the hydrologic cycle, unidirectional charter of the dissolved salts and

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Soil Forming Factors and Processes from the Temperate Zone 11(2012) 49-56
THE HYDROLOGIC CYCLE, UNIDIRECTIONAL CHARTER OF THE
DISSOLVED SALTS AND SUSPENDED LOAD
a
Nicolae Floreaa, Valentina Coteţa,*
National Research and Development Institute for Soil Science, Agro-chemistry and Environment – ICPA
Bucharest, Bd. Marasti 61, sect.1, cod.011464, Bucharest,
Circuitul apei în natură, cărăuş unidirecţional de săruri dizolvate şi aluviuni în suspensie
Abstract
In this paper it is underlined that the hydrologic cycle in nature, reversible and regenerating of fresh
water, carries out also an unidirectional and irreversible circulation – by means of a fragment of the hydrologic
cycle – of the dissolved salts and stream’s suspended load, entailed by the water drained from continents to
ocean.
The trend is to transfer soluble salts from land to ocean in the same time with the running water on
land in the portion of the hydrologic cycle which refers to the water transfer from continents to ocean in order
to equilibrate the annual water balance of the hydrologic cycle. But, one can realize here and there some local
salt accumulations in salt soils or in salt lakes within areas without drainage in arid climate; these salts
accumulations are cases of local hydrologic cycles „grafted” along the way of water on land (to ocean).
The energy necessary to the hydrologic cycle in nature is delivered by the Sun, and the entropy
remains at a low level as a consequence of the elimination in this cycle of water vapors with high entropy, and
of the receiving of liquid or solid water with low entropy, so that the annual level of entropy is maintained at a
low level.
©2012 Author(s) CC Attribution 3.0 Unsuported License.
Keywords: hydrologic cycle, soluble salts, stream’s suspended load, unidirectional and irreversible transport
1. INTRODUCTION
The continuous movement of water between hydrosphere and the other geospheres, well
known under the name of hydrologic cycle in nature, is of huge significance for all the processes that
take place at the Earth’s surface (crust), and for existence of the living beings, because this cycle
assures the permanent regeneration of the fresh water in nature.
Although on Terra there are over 1400 millions km3 of water (so that could cover all the
terrestrial globe with a layer of 3 m thick) only a tiny part of this amount is involved in the
hydrologic cycle in nature which supplies the atmosphere, rivers, lakes, soils and beings. The rest of
water is inaccessible, over 97% are found in the planetary ocean, and is salted, and near 2% are
under ice state, having a residence time of thousands years; about 0.7% is ground water with a
residence time of several hundred years in average (Brady & Weil, 2008; Montgomery, 1995; Pişota
& Zaharia, 2001; Pișota et al., 2005).
Corresponding author:
*e-mail address: [email protected]
Florea and Coteţ / The Hydrologic Cycle, Unidirectional Charter of the Dissolved Salts and Suspended Load
The hydrologic cycle fulfils, besides the continuous refreshing of the drinking water
reserves, and other essential processes such as actual unidirectional movement of some compounds
either dissolved in water or carried on in suspensions by water in its uninterrupted running towards
low relief areas; the dissolved salts arrive by such way in most part in the planetary ocean, where
they accumulate.
In general, these aspects are not discussed in relation with hydrologic cycle or are only
mentioned in short (Brady & Weil, 2008; Șerban et al., 1989), or it is mentioned that high amounts
of dissolved substances and compounds in suspension are transported by the hydrologic cycle
(Maidment, 1992). In the hydrochemistry works, even though it is clearly shown that hydrologic
network supplies with salts the ocean basins, however a close connexion with hydrologic cycle does
not made.
Approaching the problem of the global earth’s surface deposits, Grecu (2000) compares this
system with the water cycle in nature, and correlates them; indeed, considering the actual movement
of materials, this is oriented, the movement taking place together with running water, in the same
direction, from upstream to downstream. Yaalon (1967) attracts attention on geochemical cycle from
solusphere (soluble salts of hydrosphere).
In the following pages, it is attempted a completion of the describing of the hydrologic cycle
in nature with the unidirectional moving of the salts and fluvial materials, insisting on the first.
2. RESULTS AND DISCUSSION
2.1. Hydrologic cycle in nature, regeneration of the utilizable water resources
The hydrologic cycle in nature represents the cyclic movement of water from atmosphere to
earth (ocean and land) and back in atmosphere, involving different processes such as evaporation,
condensation and precipitation fall, interception, running on the land surface, infiltration and
percolation in soil, storage, and transpiration.
