79
18.9
18.4
0.5
48.0
40.8
38.8
6.6
25.2
5.4
4.8
27.1
34.2
68.0
8.8
59.2
14.7
0.6
4.5
31.9
2.5
13.6
1.1
4.3
0.2
-3.8
2.2
2.2
194
205
195
52.5
20.7
19.1
1.6
21.2
5.8
5.4
0.4
20.0
0.0
2.7
2.7
10
305
3.7
1.1
1.6
1.2
0.0
1.6
0.0
0.4
14.2
9.2
7.2.
15
155
17.6
37.5
11.4
817
27.3
191
487
-24.6
-614
7.2
0
3.7
-3.7
0
4.1
-5.1
2.3
2.3
-4.1
2.1
2.1
6.0
-4.2
4.8
8.2
4.8
8.1
0.0
3.4
3.4
0
12.4
0.3
0.0
1.8
1.8
0
9.3
0.3
0.1
11.3
11.2
1.0
0.9
0.0
4.9
4.9
0
6.9
0.5
0.8
0.0
0.0
0.0
0.0
5.7
6.9
25.8
6.0
7.4
29.2
-24.5
-32.1
0.0
0.0
8.4
0.1
5.2
0.7
0.6
0.0
0.1
26.8
26.2
0.2
1.3
54.9
52.9
2.0
61.9
58.9
3.0
12.3
8.4
0.4
4.7
2.8
0.5
2.0
6.9
6.8
0.4
0.0
0.6
0.0
2.0
0.0
16.5
20.4
25.1
19.9
24.7
50.2
-70.7
10.4
40.4
10.0
12.4
47.7
20.4
55.5
-51.0
-63.5
-83.8
4)
32.9
52.6
-82.0
3.0
37.9
35.6
2.3
58.3
54.8
3.5
17.7
7.7
0.8
0.0
3.5
19.5
24.0
48.2
53.6
34.4
48.7
3.8
20.6
6.5
1.0
0.0
3.8
18.0
22.0
42.9
-62.0 -51.0
Estimated Water Balance of the Texcoco Basin
-41.7
29.7
-21.1
0
16.7
0.7
1.2
0.0
0.0.
290
15
154
40
10
0
TABLE XV (continued)
Row
No. Variable
Jan.
Feb.
Mar.
Apr.
May
June
July
12.0
11.6
0.4
15.6
14.2
16.5
14.2
1.4
2.3
40.2
29.4
10.8
104.1
63.3
40.8
101.0
53.6
47.4
1
2
+P.m
-Ept.m
7.5
7.5
3
=S.m
0.0
4
5
-A.m
=0.m
+P.p
-
-
-
-
-
0.0
2.1
0.4
3.4
1.4
4.5
2.3
4.7
10.8
40.8
11.4
2.1
3.4
0
4.4
0.1
4.5
0.2
10.4
1.0
29.7
26.4
3.5
4.6
4.5
6.0
5.8
0.2
6.4
15.6
6.1
14.4
1.2
6
7
8
9
-Epr.p
=S.p
0
+P.T
2.9
10
11
12
-Epr.l
=S.l
+R.m
2.9
13
14
15
+Q.m
+R.p
+Q.p
16
17
18
+S.1
-A.p
19
20
0
10.4
1.1
0.1
6.7
0.1
5.6
0.1
0.1
5.8
-Esc.p
0.0
4.7
5.2
-Esc.l
= 1.1
15.0
-6.6
17.0
-11.8
7.1
9.1
2.5
6.4
2.6
4.2
0.1
0.2
7.8
8.6
23.5
-25.7
5)
Including imports from Pachuca
0.3
6.1
1.5
6.0
7.4
-
-
Aug.
85.9
39.5
46.4
-
Sept.
Oct.
Nov.
Dec.
