The spatial variability of runoff and precipitation in the Rio de la

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The spatial variability of runoff and precipitation in the Rio de la
Hydrological Sciences -Journal- des Sciences Hydrologiques, 41(3) June 1996
279
The spatial variability of runoff and
precipitation in the Rio de la Plata basin
N. O. GARCIA
Facultad de Ingenieria y Ciencias Hi'dricas, Universidad National del Litoral,
CC 495, 3000 Santa Fe, Argentina
W. M. VARGAS
Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires,
Departamento de Ciencias de la Atmôsfera (CONICET), Ciudad Vniversitaria
Pabellôn 2, 1428 Buenos Aires, Argentina
Abstract A few aspects of spatial variability, connected with climate
variability, in the Rio de la Plata basin are analysed through changes in
the basin runoff considered as indicative of the annual integrated precipitation. Those changes contain better information on climate variability
than the sparse records of precipitation and temperature in view of the
high spatial and temporal variability of those parameters. The precipitation data are applied in the most active zones of the basin, however, in
order to assess aspects of hydrological variability that the lack of data
causes in the studied region. The hydrological series employed correspond to average monthly and yearly flows at stations selected along the
main rivers, both due to stability in their gauging stations and to the
availability and quality of data for the 1931-1992 period. This period
includes the longest series of simultaneous observations for the stations
selected. Similar approaches were applied to the series of precipitation.
Through basic statistical analysis, and from the annual evolution of flows
and precipitation, the study of the relationship between rainfalls in the
area and flows at separate hydrological stations (taking also the annual
cycle of precipitation and flows into account), nine hydrological areas
were detected. From those areas, well separated from each other, the
spatial variability for the Rio de la Plata basin was characterized.
Variabilité spatiale des débits et des précipitations dans le
bassin du Rio de la Plata
Résumé Quelques aspects de la variabilité spatiale dans le bassin du Rio
de la Plata, en liaison avec la variabilité climatique, ont été analysés à
partir de modifications du débit des rivières considéré comme représentatif
de la précipitation annuelle. Ces modifications de débit apportent davantage
d'information sur la variabilité climatique que les rares données de précipitation et de température, à cause de la grande variabilité de ces grandeurs
dans l'espace et dans le temps. Les précipitations ont toutefois été utilisées
dans la région la plus active du bassin, mais il s'agissait seulement
d'apprécier certains aspects de la variabilité hydrologique en raison du
manque de données dans la région étudiée. Les séries hydrologiques utilisées sont les moyennes mensuelles et annuelles des stations sélectionnées
sur les principales rivières en tenant compte de leur stabilité, de la
disponibilité et la qualité des données pour la période 1931-1992. C'est sur
cette période que l'on possède les plus longues séries d'observations
simultanées pour les stations sélectionnées. Des critères de sélection
analogues ont été appliqués aux séries de précipitations. Grâce à une
Open for discussion until 1 December 1996
N. O. Garcia & W. M. Vargas
280
analyse statistique de base, et à partir de l'évolution annuelle des
écoulements et des précipitations, l'étude de la relation entre les
précipitations et les écoulements des différentes stations hydrologiques de
la région (compte tenu du cycle annuel des précipitations et des écoulements), a permis de mettre en évidence neuf régions hydroclimatiques. Ces
régions bien différenciées caractérisent la variabilité spatiale du bassin du
Rio de la Plata.
INTRODUCTION
The Parana River (Fig. 1) is the most important component of the "del Plata"
system due to the magnitude of its flows, the size of the area covered by its
tributaries, its length and the different dimensional features characterizing a
hydrological basin. Even when including the Paraguay River to create a single
basin, integrated to form the Parana system, from the hydrological point of
view it is appropriate to analyse the rivers separately. With such a purpose, the
gauging stations at Posadas (Parana River), Puerto Bermejo (Paraguay River)
and Corrientes (both rivers) become significant.
The aim of this study was to investigate the climatological variability in
the area, documenting it through its runoff and precipitation. Flow records
have previously been successfully employed in forecast-oriented studies in some
other hydrological basins (Hastenrath, 1990; Silber & Vanlesberg, 1993). In
the "del Plata" basin, however, they have not yet been employed even though
Fig. 1 Geographical location of the Rio de la Plata basin in South America.
Runoff and precipitation in the Rio de la Plata basin
the relationships are likely to be more stable due to the nature of the basin in
acting as time-space filter for precipitation.
The fluvial system of the Rio de la Plata, one of the largest in the world
(Fig. 1), extends over an area of approximately 3 100 000 km2, covering
territories in Argentina, Bolivia, Brazil, Paraguay and Uruguay. Its limit
coordinates range from longitude 67°00' to 43°35'W and latitude 14°05' to
37°37'S extending from the Bolivian Altiplano and the Chapada de Parecis in
the Brazilian Planalto to its confluence with the Atlantic Ocean.
The main portion of this large drainage area is in Brazil, where it covers
1 415 000 km2 with 920 000 km2 in Argentina, 410 000 km2 in Paraguay,
205 000 km2 in Bolivia and 150 000 km2 in Uruguay.
Fig. 2 The hydrological system with its three main units.
