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The method of groundwater recharge evaluation
on the quality of soils as well. Therefore it is necessary to investigate their relationship with the characteristic curve of groundwater household. Regarding equation 4 , it can be stated that the difference of the recharge and the loss of water is equal to the sum of the other effects. Let us take several small basins filled up with soil. If their groundwater space is influenced only by the effects of infiltration recharge and évapotranspiration through the surface, the groundwater table will develop at their equilibrium level. But the groundwater table can be raised by artificially supplying the basin, or it can be lowered by tapping the groundwater space. The k n o w n values of supply or tapping are equal to the difference of the infiltration and evaporation characterizing the groundwater table that has developed in the basin observed. Thus the characteristic curve of the groundwater household belonging to the investigated conditions can be determined by these data.
The method of groundwater recharge evaluation in sand desert by investigation of moisture exchange in the zone of aeration
V . N . Chubarov
All-Union Scientific Research Institute of Hydrogeology and Engineering Geology, M o s c o w , U . S . S . R .
A B S T R A C T : 1. Study of the moisture content distribution through depth in the zone of aeration in small areas of sands covered with vegetation in the desert showed that this distribution is subject to, even at great soil thicknesses (down to 40 m ) , the relationship y + y S 0, where: <p is the potential of gravitation field, yi is the capillary potential. This is indicative of the impossibility (in the first approximation) of descending and ascending movement of liquid moisture.
2. The moisture exchange of groundwater with the atmosphere occurs when rp + y> •£ 0.
3. According to theoretical and experimental data, the relationships if = f\(W) and k — fi{W) are determined, where W and k are the moisture content and soil moisture conductivity coefficient respectively.
4. The relationships determined allow special pallets (as a series of moisture content curves) to be calculated by which the direction and value of moisture flow in the zone of aeration m a y be determined.
5. Comparison of a moisture curve obtained for the small areas of barkhan sands with pallet curves showed that in this case the groundwater is recharged by atmospheric moisture at a rate of 10-20 m m per year.
6. The processes of liquid moisture evaporation and desuction are not very intensive when reaching some 'critical' moisture contents (the moisture of capillary rupture and fading moisture respectively).
7. Supposition of the 'critical' moisture contents on the pallet data permits us to maintain that the groundwater evaporation at the expense of liquid transportation and desuction under the conditions of the sand desert cannot occur at depths of more than 3 and 12-14 meters to the groundwater respectively.
809
V.N. Chubarov
8. Absence of the groundwater distribution elements at depths of more than 12-14 m (besides the diffusion process of 'underground' evaporation) accounts for the fact that the groundwater in the desert, as a rule, occurs at more 'critical' depths (more than 14 m ) and is salinized to a smaller degree.
R É S U M É : 1. L'étude de la distribution de l'humidité du sol en profondeur, dans la zone d'aération, sur de petites étendues de sable couvert de végétation, dans les zones désertiques, a montré que cette distribution est caractérisée par la relation q> + y> = 0 , m ê m e sur de grandes profondeurs (jusque 40 m ) , relation dans laquelle <p est le potentiel du c h a m p de la gravitation et y> est le potentiel de la capillarité. Ceci est indicatif de l'impossibilité (en première approximation) de mouvements descendants ou ascendants de l'humidité du sol.
2. L'échange d'humidité de l'eau souterraine avec l'atmosphère ne se produit que pour <p + y> ?¿ 0 .
3. E n se basant sur les données théoriques et expérimentales, les relations y> = fi(W) et k = fi(W) sont déterminées, dans lesquelles W et k sont respectivement la teneur en humidité du sol et le coefficient de conductivité (de perméabilité).
4. Les relations déterminées permettent de calculer des séries de courbes de teneurs en humidité grâce auxquelles la direction et la valeur de l'écoulement de l'humidité dans la zone d'aération peuvent être déterminées.
5. L a comparaison d'une courbe d'humidité obtenue pour de petites étendues de sables «barkhan» avec les courbes dont il est question au paragraphe précédent, montre que, dans ce cas, l'eau souterraine est rechargée par l'humidité atmosphérique à un taux de 10 à 20 m m par an.