The energy which puts in action the hydrologic cycle is the solar energy. About a fifth part
from solar energy that arrives on earth (or even more) is absorbed by water from or near surface
causing evaporation, that is the liquid water conversion in watery vapors; these vapors rise in
atmosphere where form clouds which move from a region to other, and after a short time they
condense, coming back on earth as rain or snow, under action of law of gravity.
The hydrologists estimate that about 500,000 km3 of water are annually evaporated from
Terra’ surface (the amounts differ depending on authors, see Table 1) from which Eo = 430,000 km3
represents the water evaporated on ocean surface, and Eu = 70,000 km3 water evaporated on land
(continent surface). From these vapors quantities, about 390,000 km3 come back as precipitations on
oceans (Po) and about cca. 110,000 km3 as precipitations on continents (Pu) (see Figures 1 and 2).
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Florea and Coteţ / The Hydrologic Cycle, Unidirectional Charter of the Dissolved Salts and Suspended Load
Fig. 1 Hydrologic cycle in nature (by Brady and Weil, 2008)
About 110,000 km3 water fall on land, from which about 40,000 m3 water come back from land to the ocean
(as surface water – runoff – or ground water seepage); the processes that occurring on land areas where the
soils are influential have impact not only on human but on all other forms of life, including those residing in
the sea.
Because more water is evaporated from ocean surface than comes back as precipitations (Eo
> Po) and more precipitations fall on continents than evaporated water (Pu > Eu), the balance of the
hydrologic cycle in nature is equilibrated by the shifting of a water amount from continents to oceans
(Tuo), equal with about 40,000 km3, equivalent with the quantity of water vapors transported from
ocean on continents (Tou):
Eo + Eu = Po + Pu
but
Eo = Po + Tuo
and
Tuo = Pu – Eu
Tou = Eo – Po
These relations define the balance of water in the hydrologic global cycle, the quantities
being those presented in the Table 1.
Concerning the salts (and also the load with material in water suspension) one cannot speak
on reversible cycle with equilibrated balance. Indeed, even though Eo and Eu represents watery
vapors, practically pure, and also Po and Pu can be considered without salts (and other load) so that
they do not interfere in the balance of mentioned substances, however the Tuo – amount of water
transferred from continents to oceans plays a certain role in the circulation of the above mentioned
substances, which deserves to be made evident, more to be pointed out.
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Florea and Coteţ / The Hydrologic Cycle, Unidirectional Charter of the Dissolved Salts and Suspended Load
Elements
of the
cycle
(balance)1)
Eo
Po
Eu
Pu
Tuo
Et = Pt
Table 1. Amounts of water – in km3/year – participating in the water cycle on Terra
(according to various sources)
Authors
Pişota et
Baumgartner
Maidment
Brady
Dediu,
Hubbart Wikipedia
al.,
and Reichel,
et al.,
and Weil,
2010
2011
1990
2001,
2)
2008
1975
1992
2005
424,700
504,560
449,000
430,000
425,000
434,000
385,000
458,150
412,000
390,000
385,000
398,000
71,000
72,590
62,000
70,000
71,000
71,000
110,700
119,000
98,800
110,000
111,000
107,000
39,700
46,410
36,800
40,000
40,000
36,000
495,000
519,000
577,150
511,000
500,000
496,000
505,000
1)
Eo – annual average amount of evaporated water from planetary ocean;
Eu – annual average amount of evaporated water from land (continents);
Po – annual average amount of precipitations fallen on oceans surface;
Pu – annual average amount of precipitations fallen on continent surface;
Tuo – annual average amount of water flwed out from continents to oceans;
Et – annual average amount of evaporated water on earth;
Pt – annual average amount of precipitations fallen on earth.
Et = Pt = Eo + Eu = Po + Pu
Eo = Po + Tuo
and
Po = Eo – Tuo
Eu = Pu – Tuo
and
Pu = Ec + Tuo
2)
Quoted by Schram and Pantazică (1983), at their turn quoting J. Bethemont, 1977
2.2. The unidirectional movement of salts in nature, attached to hydrologic cycle
Through the agency of the shifted water from continents to ocean, the hydrologic cycle
fulfils in nature the function of unidirectional movement of the materials incorporated in water
during its route on land, and especially of the salts. Therefore, the hydrologic cycle in nature is in the
same time in one of its part (segment), between continents and oceans, an unidirectional carter of the
materials taken away from land, action that entails important changes of relief and water chemistry.