Total
108.8
40.4
68.4
39.9
15.4
24.5
19.5
8.9
10.6
7.0
7.0
558
323
0.0
235
-
-
-
24.5
11.4
9.2
2.2
10.6
5.6
0.0
2.0
235
159
4.7
2.0
0
137
22
15.4
12.6
7.6
6.4
1.2
2.7
216
2.7
0
11.7
11.3
2.6
187
29
111
-
-
47.4
28.7
46.4
24.5
68.4
31.0
24.6
4.1
20.5
25.0
4.0
40.2
39.2
35.6
33.8
33.3
27.9
6.0
42.1
4.6
6.7
5.4
9.2
5.4
10.4
34.3
7.8
11.3
21.9
3.5
1.1
22.6
18.1
25.4
2.8
11.7
11.2
5.0
1.2
7.3
1.2
7.7
0.6
5.4
6.5
0.8
5.4
7.8
2.8
7.1
7.7
0.3
1.2
13.7
15.1
11.0
12.2
9.8
10.8
7.1
7.9
5.9
6.5
115
127
28.0
-15.6
24.9
4.9
21.8
-8.4
17.3
-4.3
15.8
-6.9
283
-187
3.3
0.1
0.3
3.1
0.4
1.2
8.0
8.9
11.5
4.6
15.1
12.7
16.6
14.6
16.1
27.7
-33.3
32.3
-38.4
30.6
-24.5
29.1
-16.4
Estimated Water Balance of the Cuautitlan Basin
0.9
7.4
0.0
0.0
124
68*
6
29
TABLE XV (continued)
Row
No. Variable
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
Total
10.1
10.1
0.0
21.9
21.9
0.0
28.8
27.5
1.3
31.9
28.8
3.1
67.7
55.0
12.7
142.0
103.0
39.0
118.3
77.2
41.1
_
107.4
63.0
44.4
_
136.9
65.7
33.5
69.4
31.9
17.5
67.5
33.8
16.0
_
_
_
_
14.1
11.1
3.0
_
778
516
262
_
1
2
3
4
+P.m
-Epr.m.
=S.m
-A.m
5
=Q.m
0.0
0.0
1.3
3.1
12.7
39.0
41.1
44.4
67.5
33.8
16.0
3.0
262
6
7
8
9
10
11
+P.p
-Epr.p
=S.p
+P.1
-Epr.l
=S.l
4.2
4.2
0.0
0.1
0.1
0.0
9.1
9.1
0.0
0.1
0.1
0.0
12.1
12.1
0.0
0.1
0.1
0.0
13.4
13.1
0.3
0.2
0.2
0.0
28.4
27.9
0.5
0.3
0.3
0.0
59.4
44.4
3.9
0.7
0.7
0.0
49.6
45.5
4.1
0.6
0.6
0.0
45.0
40.6
4.4
0.6
0.6
0.0
57.4
50.7
6.7
0.7
0.7
0.0
27.5
24.1
3.4
0.3
0.3
0.0
14.0
12.4
1.6
0.2
0.2
0.0
5.9
5.6
0.3
0.1
0.1
0.0
326
301
25
4
. 4
0
12
13
14
15
+R.m
+Q.m
+R.p
+Q.p
23.3
0.1
2.2
0.0
18.0
0.3
1.8
0.1
12.9
0.4
1.4
0.1
10.1
0.3
1.0
0.0
10.4
0.9
0.8
0.2
13.0
2.1
1.0
0.5
20.1
1.8
1.3
0.5
25.9
1.6
2.0
0.4
28.8
2.2
2.5
0.6
30.1
1.2
2.7
0.3
29.6
0.8
2.7
0.2
27.8
0.3
2.6
0.1
250
12
22
3
16
17
18
+S.1
-A.p
-Esc.p
0.0
0.0
5.0
0.0
0.0
7.6
0.0
0.0
10.4
0.0
0.0
11.5
0.0
0.0
18.7
0.0
0.0
27.7
0.0
0.0
24.8
0.0
0.0
23.1
0.0
0.0
22.7
0.0
0.0
17.0
0.0
0.0
10.0
0.0
0.0
6.5
0
0
185
19
20
-Esc.l
=1.1
0.3
20.3
0.4
12.2
0.5
3.9
0.6
-0.7
0.7
-7.1
0.6
-11.7
0.6
-1.7
0.6
6.2
0.5
10.9
0.5
16.8
0.4
22.9
0.3
24.0
6