6. Le processus de l'évaporation de l'humidité du sol n'est pas très intense quand on atteint une certaine teneur «critique» en humidité (respectivement l'humidité de la rupture capillaire et l'humidité disparaissant graduellement).
7. L'introduction de la teneur en humidité «critique» sur les courbes dont il est question au paragraphe 4 , permet d'assurer que l'évaporation de l'humidité du sol aux dépens de liquide transporté, dans les conditions du sable du désert, ne peut pas se produire à des profondeurs respectives de plus de 3 et 12-14 mètres.
8. L'absence des éléments de distribution de l'eau souterraine à des profondeurs de 12-14 m intervient pour expliquer le fait que l'eau souterraine dans le désert, en général, se présente en dessous des profondeurs critiques (plus de 14 m ) et qu'elle est moins saline.
T h e zone of aeration is the m e d i u m through which moisture exchange between groundwater and the atmosphere occurs. H e n c e , a principal possibility appears for determination of the direction a n d rate of this moisture exchange from the character of the moisture field in the zone of aeration. In particular, solution of the problems of moisture migration in soils is of keen interest to hydrogeologists w h e n estimating the recharge, discharge and formation in general of groundwater and, in the end, deeper underground water. In the zone of aeration, specific features of climate a n d other environmental factors causing groundwater zoning manifest themselves in their most complete form. In arid zones, the effect of climate o n groundwater accumulation and discharge is so great that the problem of fresh water availability in the desert can be reduced to solving the problem of moisture exchange in the zone of aeration. In this connection, the author believes that specific, purely 'hydrogeological' aspects of the general problem of water in the zone of aeration are particularly convenient w h e n considering the arid zone.
T h u s , in the present report, t w o m a i n tasks are set: 1) to apply the current theory of moisture m o v e m e n t in soil to the solution of the hydrogeological problem of fresh groundwater accumulation in the desert; and 2) to check s o m e theses of soil hydrology by comparing a n u m b e r of hydrogeological facts and moisture migration conditions in the zone of aeration.
810
The method of groundwater recharge evaluation
The specific 'hydrogeological' aspects of the water problem in unsaturated soils m a y include the following: a) moisture movement in fairly deep horizons of the zone of aeration where flow discharge are considerably less than in the surface zone (from fractions of a millimetre to hundreds of millimetres of a water layer annually) and where the porous m e d i u m has considerably greater thicknesses (occasionally up to tens of metres); b) inter-relation of the capillary fringe and overlying soil moisture horizons; c) consideration of moisture exchange processes on a smaller time scale as compared with that used by soil scientists and agrophysicists (from several to tens and hundreds of years).
The problem of moisture movement in soils for one of the sandy desert areas was partially discussed in works by Ogilvy and Chubarov (Ogilvy, 1963 ; Ogilvy and Chubarov, 1963; Chubarov, 1963). The general scientific supervision of the desert investigations was carried out by Ogilvy. H e also worked out some problems of vapourous moisture migration and gave a theory and estimation of the process of intrasoil condensation. In the present report, on the basis of the author's latest experiment an attempt is m a d e to discuss more completely the problem of atmospheric recharge of groundwater in the arid zone.
Under sandy desert conditions where the thickness of the zone of aeration is more than 10-15 m , two principal types of moisture distribution m a y be distinguished (fig. 1): a) in sites with vegetated sands (the aôx épure); b) in areas with barkhan non-vegetated sands (the a "%' epure). In addition, in the zone of barkhan sand movement up to vegetated sand an intermediate type of moisture distribution (the a'è'z epure) is distinguished. In the upper part of the zone of aeration (for example, the epure sections aô or a'd') moisture content is most variable (below this problem will be discussed in some detail) which is due to the great influence of meteorological factors on soil. With depth this influence weakens, therefore, soil moisture content does not vary in time at all under these conditions or varies within insignificant ranges (the zone of quasi-stationary moisture).