281
282
N. O. Garcia & W. M. Vargas
Within the basin, three large hydrological units can be differentiated, that
of the Parana River, that of the Paraguay River and that corresponding to the
Uruguay River (Fig. 2). The former and the latter constitute the Rio de la
Plata, while the Paraguay River outflows into the Parana River, some few
kilometres north of the city of Corrientes.
These units individually cover very different geographical areas:
Upper Parana River basin
934 000 km2
Lower Parana River basin
576 000 km2
Paraguay River basin
1 095 000 km2
Uruguay River basin
365 000 km2
Rio de la Plata basin proper
130 000 km2
Rio de la Plata total basin
3 100 000 km2
Figure 2 shows the gauging stations used in this study with pluviométrie
stations assigned a • beside a number, and the gauging stations indicated by
a • beside a number in a circle.
DATA AND METHODOLOGY
Differences among hydrological statistical structures have been analysed for the
1931-1992 period as this is the longest period with simultaneous observations.
Variations in discharge from one year to another were considered. Monthly
series were considered to analyse the annual cycle of river discharge.
It is worth mentioning that, in the basin, there are more than 60 dams
each with more than 1 km3 volume, and with a total retention capacity of
280 km3. (Comision Mixta Paraguayo-Brasileha, 1974). Their effect, however,
is almost negligible on mean monthly flows for floods, being within gauging
errors downstream, even with the retention they may produce. They have no
effect on mean yearly flows (Garcia et al., 1992). This behaviour is due to the
characteristics both of topography and of the dams themselves (which are
mainly intended to generate power), located one after the other on the more
important rivers running from east to west (Grande, Tieté, Paranâpanema,
Iguazu, etc.), exceptions being Itaipu and Yaciretâ on the Parana River and
Salto Grande on the Uruguay River.
From the time closures were introduced by the dams, daily flow values
were corrected at these sites using the following relationship:
Natural flow = Discharge + Volume difference
It is obvious then that the slight effect that dams have on monthly and yearly
average flows is due to the characteristics of infrastructure works such as
run-of-the-river powerplants with small regulating capacity (Garcia & Vargas,
1994). This means that average values may vary from year to year as a function of variability in precipitation, conditions of vegetation and soil, etc. The
large dams are not used to enhance irrigation.
Runoff and precipitation in the Rio de la Plata basin
283
Selection of the flow series
Most of the gauging stations installed in the Rio de la Plata basin have as their
main objective the obtaining of statistics on flows, since they give information
of great importance in the programmes of development and use of water
resources.
In order to assess this information, for its proposed use, it was necessary
to adopt or define some norm. In this work the following arbitrary conditions
were adopted:
A
well controlled station, without missing data, good stage/discharge curve,
reliable readings or existence of limnigraph, or existence of a dam; and
B
less well controlled station that could need some small data adjustments
or have some small periods of missing data, hopefully with an error not
greater than the 10% in the monthly mean flows.
The selection process for the series of discharges from those available
was based on the stability of gauging cross-sections as well as on the reliability
evidenced by coherence and consistency tests viz. Kolmogorov-Simirnov's test
(Yevjevich, 1972) and accumulated flows test (Remenieras, 1974), when the
series were individually treated. Double-mass curves and double accumulated
flows (Remenieras, 1974), displaced in time, were applied to those stations
consecutively located on the same river. Finally, those stations having at least
60-year long records were selected. The same requirements were placed on
other climatic elements employed in the study.
Basically, measurements of river discharge were employed since they
reflect spatial integration of meteorological data thus providing more significant
information on climatic variability, and because this information is less affected
by the local time at each station.
For the selection of precipitation series a similar approach was adopted,
always observing the recommendations of the World Meteorological Organization (Buishand, 1982; WMO, 1986; Conrad & Pollak, 1950; Brooks &
Carruthers, 1953).
Here it is necessary to emphasize that an extensive region exists at the
west side of the sub-basin of the Paraguay River (west from Paraguay and the
whole Bolivian territory) that has few stations with series that fulfil the
demands imposed on the data used in this study. The Argentinean gauging
stations on the Pilcomayo River, the Bermejo River and the Salado River are
not good, and have not been used in this paper. There are several Argentinean
meteorological stations whose series are very good, but they have not been
used in this paper because the region is not hydraulically active and its
precipitation does not affect the flows of the hydrographie system significantly.
Methods of analysis
First, standard statistics (WMO, 1966A; Chou, 1977) were computed with the
284
N. O. Garcia & W. M. Vargas
flow series obtained at the following stations: Jupiâ ("1), Guairâ (»2), Itaipu
(•3), Posadas ("4) and Corrientes ("5) on the Parana River; Rosana ("6) on the
Paranâpanema River; Puerto Bermejo ("7) on the Paraguay River; Santo Tome
(•8), Paso de los Libres (>9) and Monte Caseros ("10) on the Uruguay River;
and Capanema ("12) on the Iguazu River, upstream from the Iguazii Falls
(Fig. 2).
In addition, a short series (less than 20 years) of flows was employed
from a station that, due to its location and to the quality of its records, was
highly useful to study river behaviour. This station is San José ("11) on the
Parana River, downstream from the confluence of the Paranâpanema River.