96
6)
Estimated Water Balance of the Pachuca Basin
TABLE XV (continued)
Row
No. Variable
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
Total
10.4
10.4
18.2
38.2
32.2
80.7
57.3
143.2
143.7
135.4
148.0
75.5
31.7
13.5
18.2
29.5
27.6
37.0
15.5
9.3
9.9
0
1.9
6.0
23.4
66.9
76.8
49.0
0
83.7
59.5
868
417
86.4
111.0
60.0
22.4
3.6
451
=0.m
+P.p
4.4
12.6
61.1
61.2
57.7
13.5
5.7
370
57.8
58.5
54.7
0
0.6
0.6
3.3
4.9
4.8
2.7
0.8
1.1
1.0
0.5
n
=S.l
0
0
0.1
0.1
3.9
5.1
4.9
0.2
353
17
5.0
4.8
0.2
3.0
4.7
4.5
0.2
5.6
0.1
0.5
10
0
0.4
0.4
30.1
2.1
12.7
=S.p
+P.1
-Epr.1
33.6
0.8
2.8
2.7
63.1
59.2
32.2
4.4
16.3
16.1
0.2
34.4
-Epr.p
7.8
7.8
12
+R.m
+Q.m
+R.p
+Q.p
38.6.
0.6
29.4
0.8
17.1
21.7
33.5
1.5
16.7
2.2
3.8
2.5
0.0
0.3
0.0
0.5
0.0
0.5
0.0
0.0
9.8
1.4
0.0
0.0
14.2
0.0
2.5
0.2
0.0
3.1
0.2
0.0
1.8
0.1
0.0
1.0
0.1
0.0
0.4
0.0
0
17
1
2.0
0.0
0.0
17.3
2.4
0.0
1.4
0.1
5.9
0.0
2.8
0.2
38.1
5.8
0
46.1
0.2
0.0
0.1
0.0
0.0
7.8
6.3
0.0
2.6
43.1
5.9
0.1
49.4
0.5
19.3
6.9
-0.3
1
2
+P.m
-Epr.m
3
4
5
=S.m
6
7
8
9
13
14
15
16
17
18
19
20
-A.m
+S.1
-A.p
-Esc.p
-Esc.l
= 1.1
1.3
30.2
7)
12.6
0
1.0
1.0
0
21.1
1.3
1.3
0
0.0
29.5
2.7
-9.8
2.6
2.5
0.1
50.2
30
29
1
415
36
0.1
0.0
43.1
2.6
0.0
39.3
2.5
0.0
37.3
2.4
0.0
37.6
2.1
0.0
28.3
1.8
0.0
15.3
1.5
0.0
9.5
1.3
0
289
24
-15.0
0.6
12.0
17.5
24.5
34.2
35.9
156
Estimated Water Balance of the Northeastern Basins
260
unclear.
Figure 44 shows the mean monthly levels of Lake Tlahuac as
observed and the fluctuations of inputs estimated for the antique
Lake Chalco, calculated by summing the monthly values of streamflow
(Q), precipitation (P), and undifferentiated runoff (R), minus the
volumes estimated for primary evaporation.
The two curves correspond
closely but, because of the uncertain resemblance of modern to preHispanic conditions and because the values shown for Lake Chalco do
not include the effects of secondary evaporation, the correspondence
may not entirely validate the procedures employed in this reconstruction.
261
Figure 44.. Observed stage of Lake Tlahuac and estimated
inputs of Lake Chalco (Boletin Hidrologico,
n.d.).