Each of the types of moisture distribution distinguished corresponds to a definite case of moisture exchange between groundwater and the atmosphere. T o establish the nature of this moisture exchange and conduct any studies on soil hydrology it is necessary to have two groups of information: a) the relationship of the capillary (carcass) potential and moisture content and b) the relationship of the coefficient of transmissibility (hydraulic transmissibility) and moisture content of a soil or the capillary potential (the terms recommended by the VIIth Soil Science Congress in 1960 are given in brackets, see also Rode , 1962).
The relationship of the capillary potential and moisture content m a y be defined in the following way . Suppose that under the conditions of vegetated sands in the upper soil layer up to 7-8 m thick liquid moisture exchange with deeper horizons is excluded. The reason for this supposition is the fact that desert plant roots, as a rule, are at depths of 0 to 7-8 m and at greater depths they are encountered as an exception (according to Shavyrina's oral communication). In this case, within this zone, plant roots intercept all the moisture coming from the atmosphere as in the agrophysicists' opinion the rate of plant roots penetration deep into soil strata considerably exceeds the rate of moisture supply when soil moisture contents are low which are observed at depths of 4-5 m and more (see the lower part of the aô epure in fig. 1). O n the other hand, it is difficult to assume that root hairs suck moisture from practically dry lower sand horizons with a moisture content of 0.5-0.8 percent when from upper horizons with a moisture content of 1.5-3.0 percent moisture can flow at a considerably higher rate. But, if it is held that under the conditions of vegetated sands atmospheric moisture cannot penetrate into the soil strata occurring at a depth of more than 7-8 m and, vice versa, the moisture of these strata is not discharged into the atmosphere, then the moisture distribution in deep soil horizons of the zone of aeration in this case must be in equilibrium and follow the
811
V.N. Chubarov
law (Nerpin, 1955):
<p + i¡i = 0 . (1)
where: (p is the gravitational field potential, counted off from the water table; \¡/ is the capillary potential. A similar mcisture distribution would take place, for instance, at the final stages of prolonged fiee moisture flow from the so-called high soil column.
F I G U R E 1. h is the height above the water table, m; w is moisture content (per cent of weight);
k(log) is the coefficient of transmissibility (at logarithmic scale); Max is the plot characterizing
moisture flow at full saturation; I, II and III are parts of the equilibrium epure for pellicular, capilla-
ry-pellicular and capillary moisture contents, respectively; BPK is the moisture content of capillary
rupture; B3 is the wilting point; 1 — moisture content data for equilibrium epure obtained from well
drilling, with different thicknesses of the zone of aeration; 2 — moisture content data for the equi
librium epure calculated theoretically ; 3, 4, 5—points for plotting sections of the graph k = q>2 (w)for
capillary, capillary-pellicular and pellicular moisture contents, respectively; 6 —points for plotting
pallet curves; 7 — ground surface
If our supposition is confirmed, in the zone of aeration there will be a model of a very high soil column (35 m ) created by nature itself characterizing the height distribution of almost all moisture categories.
812
The method of groundwater recharge evaluation
With the aim of verification of the supposition concerning moisture equilibrium in deep soil horizons in sites with vegetated sands, an average moisture epure was constructed (the (¡T epure in fig. 1). T h e data on moisture content obtained from the results of well drilling in several sites differing in thickness in the zone of aeration were plotted. A s seen from the figure, all the data are grouped around one curve showing a decrease in moisture content as the height of the section increases above the water table.
This makes it possible to present a number of good reasons in favour of the fact that the moisture distribution characterized by the fix epure (fig. 1) is in equilibrium or very close to equilibrium and follows the relationship (1):
1. The character of the height distribution of moisture. (As further calculations will show, moisture épures, characterizing downward or upward moisture movement , i.e. non-equilibrium moisture distribution, have quite a different form).
2. Coincidence of moisture content épures pertained to sites with different thicknesses
of the zone of aeration which can be imagined only if the moisture is immovable.
3. Invariability in time of moisture content in the zone under discussion, established as a result of highly accurate moisture measurements with electrical transducers (blocks) m a d e of fibre glass (the accuracy of these devices within a moisture range of 0.6-2.0 percent of weight amounts to hundredths of a percent).