The degree of randomness in the series was assessed by the accumulated
periodogram method (Anderson, 1977). To assess spatial variability the annual
evolution was analysed for each hydrometric station (WMO, 1966B; WMO,
1988). The annual cycles corresponding to every station located along the main
channel of the Parana and Uruguay Rivers as well as the stations corresponding
to the main tributaries for which data were available were also compared. This
was complemented with a study of the annual evolution by harmonic analysis
(Panofski & Brier, 1968) to be completely assured of the results on the regime
of the rivers in the area. The harmonic analysis aimed to explain the variance
corresponding to each sub-period of the basic period, viz. a year. The second,
third, fourth, fifth and sixth harmonics correspond to sub-periods of 6, 4, 3,
2.4 and 2 months respectively.
RESULIS
First, after considering the coherence of the series employed, it was verified
that the series follow a white noise process, i.e. a process where the members
of the series are independent of each other (Anderson, 1977).
Taking into account that the main objective of this work was to advance
toward an understanding of the climatological variability in space in the Rio de
Table 1 Basic statistics for discharges in the Rio de la Plata Basin for 1931-1992
River/Station
Drainage
area
(km2)
Annual
discharge
(m3 s"1)
Standard
deviation
(m3 s'1)
Skewness
Paranâ/Jupiâ
Paranâ/Guairâ
Paranâ/Itaipû
Paranâ/Posadas
Paranâ/Corrientes
Paranâpanema/Rosana
Iguazû/Capanema
Paraguay/Pto. Bermejo
Uruguay /Santo Tome
Uruguay/Paso de los Libres
Uruguay/Monte Caseros
477 885
802 850
830 580
933 360
2 051 720
100 120
64 430
1 095 000
123 120
191 800
217 360
6125
9275
9563
12329
17037
1077
1313
3893
2468
4058
4558
1490
2618
2573
3073
4467
565
483
1319
963
1491
1787
1.292
2.100
1.944
2.321
1.819
3.526
0.483
0.245
0.368
0.600
0.742
Kurtosis
8.467
12.050
10.702
13.562
10.554
20.352
3.174
2.589
2.727
3.023
4.337
Standard
error
(m3 s"1)
189.230
332.486
326.771
387.162
567.310
71.755
62.319
166.178
125.372
187.848
234.645
Runoff and précipitation in the Rio de la Plata basin
285
la Plata basin by means of its discharges, the values for the standard statistics
shown in Table 1 were initially computed to assess an initial reference for the
1931-1992 period. The most significant values in Table 1 are in the skewness,
since that parameter is associated with the extremes of the series. Note that
they indicate highly variable maximum discharges, since the kurtosis results
indicate that there is a skewness to the right. This is especially evident in rivers
coming from the east in the upper basin of the Parana River, such as the
Paranâpanema and Iguazii Rivers, which show a high incidence for stations
located immediately downstream from its confluence with the Parana River.
Converting the values of Table 1 into values per unit of area and constructing bar-diagrams showed more clearly the differences between the
different rivers or between parts of a given river. Indeed, Figures 3(a) and 3(b)
show what cannot easily be perceived in Table 1 viz. a great similarity in the
behaviour of the Parana River up to Posadas. The Corrientes gauging station
H Annual Discharge [
|SD
I 0,025 —
to
w
0,02
a
•
V)
3 0,015
E
j j 0,01
g
|
0,005
FTHIF T,
Jupla
Itatpu
Qualra
(b)
2.1E-04
1 !1
|• 1 ,• 1 j• 1
Contantes
Capanama
P.Ubres
Sto.Tome
Posadas
Rosana
P.Betmejo
M.Caseros
2 J Skewness •
Kurtosis
1.8E-04
•O 1.2E-04
S 9,0E-O5 -c
|
6.0E-05-
-
3.0E-O5
•
I
i, -jj
J
, - c J L „ i , M I -L^JLj-^i^JLi 1.-1—I,
Jupla
Italpu
Contentes
Capanema
P.Libres
Sto.Toms
Guaira
Posadas
Rosana
P.BenneJo
M.Caseros
Gauging Station
Fig. 3 (a) Annual discharge and standard deviation; and (b) skewness and
kurtosis of each gauging station.
N. O. Garcia & W. M. Vargas
286
shows the effect which the Paraguay River produces over the Parana River. In
the Uruguay River one type of behaviour up to Santo Tome may be observed,
and a slightly different one downstream from that station down to the outlet.
These particularities may be observed clearly from the skewness and the kurtosis while being not so notable via the discharges and the standard deviation.
The structures of the bar-diagrams also manifest the differences of behaviour that exist between the different courses of water that run from east to
west, like the upper Uruguay River, the Iguazu River and the Paranâpanema
River. These notable differences in the behaviour of the rivers justify a hydroclimatic classification of the region. Synthetically, the structures of the bardiagrams are similar along each river, but they are different between them.
The annual cycle of the Parana River
This river, starting at the confluence of the Grande and Paranâpaiba Rivers,
runs along the whole basin in a general north to south direction to a point
shortly before Posadas, and taking a north to south direction from Corrientes.