These changes are determined by the transport towards low areas both of the materials in suspension
often deposited as fluvial sediments, and of dissolved salts transferred in the last analysis in ocean
basins (or in lakes or low land where the salts can stay a period retarding the arrival in ocean). This
function of the mentioned segment of the hydrologic cycle produces the continuous enrichment of
the ocean water with salts.
The evaporated water (vapors) from soils and oceans and lakes, forming clouds, does not
content salts; it comes back on land or ocean practically also without salts (if one makes abstraction
of the very few amounts of salts taken from atmosphere). The water fallen or infiltrated on land
loads with salts and then forms the surface running water and ground water which at last arrive in
oceans and lakes together with dissolved salts which accumulates in the water of these water bodies.
Also the water accumulated in soil return in a great extent in atmosphere by vegetation
transpiration without salts, these ones remaining in soil and in some circumstances – lack of drainage
of the territory and arid climate – accumulate and determine the soil salinization.
Even thought the water movement forms an active cycle, recurrent and therefore redeemable
– the well known hydrologic cycle – the contemporary movement of salts associated to the
mentioned segment of the cycle does not implicate in this cycle. The salts move in one way
irreversible together with water from high to low part of the land relief (within the cycle segment
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Florea and Coteţ / The Hydrologic Cycle, Unidirectional Charter of the Dissolved Salts and Suspended Load
from continents to oceans). Therefore, in a part of hydrologic cycle, the water is the carrier of salts
and particles carried by water, which is the segment between land and ocean (Figure 2).
Making the water balance for ocean and for land, using the evaluation from the Figure 2 one
obtains for ocean:
390,000 + 40,000 – 430,000 = 0
and for continents:
110,000 – 70,000 – 40,000 = 0
so that the balance of water remains equilibrated in average.
The situation is changed for the salts balance. In this case, considering zero the salt
concentration of precipitations and watery vapours, the salt balance for ocean is:
390,000  o + 40,000  c – 430,000  o = 40,000  c
(c - being the salt content of transferred water from land to ocean), and the salt balance for continents
is:
110,000  o – 70,000  o – 40,000  c = – 40,000  c.
Fig. 2 Actual hydrologic cycle on earth, recurrent and reversibil and actual recurrent and unidirectional
movement of the in suspension particles and of the dissolved salts from land to low parts of relief and to ocean
The global
balance of
water:
Eo + Eu = 500,000 km3
Po + Pu = 500,000 km3
The differenciated balance
of water: on ocean
on land
Eo – Po = + 40,000 km3
Eu – Pu = – 40,000 km3
The unequilibrated balance of water between land and ocean is compensated trough transferring of watery
vapours from ocean to land (Tou) and transferring of water from land to ocean (Tuo). The transferring of water
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Florea and Coteţ / The Hydrologic Cycle, Unidirectional Charter of the Dissolved Salts and Suspended Load
from land to ocean (Tuo) is accompanied also of an unidirectional transferring of material towards lower land
parts or in water basins, as well as of salts which are passing continuously and ireversibil from land to ocean or
to low endorheic areas and lakes (can return on land only after geological epochs by orogenetic movements)
(Explanation of symbols in the Table 1).
It results therefore, an increase of the salts amount of ocean, annually, with a salt quantity
brought from the continents, especially by running waters. Indeed, the hydrographic network brings
yearly in ocean and lakes important amounts of dissolved substances and materials in suspension
(alluvia). The average salinity of the river water from former USSR is 111 mg/l, with variations
between 91.8 mg/l for the rivers which flow into oceans and 303.5 mg/l for rivers which flow in
lakes from areas without letting out (within continents) (Alekin, 1952). Each year, the rivers convey
on the territory of former USSR 335  106 t salts, of which 235  106 t arrive in ocean, and about 100
 106 t enter and remain in regions without drainage.
The specific index of the salin discharge for the same territory is – as average – 16.8 t/km2
with variation from 11 – 14 t/km2 in the case of the basins of the Baltic Sea and the Arctic Ocean till
53 t/km2 in the case of the basin of Aral Sea (Alekin, 1952).