But, if the generalized epure /?T in fig. 1 obeys the relationship (1), it is simultaneously
the relationship of capillary potential to be found (expressed as the gravitational field
potential, i.e. water column height) and moisture content. The described method of determining the moisture potential was usually used for
low soil columns (1-3 m ) and in a number of cases it was considered that in higher sections the relationship (1) was not observed and the epure of moisture over the capillary fringe in this case is linear and parallel to the vertical axis. Nerpin (1955) showed that when the equilibrium of pellicular and meniscus moisture is considered simultaneously, the moisture content, provided moisture movement is absent, must decrease with height above the water table.
The generalised moisture epure given in fig. 1 supports Nerpin's assumptions and allows the moisture of all the unsaturated soil strata to be related to the hydrodynamic system c o m m o n with groundwater.
The author also held it expedient to calculate, according to Nerpin's theory, a theoretical equilibrium moisture content epure and compare it with the one obtained. A s seen from fig. 1, the agreement is fairly good.
The calculation was m a d e for a soil with the grain-size distribution, given in table 1.
T A B L E 1. Grain-size distribution of the soil used in the calculation
Fraction diameter, m m
Fraction content (percent of weight)
0.1-0.25
70
0.05-0.1
24
0.01-0.05
2
0.005-0.01
2
<0.005
2
Table 2 gives calculations of the moisture content for individual fractions, assuming the particles to have a ball form. The coefficient of surface tension was assumed to be equal to 72.5 dynes/cm, the coefficient e in calculating pellicular moisture content was equal to 2.105 (g . /cm) 1 / 3 .
The moisture content for construction of the equilibrium epure was calculated from data of table 2 with regard for the percent content of individual fractions presented in table 1.
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814
The method of groundwater recharge evaluation
Depending on the predominance and content of a certain kind of moisture, one can distinguish pellicular, capillary (joint) and capillary-pellicular parts of the equilibrium moisture content epure (fig. 1).
The coefficient of transmissibility (hydraulic transmissibility) was determined also by using Nerpin's theoretical relationships and assumptions (1955, 1962) separately for pellicular and capillary (joint) moisture. Here, a soil ball model was assumed. T h e data on the hydraulic transmissibility used below for direct estimation of groundwater recharge in the desert were obtained experimentally. It is also due to the fact that in the moisture range of 1-2 to 4-5 percent (fig. 1) a combined capillary—pellicular mechanism of moisture transport acts. In this case, a theoretical approach to estimation of transmissibility is extremely hindered.
Calculations of transmissibility of individual soil fractions for pellicular moisture are given in table 3.
It follows from table 3 that with a decrease in height above the water table from 50 to 10 m (i.e. with an increase in the thickness of the water pellicle) the hydraulic transmissibility increases. However , in sections of 5 metres and less in a natural soil consisting of a number of fractions, in addition to pellicular moisture, capillary water appears which sharply increases the total transmissibility of the porous m e d i u m . This is somewhat similar w h e n high electric resistance (a thin water pellicle) is shunted by a small value resistance (a capillary 'tube').
In this case, the total hydraulic transmissibility of soil evidently cannot be estimated without regard for capillary moisture (which is clearly seen w h e n comparing values of the total transmissibility for pellicular moisture kp in sections 5 m thick and more (see column 6 in table 3).
T A B L E 3. The coefficients of transmissibility of different soil fractions for pellicular moisture depending on the carcass potential (in mm/year , head gradient = / m m / m m )
Fraction diameter, m m
Carcass potential, m of water column
0.5 1.0 2.0 3.0 5.0
10.0 30.0 50.0
0.1-0.25
1.77 0.232 0.0667 0.0362 0.0162 0.0067 0.00153 0.00077
0.05-0.1
0 6.55 0.386 0.158 0.062 0.018 0.00442 0.00214
0.01 -0.05
0 0 0 1.45 0.32 0.0726 0.01247 0.00565
0.005-0.01
0 0 0 0 0 4.62 0.0837 0.0523
Total transmissibility for pellicular moisture
1.239 1.734 0.1393 0.0923 0.0262 0.1953 0.00573 0.00280
In calculating the capillary moisture transmissibility (£„), the following relationship was used (Nerpin, 1962):
xo3 , aw
t̂. — C„ (2)
where: c is the constant coefficient incorporating coefficients of surface tension and water viscosity; O is soil porosity; w is the system moisture content determined by the quantity of water filling pores of a certain size; A Pcr is the critical capillary pressure corresponding to the rupture of pore capillaries of the same size; x ¡s m e coefficient depending on the capillary potential.