From Posadas to Corrientes, it runs in an east to west direction. The river was
studied at each of the selected stations, analysing for each one the annual
evolution for the 1931-1992 period.
Figure 4 shows the behaviour of the Parana River for the selected series
of discharges. At all stations, February and March appear as months corresponding to flood periods, with August and September being low-water periods.
At Posadas and Corrientes, a slight trend towards increasing the monthly
average flow in June and October can be observed. This, however, can only
be attributed to regimes in the riversflowinginto the Parana River immediately
upstream from such stations, viz. the Iguazû River between Itaipii and Posadas
25000
JAN
MAR
FEB
-m- Corrientes + Posadas
MAY
JUL
SET
NOV
JUN
AUG
OCT
DEC
Months
-&- Itaipû
-a- Guairâ
- » Jupiâ
APR
Fig. 4 Monthly average flows for the Parana River during 1931-1992.
Runoff and precipitation in the Rio de la Plata basin
287
and the Paraguay River between Posadas and Corrientes. Consequently, it may
be inferred that the Parana River behaviour is homogeneous over its whole
length and each station shows the essential features of the annual evolution in
spite of the small differences manifested in the bar-diagrams.
Annual cycles of the tributaries
The tributaries of the Parana River show highly differentiated characteristics
among themselves, evident from the magnitude of their discharges (Fig. 5). It
is worth mentioning that the Paraguay River (running in a north to south
direction) differs from the rest of the tributaries due to the magnitude of its
discharges. This river, even though its origins are at the same latitude as those
of the Parana River, receives smaller rainfalls (Hoffmann, 1975). Hence, it
shows considerably smaller flows even when its drainage basin area is comparable with the one corresponding to the Alto Parana (upstream from the
confluence with the Paraguay River).
5000 j -
4000 - -
g 3000S
E2000 - -
1000 -
0 J—i
1
JAN
1
1
MAR
FEB
1
MAY
1
1
JUL
1
i
SET
1
1
1 — '
NOV
APR
JUN
AUG
OCT
DEC
Months
- - Pto.Bermejo (Paraguay River) — Rosana(Paranapanema River)
Capanema(lguazu River)
Fig. 5 Monthly average flows for the tributaries of the Parana River during
1931-1992.
The Paraguay River regime is highly regular and shows high waters in
June, with well defined low water periods ranging from September to January.
The cross-section at Puerto Bermejo, some few kilometres upstream from its
confluence, is, according to a study by Motor Columbus and Associates (1979),
a more adequate section to perform such measurements because both shore
lines are not well defined along the river upstream from Asuncion. Over its
upper basin there is a large area - the so-called El Pantanal - with low lands,
exceeding 100 000 km2, acting as a regulating discharge area. This means that,
288
N. O. Garcia & W. M. Vargas
even if there are no records of discharges in that area, it can be stated that at
the outflow of the area, the river shows a well-regulated regime and that rainfalls on its lower basin are responsible for the extraordinary out-of-season
floods produced. For this reason, when analysing the spatial variability in this
basin, comprising 1 095 000 km2, it is necessary to use records of precipitation, comparing mainly stations at Câceres (1) at the northern end of El
Pantanal; Corumbâ (2) at the southern end but within the area; Puerto Casado
(3) in El Pantanal; and Asuncion, almost at the southern end of the sub-basin
(Fig. 6).
FEB
APR
JUN
AUG
OCT
DEC
Months
•-«r Câceres
-»• Corumbâ — P. Casado -•- Asuncion
Fig. 6 Annual evolution for precipitation at Câceres, Corumbâ, Puerto
Casado and Asuncion.
At the two pluviométrie stations in the north (Câceres and Corumbâ) a
well defined annual evolution can be observed, associated with sun declination,
with a maximum in January and a minimum in July. Puerto Casado and
Asuncion show a minimum in August, but with monthly precipitation values
slightly larger than those observed during the dry season at Câceres and
Corumbâ. Maximum values in the southern stations correspond to a rainfall
season extending from October to April, with two maximum values slightly
different: in March and November at Puerto Casado, while at Asuncion, they
appear in January and March.
The amount of monthly precipitation decreases appreciably from north
to south up to Puerto Casado, with a small increase thereafter up to Asuncion,
during the summer months, while during the remaining part of the year (from
April to September) the decrease is produced from south to north. The
out-of-season floods of the Paraguay River are generally associated with
rainfalls in September, October and November over the lower basin.
Another of the tributaries that, as previously noted, appear to be different
Runoff and precipitation in the Rio de la Plata basin
289
from the flows of the Parana River, is the Iguazu River which, running in an
east to west direction, originates in the hills near Florianopolis, joining with the
Parana River some 250 km upstream from Posadas, after generating the Iguazu
Falls. The discharge record from the Iguazu River is long enough to understand
disturbances produced by the variability of its discharges into the Parana River
at Posadas. Thus, the Posadas station shows an annual evolution not strictly
following the behaviour observed upstream from Itaipu, viz. during June.
The annual evolution of the Capanema flows, in the period under consideration, shows great variability, with a well-defined low water season from
January to April, a major maximum from May to July, and a secondary one
in November. This behaviour will be further discussed when precipitation in
relation to discharge will be considered.