This increase of the salt content of ocean water is however extremely low, having in view
that the proportion of water transferred yearly from continents is under 0.003% of the ocean volume,
and salt quantity transferred is 50  1015 t of chlorides (Yaalon, 1967) and the ratio between Cl–
content of ocean water and river water is about 2500; consequently, a significant increase of salt
content of ocean water is expected after thousands of years. According to Alekin (1952) a change of
Cl– content in the planetary ocean with 0.02% (the limit of the analytical detecting of this ion) would
necessitate about 3200 years (quoted from Trufaş and Trufaş, 1975). (Moreover, a part of the salts
brought by running water precipitates in the saline condition of ocean water; for example, the
bicarbonates (of Ca, Mg) pass in carbonates and fall out, and the sulphates could pass in insoluble
sulphide in redox conditions, fact that explains, at least partially, the dominance of the sodium
chloride in ocean water).
The accumulated salts in oceans during geological periods can come back only by
orogenetic geological processes, as saline marine sediments.
2.3. Salts accumulations in lakes and saline saoils, local cases of hydrologic cycle
In its route on land, the water (of the hydrologic cycle) can move in aride regions towards
local low areas, without drainage – endorheic areas – that have frequently patches of land with saline
soils or sometime even salt lakes in the lowest zones. These endorheic areas are practically territory
in which local hydrologic cycle develop, being as „whirlpool” on the route of water on land.
In the case of lakes, the water and salts balances are similar to those described for ocean, but
limited – of course – to the their hydrographic basins. The lake water is concentrated in salts due to
periodical evaporation, so that in time solid salts can be accumulated in the lowest part of the
lacustrine sink. The solid salts accumulation begins with the calcium and magnesium carbonates, the
least soluble salt, followed by gypsum and at long last common salt (NaCl) if the lake becomes very
saline, forming often even salt crusts. Many lakes remain with salt water, continuous, being supplied
with water from ground water.
In the peculiar case of saline soils (Figure 3), a salt accumulation took place in some low
areas, as a rule more higher if a shallow ground water existed. In this case, the balance of water and
salts corresponding to high parts of relief does not led to salt accumulation, but on the low part of
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Florea and Coteţ / The Hydrologic Cycle, Unidirectional Charter of the Dissolved Salts and Suspended Load
relief an increase of evaporated water takes place (on account of water accumulated by running or
originated from ground water) with the production of a corresponding salt accumulation (Figure3).
It is to mentioned that by natural or artificial draining these territories will be included in the
general hydrologic cycle.
Fig.3 Water and salt balance in an area without drainage from aride regions
ETp = P – C
ETd = P + C + F
Water balance
O = O – Csp
O = O + Csp + Fsf
and
salt accumulation
salt accumulation
Salt balance
does not take place
takes place
(Salt accumulation is found in low part of relief or/and with shallow ground water)
ETp = evapotranspiration on flat relief; ETd = evapotranspiration in micro or mesodepressions; P =
atmospheric precipitations; C = running water; F = flow from ground water; sp = salt content in running water;
sf = salt content in ground water.
2.4. Some considerations on energy
A great amount of energy is used up in the hydrologic cycle; this energy is delivered
by Sun, which in fact is the mover of all the processes from nature.
It is estimated that the evaporation of water implicated in the hydrologic cycle needs
2.2  1020 kcal/year, that is 16.9% of solar energy received on earth, that is much more than
energy utilized by plants in photosynthesis that is 0.009  1020 kcal/year representing only
0.1% from the energy received on Terra (Petrescu & Petrescu, 1981).
The hydrologic cycle in nature necessitates mechanical work for the transport of
water and its loading, with consumption of energy that entails entropy increase. But this
increase does not led to a maximum size of entropy that would mean the activity ceasing,
because continuously watery vapours with high entropy are eliminated in this cycle and
liquid or solid water with low entropy is received in exchange, so that size of entropy in the
hydrologic cycle remain al low level. This fact is possible due to a continuous adding of
energy from the Sun, which permit the water heating (therefore energy out of the system of
water cycle), compensating in this way the loss of energy and maintaining the entropy at
low level.
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3. CONCLUSIONS
The description of the hydrologic cycle in nature, reversible and regenerator of fresh water,
is completed with the characterization of the movement, unidirectional and irreversible, of the salts
(dissolved) and material (in suspension), transferred by running water in one part of the hydrologic
cycle, namely the segment between continents and oceans.
The tendency is to transfer salts from land to ocean together the water of the mentioned
segment of the hydrologic cycle (Figure 2).
Some isolated accumulation of salts can be produce on land in endorheic areas as local
hydrologic cycles (Figure 3).
The energy necessary for the hydrologic cycle is delivered by Sun. The entropy in this
system (hydrologic cycle) remains at low level because continuously the watery vapors, with high
entropy, are eliminated from system and liquid or solid water, with low entropy, is received in
exchange.
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