T h e coefficient x was assumed to be equal to unity. In this connection the hydraulic transmissibility with full saturation (the coefficient of permeability), calculated using formula (2), appeared somewhat different from that estimated experimentally.
815
V.N. Chubarov
Therefore, on the graph k = q>2 (w) (fig. 1) transmissibility data were plotted, reduced (by multiplication by a constant value) to the coefficient of permeability equalling / m / d a y . The relationship w — q> (A Pcr) which is necessary for calculations using formula (2) is the same as the equilibrium curve (fig. 1).
Experimental data on capillary-pellicular transmissibility were obtained in experiments with long (up to two months) free moisture flow in soil 'prisms' 2 m high, with a section of 1.2-1.2 m and moisture-proof at their sides. It was assumed that beginning from a certain depth all the moisture was discharged only for flow and the pressure gradient was equal to unity. In this case, to calculate the coefficient of transmissibility (kw) the following relationship is proposed:
h
L(w»-wn+i) K = : (3)
At
where: h is the distance along the soil prism height from the zero section (for which moisture exchange is taken to be equal to zero) to the section h ; wn and í¿'n+ j are average moisture contents at the time moments n and « + 1 for a depth portion Ah\ wn_ivn+l
is the moisture content increment in the section h during the time At. The value of the coefficient of transmissibility is related to the average moisture content wav in the section h:
_ w„ + wn + 1
Calculations of the relationships of the capillary potential and capillary transmissibility and the soil moisture content allow all the moisture in a thick zone of aeration (35 m ) to be considered as a whole. In this case, w e are entitled, using Darcy's law, to calculate a series of curves (in the form of pallets), each of them for a given type of soil would correspond to the moisture flow, which is strictly definite as to its value and direction, in the zone of aeration (fig. 1).
If the moisture movement in fairly deep soil horizons is considered to be quasi-stationary or similar to that, by superimposing a natural moisture content épure on a pallet curve, it is possible to find the values of moisture discharge through the zone of aeration and, therefore, to estimate the atmospheric recharge of groundwater. Also, the pallet obtained is the basis for any investigation of the problem of moisture exchange between groundwater and the atmosphere.
The curves were calculated in keeping with the relationship:
ft,+ 1 = f c , - M f c ± - ^ - (4) Si +1 + ^n
where: q is the value of moisture flow in m m / y e a r ; \//„+l and \¡/„ are capillary potentials (expressed as a water column height), acting on the sections (from the water table) n + 1 and n, respectively, with a given flow discharge q\ h is the difference between the heights of the upper and lower sections ; kn + , and kn are hydraulic transmissibilities with moisture contents in sections n +1 and n, respectively, and with a given flow discharge q.
The calculation is performed in such a way that data for each subsequent section are determined from the results of estimates for the preceding section (beginning from the water table). The relationships <p + (p¡ (w) ar*d k = q>2 (u) were used. The values ¡¡/n+1
are calculated with one or several approximations until \j/„+i becomes identical for two neighbouring approximations (within the accuracy prescribed). In the relationship (4) a plus is used when constructing plots to the left of the equilibrium curve (which
816
The method of groundwater recharge evaluation
corresponds to upward moisture flow); a minus is used when constructing plots to the right of the equilibrium curve (in case of downward moisture movement).