The Paranâpanema River is the third tributary to be analysed in this
study through measurements at the Rosana gauging station, near its confluence
with the Parana River. Its intra-annual variability, during the 1931-1992 period,
is shown in Fig. 7. This river has a highly irregular regime, with floods in
February, July and October (in decreasing order of importance) and shorter low
water periods in April, August and November. These smaller basins (the
Iguazu and Paranâpanema Rivers) show that winterfloodsreflect the ability of
cold fronts to reach lower latitudes.
JAN
MAR
FEB
MAY
APR
JUL
JUN
SET
AUG
NOV
OCT
DEC
Months
Fig. 7 Monthly average flows in the Paranâpanema River at Rosana for
1931-1992.
Even when considering previous information, these rivers by themselves
would not be responsible for the extraordinary floods in the Parana River. One
has to infer that rivers coming from the eastern portion of the Parana River
upper basin maintain quite irregular regimes and that their flows are clearly
smaller, even in the Paraguay River, though they may show appreciable floods
with short duration in any season of the year.
290
N. O. Garcia & W. M. Vargas
The annual cycle in the Uruguay River
Having a basin of 365 000 km2 and an historical average flow of approximately
4500 m3 s"1, the Uruguay River is the second largest in the Rio de la Plata
basin. It is the only important river that does not have a direct relationship with
the Parana River, its only coincidence being its confluence jointly with the
Parana River into the Rio de la Plata basin (Fig. 2). The Uruguay River has
its headwaters at Sierra do Mar and Sierra Geral, near the Atlantic Ocean. Its
upper basin, upstream from the Aguapey River (near Santo Tome gauging
station) receives annual rainfalls ranging from 1400 to 2400 mm, which
decrease to 1000 mm at its confluence. These irregular rainfalls are reflected
in the annual regime of flows in the three hydrological stations selected for this
river (Fig. 8) which show similarity. During the period considered, maximum
flows were observed ending in October with an important plateau between May
and September. Moreover, at the Santo Tome gauging station the maximum
flows were observed during September and October while the plateau was from
May to August. A small difference between Santo Tome gauging station and
the downstream gauging stations can be seen. The low water period of this
river is during the summer months from December to March. In this basin, the
tributaries coming from the east are also the most important since the inflows
from the west are almost negligible, especially in its middle and lower part.
The Cuareim River affects flows slightly in the Uruguay River at Monte
Caseros, similarly the Ibicuy River at Paso de los Libres while flows at Santo
Tome provide evidence of the irregularity of the upper basin. The small
incidence of the flood effects is due to their short duration in these small
basins.
7000 -r
5800 -
J4600 -
g
S
§3400 -
2200-
1000
1
1
JAN
1
1
1
MAR
FEB
1
MAY
APR
1
1
JUL
JUN
1
,
SET
AUG
1
1—<
NOV
OCT
DEC
Months
—- SANTO TOME
- *- PASO DE LOS LIBRES ^ M O N T E CASEROS
Fig. 8 Monthly average flows of the Uruguay River at Santo Tome, Monte
Caseros and Paso de los Libres stations for 1931-1992.
Runoff and precipitation in the Rio de la Plata basin
291
Annual cycles as displayed by harmonic analysis
As previously explained, to complete the study of the monthly regime in the
region, the annual evolution was studied by means of harmonic analysis for
each gauging station. These results, summarized in Table 2, include up to the
third harmonics only, which accounts for more than 95% of the variance. The
harmonic analysis of the annual evolution at each gauging section shows that
flows both in the Parana River and in the Paraguay River are coherent, being
definitely affected by an astronomical component with an annual period caused
by the changes in sun declination. In the main channels of these rivers, the first
harmonic explains more than 90% of the variance, even in the case of the
Paraguay River, whose annual wave is displaced because of the reasons previously mentioned. The amplitude of the second harmonic decreases from north
to south, being more appreciable at Jupiâ. This harmonic probably represents
the double culmination of sun declination and variability of rainfalls at a
synoptic scale. The third harmonics in the flow series of the Parana River are
almost negligible. Only those stations having major tributaries upstream show
significant explained variance for this harmonic, which is due to the variability
in inflows. The remaining harmonics are not significant and the explained
variance is caused by noise in the data series.
Table 2 Explained variance for the first three harmonics for the annual evolution of flows
for 1931-1992
Station/River
First harmonic
Second harmonic
Third harmonic
Jupiâ/Paranâ
Guairâ/Paranâ
Itaipû/Paranâ
Posadas/Paranâ
Corrientes/Paranâ
90.93
92.65
93.04
89.50
91.53
8.50
5.72
5.32
4.10
1.24
0.38
1.42
1.47
6.05
6.47
Pto. Bermejo/Paraguay
Rosana/Paranâpanema
Capanema/Iguazû
91.53
55.95
55.16
5.91
16.98
9.83
2.08
24.63
34.19
Santo Tomé/Uruguay
P. de los Libres/Uruguay
Monte Caseros/Uruguay
86.79
86.05
89.19
5.60
7.63
10.49
7.21
5.33
5.38
The Uruguay River also shows an important annual evolution accounting
for 86-89% of the variance, while the remaining portion is distributed between
the second and third harmonics. The harmonics corresponding to periods
smaller than four months (third harmonic) show a negligible contribution to
explaining the variance. The high synoptic variability in these areas, with
frequent fronts and cyclogenesis, may be related with four and six months
periodicity. Topography and basin location, with headwaters near the Atlantic
area, are most significant aspects in this respect.