262
Part IV
Estimates of Variation in Lake Stage
1. Assumptions
In order to produce estimates of the variations in stage of
the antique lakes, the following procedures were employed:
a)
The areas of the lakes were derived from maps and are assumed
to have been as shown in Table XVI, column 2.
b)
The relative elevations of mean lake surfaces are assumed to
have been as shown in Table XVI, column 3.
c)
The average elevation of all five lakes are assumed to have
been 2240 m above mean sea level, which coupled with the two preceding
assumptions yields the values for individual lakes shown in Table XVI,
column 4.
d)
The shape of each lake depression was.approximated as a spheri-
cal segment with a mean volume equal to the mean area of the lake
multiplied by one meter.
A graph was then constructed for each sphere
lake that related lake stage to volumes in storage, and stage values
for calculated volumes in storage were then read from the graph.
2.
Estimated.Variations of Lake Stage with Normal Precipitation
The volumes of water entering a lake complex in any month i
were estimated using the figures from the Table XIV of this Appendix,
and were taken as:
263
TABLE XVI
ASSUMED AREAS AND ELEVATIONS OF THE ANTIQUE LAKES
(3)
(2)
(1)
(4)
mean area
relative mean
mean elevation
Lake
(km )
elevations (m)
above m.s.1 (m)
Texcoco
483.7
0.0
2238.1
Mexico
178.8
+0.5
2238.6
99.7
+2.0
2240.1
Chalco
140.9
+2.5
2240.6
Cuautitlan
317.2
+5.0
2243.1
2
XochimiIco
I.
= R. + Q. + S.l. - A.p - Esc.p - Esc.l. +Q.1.
i
i
where, in each month i
I.
= total inputs
R.j
= undifferentiated runoff
Q.
= streamflow
S.1.
= lake surplus
A.p.
= agricultural evaporation supplement from plains,
valleys and the Teotihuacan Valley
Esc.p.
1
= secondary evaporation from plains, valleys and the
Teotihuacan Valley
Esc.l^
= secondary evaporation from lakes
Q.l.
= discharge from lakes upstream
264
For models of uncontrolled conditions, the discharge from a lake in
any month i was estimated as:
Q.1.
= 0.5 Q.1. + 0.5 I.
where Q1
. y is equal to the annual discharge from the lake, per text
Figure 2, divided by 12.
The volume remaining after discharge was
then located on the graphs described in 1. d) above to give the corresponding value of stage.
3.
Estimates of Variation of Lake Stage.with Precipitation at
Exceptionally High or Low Values
Only one variable was modified in order to produce these
estimates, namely precipitation itself.
to be as they were in normal years.
All other values were assumed
There is some justification for
this with regard to evaporation rates.
Lines of equal potential evapor-
ation in 1957 and 1958, which were exceptionally dry and wet years,
respectively, do not vary consistently; in some of the basins losses
were higher in the wet year than in the dry and in others, lower
(C.H.C.V.M. 1964 V:257, 259)
In order to produce the estimates shown in text Figure 19, 28
and 29, the annual value for precipitation for the whole of the Basin
was increased or decreased by 25% and distributed between basins and
physiographic regions in the same proportions as in normal years.
For each region, the annual precipitation value was reduced by the
normal value for primary evaporation, to yield an annual value for
surplus.
In the lakes, the value for surplus was pro-rated against
265
the normal monthly distributions.
In the remaining regions values for
surplus were distributed against normal values for discharge (Q) and
runoff (R).
Monthly values for discharge were obtained by proportioning
the volumes against normal values, and monthly values for runoff were
calculated from Figure 43.
The procedure was carried out individually
for each year of the sequence and overlapping values in consecutive
years were summed. The resulting figures were then reduced by normal
monthly values for secondary evaporation, and the agricultural
supplement, to provide estimates of the total inputs to the lake
complex in each successive year.
For text Figure 19 (page 50), fluctuations of Lake Chalco over
a 10-year period, monthly lake discharge and stage were estimated in
the manner described for uncontrolled normal variations in Part IV, 2,
above.
For text Figures 28 and 29 (pages 85 and 89, respectively),
stage was estimated in the same way but inputs of water were either
added to lake storage or discharged, depending on the state.of the
system and the decisions imputed to the managers.
266
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