In fig. 1, several most typical pallet curves are presented allowing a number of general conclusions to be made:
1. The equilibrium curve differs sharply in form from the moisture content épures showing a moisture movement. Moisture content épures for downward moisture movement at great heights above the water table have the form of a straight line parallel to the vertical axis. Moisture content épures for upward moisture movement form a m u c h larger angle with the vertical axis as compared with that formed by the equilibrium epure, furthermore, this angle increases with height (in the case of the equilibrium epure, it decreases). Hrnce, the equilibrium distribution of moisture content in the zone of aeration m a y bî established with a sufficient accuracy from the nature of moisture content increase with depth. In a number of cases, it allows the relationship of the capillary potential and moisture content to be determined without resorting to labour-consuming and sometimes unreliable (as applied to natural soils) methods of determining the above relationship using soil samples.
2. The more the pallet curves for downward moisture movement are similar to the equilibrium epure, the smaller is the rate of moisture discharge reflected by these curves. Therefore, for discharges of moisture infiltrating through the zone of aeration (0-200-300 mm/year ) , observed under natural conditions, and particularly for arid zones it is necessary to take into account that with the same discharge of infiltrating moisture natural moisture content somewhat increases approaching the groundwater. Therefore, with small moisture discharges and relatively small heights above the water table flow discharge cannot be considered to be a single-valued function of natural moisture content without regard for the height of the section above the groundwater. It is m u c h more true in the case of upward moisture movement .
3. The notion 'capillary fringe' widely used by hydrogeologists is rather conventional and conditional. If, for instance, the equilibrium moisture distribution is considered, it would be logical to speak about capillary, pellicular and capillary-pellicular parts of a single equilibrium epure (fig. 1). In the case of moisture movement, the form of the 'capillary fringe' evidently depends on the direction and value of moisture flow passing through the zone of aeration.
Using the pallet obtained, one m a y estimate groundwater recharge in the sandy desert. T o do so, the typical natural moisture content epure of the zone of aeration characterizing the moisture exchange in the area of barkhan sands development is superimposed on the pallet. A s seen, the natural moisture content epure is mainly placed between the pallet curves + 1 0 and + 2 0 . Therefore, the areas of barkhan sands development in the sandy desert are the areas of groundwater recharge with atmospheric moisture, and the average long-term rate of this recharge is 10-20 mm/yea r . O n the contrary, in areas of vegetated sands with a fairly great thickness of the zone of aeration liquid moisture exchange between groundwater and the atmosphere is ruled out (see the equilibrium epure, fig. 1).
It is not clear, however, for what thickness of the zone of aeration the assumptions m a d e are valid. This problem m a y also be solved using the pallet obtained. In this case, it is necessary to have some data on moisture exchange in the upper layers of the zone of aeration which, in addition, allows the mechanism of atmospheric moisture penetration into deep soil horizons to be elucidated and groundwater recharge (for barkhan sands) to be estimated by another method.
With this aim in view, two processes were experimentally studied: evaporation in summer time (fig. 2,1) and infiltration in the cold period of the year (fig. 2, II).
In evaporation studies, a soil prism (dimensions 1.2 x 1.2 x 2.0 m ) with moisture-proof sides was saturated with moisture (up to 6-8 percent), then water was allowed to flow
817
V.N. Chubarov
and, finally, the prism was opened for evaporation (Chubarov, 1963). A s a result, three evaporation stages were observed: first (ej when the moisture flow to tht evaporating surface is not limited; second (e2) (the well-known period of the decreasing rate of drying in the experiments with soil samples) when a dry soil layer is formed on the surface which sharply slows d o w n the process of evaporation ; third (e3) when at a depth of 0.2-0.3 to 0.8-1.0 m a layer with a moisture content of 2.0-2.4 percent is formed, and evaporation occurs practically without liquid moisture flow from deeper horizons (only at the expense of moisture reserves loss in the surface layer). This moisture content for the process of evaporation m a y be considered critical. Abramova, Bolshakov, Oreshkina and Rode (Abramova et al., 1956) call it the moisture content of capillary
i i
F I G U R E 2 . / — Stages of soil moisture evaporation:
n —first; E 2 — second; M — third
II — Stages and cases of infiltration of precipitation in soils: u* — infiltration in made soil with an
even height distribution of the initial moisture content (the latter is shown by a dashed line); u\ —
first stage; u2 — second stage; m — third stage
III — Stages of atmospheric moisture penetration to ground water: e.g. e m i — the first stage of
infiltration at the first stage of evaporation; ^uz — the third stage of infiltration at the third stage
of evaporation
w — moisture content (percent of weight); h — depth, m; a — mulch-layer; fi — zone of moisture
content of capillary rupture (BPK); jS — zone of moisture (rapidly) flowing to groundwater.