292
N. O. Garcia & W. M. Vargas
The behaviour of the Paraguay River, as related to the first harmonic,
has been discussed already with the Parana River. The explained variance
decreases sharply with the second harmonic. The third harmonic along with
those corresponding to the fourth, fifth and sixth harmonics can be considered
as caused by noise in the series.
The tributaries of the Parana River coming from the east show similarities with the upper Uruguay River, showing a variability that accounts for
55 % of the variance in the first harmonic, especially the Iguazu River where
the second and third harmonics have significant contributions (Table 3). The
small values of variance explained by harmonics with higher than third order
are also caused by noise in the series. The Paranâpanema River also shows a
high incidence of a six-month period, and also one of four-month period,
explaining 25% of the variance. In both rivers, as probably occurring with
those coming from the east, the synoptic variability seems to have a definite
influence. For these tributaries, comments similar to those made for the
Uruguay River can be made, but showing a higher incidence of topographic
characteristics, synoptic variability and origin near the Atlantic Ocean.
Table 3 Percentage of variance for annual evolution at Rosana
(Paranâpanema River) and Capanema (Iguazû River)
Station
Rosana
Capanema
55.95
16.98
24.63
51.95
16.98
7.45
Harmonic
First
Second
Third
A similar analysis was made with the annual evolution of the
precipitation obtained from the complete series, at the selected stations in the
basin (Fig. 2): Câceres (•!.), Corumbâ (*2), Puerto Casado (*3), Asuncion
(•4), Resistencia (*5), Corrientes («6), Posadas (*7), Parana («8), Buenos
Aires («9), Concordia (»10), Près. Murtinho («11), Campo Grande (»12),
Foz de Iguazû (»13), Cruz Alta (»14), Bagé («15), Aracatuba (»16),
Jaguariaiva («H), Valoes («18) , Vacaria («19), Guaiania («20), Monte
Alegre («21), Ribeirao Preto (»22), Campinas (»23), Formosa (»24), Patos
do Minas (»25) and Caxambu (•26).
Even when a clear view of spatial variability is acquired with the study
of discharges, that view is confirmed with the variance explained by the
harmonics of the monthly data of precipitation in the Rio de la Plata basin
(Table 4). The variability of the precipitation at all the stations is similar in the
upper Paraguay River basin where the first harmonic explains more than 94%
of the variance, while in the lower basin, it explains 80-82%, the second
harmonic accounting for 13-19% of the variance. This obviously indicates that
within the Paraguay River basin, which could be considered as a single hydro-
Runoff and precipitation in the Rio de la Plata basin
293
climatic unit by itself, there is a need to differentiate clearly between the upper
and lower basin. Both individually constitute sub-regions showing hydrological
differences and, hence, differences in climatic behaviour too. A similar
behaviour can be observed in the upper basin of the Parana River. Several subregions can be identified which are also clearly perceived in the rainfall regime.
Stations located downstream from some major tributaries are disturbed by these
regimes (in Posadas and Corrientes, for example). The spatial rainfall variability affects differently those tributaries inflowing from the left margin of the
river.
Table 4 Percentage of variance for the six harmonics in the annual evolution in the
complete precipitation series up to 1988
Percentage of variance
Station/basin
First
harmonie
Second
harmonie
Third
harmonie
Fourth
harmonie
Fifth
harmonie
Careres/Paraguay
Près. Murtinho/Paraguay
Campo Grande/Paraguay
Corumba/Paraguay
Pto. Casado/Paraguay
Asuncion/Paraguay
94.31
97.51
97.19
97.37
81.80
80.48
2.61
0.02
0.03
1.13
12.97
17.01
0.55
1.23
1.77
1.13
3.47
0.34
1.54
0.25
0.96
0.06
0.65
1.44
0.18
0.56
0.04
0.06
1.11
0.45
0.81
0.43
0.01
0.24
0.00
0.27
Goiania/Paranâ
Formosa/Parana
Monte Alegre/Paranâ
Patos do Minas/Paranâ
Ribeirao Preto/Paranâ
Aracatuba/Paranâ
Caxambu/Paranâ
Campinas/Paranâ
Jaguariaiva/Paranâ
Valoes/Paranâ
Foz do Iguazû/Paranâ
Posadas/Paranâ
Corrientes/Paranâ
Resistencia/Paranâ
Paranâ/Paranâ
96.99
91.35
97.13
94.33
94.12
94.33
93.26
93.04
76.87
47.44
30.25
22.36
78.06
78.67
83.94
0.55
1.91
0.48
3.35
3.79
1.93
4.28
4.99
5.19
7.10
16.99
59.66
17.01
13.91
8.13
2.20
4.86
3.52
1.46
0.00
1.55
0.07
0.86
12.87
37.04
22.62
12.37
0.70
4.17
1.75
0.10
1.52
1.15
0.76
0.80
1.66
0.06
0.63
2.60
4.81
26.85
1.99
3.60
1.23
3.59
0.14
0.36
0.71
0.11
1.29
0.32
1.49
0.48
1.31
3.26
2.32
3.11
0.52
1.12
1.76
0.02
0.00
0.02
0.00
0.00
0.22
0.05
0.00
1.16
0.34
0.98
0.51
0.11
0.91
0.83
Vacaria/Uraguay
Cruz Alta/Uraguay
Bagé/Uruguay
Concordia/Uruguay
12.03
8.75
28.94
59.33
21.56
16.25
18.42
32.00
46.93
46.42
38.47
0.87
12.07
21.58
8.61
6.88
0.11
1.37
2.89
0.68
7.29
5.63
2.67
0.24
Buenos Aires/R. de la Plata 52.73
32.58
1.91
5.85
4.78
2.15
Sixth
harmonie
The upper basin of the Parana River, north from the Paranâpanema River
basin, shows high homogeneity since the first harmonic explains more than
93% of the variance. Thus, one may conclude that this region has similar
hydroclimatological behaviour in all its constituent hydrographical units. The
latter could be analysed graphically through figures that relate monthly mean
flows of the output of a hydrographie unit vs monthly mean precipitation at
selected points of the same unit (Figs 9 and 10). Those figures show the
characteristics of the annual wave of the precipitations and flows clearly, and
294
N. O. Garcia & W, M. Vargas
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Fig. 9 Rainfall upstream from the Grande River vs discharge at Jupiâ in the
upper basin of the Parana River.
- Formosa
— Goiania
Monte Alegre
- - Patos do Minas
Fig, 10 Rainfall at the Grande River basin vs discharge at Jupiâ in the
upper basin of the Parana River.
each point in the figures is the result of the relationships between both variables
for each month. If the figures have similar structure, then the hydroclimatological behaviour of the hydrographical units is similar in all their regions,
although they could have any particular manifestation in any of their parts.
Figures 9 and 10 show this fact as rainfall vs discharges. Both figures
show that this relationship holds its structure almost constant, quantitatively and
qualitatively. For this reason, it could be considered a single hydroclimatic
unit, independent and homogeneous in the regime of the region. Moreover, it
is possible to see little difference between the relationships, and so it is then
possible to have a better regionalization if it is possible to use more time series
Runoff and precipitation in the Rio de la Plata basin
295
data of flow and precipitation. With the records available for this work,
however, it is reasonable to assume that the region north from Jupiâ is
hydraulically homogeneous within the upper basin of the Parana River, totally
agreeing with the results shown in Table 2 for discharges.
The Paranâpanema River basin shows in the rainfall at Jaguariaiva the
same variability observed in its flows. Here, the first, second and third
harmonics of the annual curve and the monthly data of precipitation in the Rio
de la Plata basin (Table 4) account for approximately 95 % of the variance. This
basin is a hydroclimatic region in itself. A similar result can be observed in the
Iguazû River basin, in explaining the variance for Valoes with those of
discharges at Capanema. Hence, it can also be considered as another unique
region within the upper basin of the Parana River.
The remaining pluviométrie stations in the upper basin of the Parana
River (Foz de Iguazû, Posadas and Corrientes) cannot be related with
discharges but do show a high variability in their regimes, especially the first
ones. For this reason, the region remaining to be considered in the upper basin
of the Parana River could be classified as another hydroclimatic sub-region.
Pluviométrie data in the middle and lower basins of the Parana River,
downstream from Itaipu, cannot be related with discharges, but can be applied
to regionalization. However for the regionalization of the territory to the south
of the Pilcomayo River basin and to the west of the Parana River is necessary
to incorporate the study of new climatological variables. This region, that
occupies a third of the total surface of the Rio de la Plata basin, contributes
less than 5 % (500 m3 s"1) of the waters that this hydrographie system yields to
the Atlantic Ocean. It can therefore be stated that this region also constitutes
a hydroclimatic region (without specifying details).
Finally, when considering pluviométrie data in the stations of the upper
and lower basins of the Uruguay River, it can be observed that the third
harmonic is the one that better explains the variance (with percentages near
40%), the remaining portion being distributed among the five remaining
harmonics. In this basin the variability of rainfall decreases from north to south
but is uniformly related to discharges (Figs 11(a), 11(b) and 11(c)). Here there
are common features in the three figures, but there is a better relationship
between Figs 5(a) and 5(b), than between those two and Fig. 5(c).
According to the facts previously explained, it is possible to define two
regions with individual hydroclimatological characteristics in the Parana River
basin, two in the Paraguay River basin and two in the Uruguay River basin.
Another two regions, by themselves, are the Iguazû River basin and the
Paranâpanema River basin.
Figure 12 shows the regions defined in this study with their limits
defined by surface water-dividing boundaries. The regions defined are: (1) the
upper basin of the Parana River, north from Jupiâ; (2) the Paranâpanema River
basin; (3) the Iguazû River basin; (4) the Parana River basin from Jupiâ to
Corrientes; (5) the upper basin of the Paraguay River, north from Corumbâ;
(6) the lower basin of the Paraguay River, from Corumbâ to its confluence;
296
N. O. Garcia & W. M. Vargas
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6000
6500
7000
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Fig. 11 Rainfall at (a) Vacarîa; (b) Cruz Alta; and (c) Bagé vs discharge at
Monte Caseros (Uruguay River basin).
(7) the upper basin of the Uruguay River, upstream from Santo Tome;
(8) the lower basin of the Uruguay River, downstream from Santo Tome; and
(9) the lower basin of the Parana River, from Corrientes to its confluence and
the western region at the south of the Pilcomayo River basin.
With the available data the justification of the regional distribution of the
basin of the Rio de la Plata could not be enlarged. One could repeat plots of
flow vs precipitation relationships for other gauging stations with several
pluviométrie stations of the same basin but they would probably only confirm
the results now shown.
Runoff and precipitation in the Rio de la Plata basin
70°
65»
( \ _ _ — 60°
55°
50°
297
45°
Fig. 12 Geographical distribution of the nine significant hydrological
regions, defined in the La Plata River.
As previously explained, it is highly probable that each basin of the
rivers outflowing into the upper basin of the Parana River through its left
margin (Grande, Tieté and some small rivers) constitute homogeneous
hydrological regions by themselves, which must be considered in the future.
A similar comment could be made for the Uruguay River basin, downstream
from Monte Caseros, or for the western region of the lower basin of the
Paraguay River and for the lower basin of the Parana River, even when the
western side of the latter two regions has no hydrological significance.
CONCLUSIONS
The basic statistics found in the series of discharges corresponding to the
selected stations give initial evidence on heterogeneity in the Rio de la Plata
298
N. O. Garcia & W. M. Vargas
basin. The unique importance of the Parana River in the basin is also indicated,
its average annual flow at Posadas being 12 200 m3 s"1 almost double that at
Jupiâ, with drainage areas following approximately the same relationship. None
of the tributaries located downstream from Jupiâ (Paranâpanema, Iguazii and
some small rivers) justify by themselves the difference found between Jupiâ and
Posadas stations. This makes evident the major role played by the small rivers
inflowing between the Paranâpanema and Iguazû Rivers, even when their
basins do not match that of the latter rivers (they jointly contribute an annual
average flow equal to 3000 m3 s"1).
The Paraguay River, with a basin comparable to that of the Parana
River, does not exceed an annual average of 4000 m3 s"1. The Parana at its
more upstream station (Jupiâ) exceeds 6000 m3 s'1. The Uruguay River, even
though belonging to the Rio de la Plata basin, but not being a tributary of the
Parana River, does show a little difference between its upper and lower basins.
At Santo Tome, it carries 2500 m3 s"1, increasing to 4520 m3 s"1 at Monte
Caseros, with a drainage area approximately double that at Santo Tome.
Taking into account the facts previously given, it may be concluded that,
as far as the average annual flows are concerned, three major hydro-climatic
regions can be identified. Disregarding the homogeneity in behaviour of the
Parana River in its main channel at its upper basin, there exists a nonhomogeneous component in its tributaries, with monthly discharge regimes at
each tributary considered, which confirms that, at least, the basins of the
Paranâpanema and Iguazu Rivers are to be considered as individual hydroclimatological regions. The latter results lead to the assumption that major
tributaries like the Paranâpaiba, Grande, Tieté and some small rivers would
constitute separate hydroclimatological units, different from those included here
if observational data were available. This conclusion is further confirmed when
studying the rainfall-discharge relationship, which shows a higher homogeneity
between the discharges at Jupiâ with rainfalls north from the Grande River
confluence than that produced between the Grande River Basin and that in
Tieté.
The analysis of the precipitation monthly regime of the Paraguay River
shows that this basin must be split into two regions: one characterized by the
presence of the El Pantanal and the other (lower basin) by the characteristics
of the annual evolution of rainfalls, whose intra-annual variability is quite
different between both basins. Discharges subjected to harmonic analysis, as
those made with the series of rainfalls, showed differences among regions of
the Parana River upper basin (two zones bounded by Jupiâ).
Precipitation in the Uruguay River basin hold similar spatial characteristics to those initially detected with discharges. Even though the high variability
of rainfalls is known, the analysis shows that precipitation stations by
themselves represent the hydrological regime.
Finally, it is inferred that when basins become excessively large, like
those of the Parana and Paraguay Rivers, it may not be enough to consider
discharges only if a good characterization of spatial variability is intended,
Runoff and precipitation in the Rio de la Plata basin
299
especially when a dense hydrological network is not available not only on main
rivers but on major tributaries as well.
Acknowledgments The authors are grateful to K. Arpe of the Max-Planck
Institute for Meteorology for his valuable suggestions and thank two
anonymous reviewers for their helpful comments on an earlier version of this
paper. This research was supported by the Secretary of Science and Technics
of the National Littoral University funding body (0435-0435-41-243) and
partially with the funding body of the Buenos Aires University (EX 274 UBA).
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Received 5 April 1995; accepted 17 November 1995

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