1 — moisture content epure at the final period of evaporation but before the beginning of infiltration
(September-October); 2 — moisture content épures for different seasons of the year; 3 — moisture
reserves are mainly spent for evaporation and flow (spring-summer); 4 — moisture reserves are
mainly accumulated at the expense of precipitation (autumn-spring); 5 — complex interrelation
between evaporation and moisture accumulation (spring-early summer); 7 — moisture reserves are
spent only for flow, i.e. for groundwater recharge
818
The method of groundwater recharge evaluation
rupture (BPK). The moisture flow through the zone BPK (fig. 2 ,1 , see intercept S) is actually negligible [fractions and several m m / y e a r according to the data k = q>2 (tv)
(fig. 1)]. The course of summer evaporation described with formation of a mulch-layer and
the zone BPK is of particular importance to moisture conservation in deep soil horizons of the desert as the evaporation rate under these conditions sharply decreases. If precipitation in one of the years was absent altogether, evaporation in such a year would amount to several millimetres.
Thus, at the end of the w a r m period (September-October) in the desert, a highly peculiar distribution of moisture content is observed as a result of evaporation (see the dashed line epure in fig. 2 , III) which is repeated each year and is taken as a basis for all subsequent analyses of data.
In the cold period of the year (October-April) moisture accumulates in the upper soil layers. According to the character of the infiltration front, one m a y distinguish three stages (fig. 2, II): in the first stage (u¡), this front is within the mulch-layer; in the second stage (u2), within the zone BPK; and in the third stage (u3), below the zone BPK (here, relatively rapid moisture flow is possible to take place). A s seen from the figure, the percolation front is fairly distinct in all three cases. This is due to the fact that in the desert the soil transmissibility in the upper layer d o w n to a depth of 2-3 m is very low (in any case, water flow to underlying horizons at this period is considerably less than atmospheric moisture inflow). T h e experimental data (fig. 2 , II, ux) show that the atmospheric moisture moving in layers with a low transmissibility does not penetrate below the part of the infiltration front seen in the plot. This allows us to calculate with a great confidence the moisture reserve increment (A w) for any period of the year and use, knowing the precipitation amount (5), the simple balance relationship for calculation of evaporation values (e):
e = S-Aw. (5)
The annual course of precipitation penetration into deep soil layers is shown in fig. 2 , III. T h e processes of evaporation and percolation, which occur simultaneously, toward, the middle of s u m m e r lead to a moisture content distribution (fig. 2, III, e3 u3) when moisture loss as a result of evaporation is ruled out as layers situated below the zone BPK are considered and the percolation front is still fairly contrasted (the third stage of infiltration). Therefore, it is possible to distinguish the value of the moisture reserve increment Aw. It is evident that this amount of moisture is spent only for flow to deep soil layers and, therefore, constitutes the annual amount of groundwater recharge. It is also clear that s u m m e r rainfall cannot penetrate, as a rule, through the mulch-layer and the zone BPK and evaporates completely. The amount of annual evaporation in the sandy desert (estimated using relationship 5) without taking into account the s u m m e r rainfall evaporation for each year is very similar to a value of 100-110 m m . This permits the following relationship for estimating long-term groundwater recharge:
C = S ' - 1 0 5 , (6)
where: Q is the annual amount of groundwater recharge in m m ; S' is the s u m of precipitation from October to M a y in m m .
If the precipitation amount is less than 100 m m / y e a r , an extremely insignificant quantity of moisture (fractions of a millimetre and several millimetres of a water layer annually) penetrates to the groundwater. O n the contrary, a sharp increase in the annual precipitation rate over the average long-term rate leads to intensive groundwater recharge (50-600 m m / y e a r ) , the annual precipitation amount not exceeding 110 m m . The average long-term recharge value, calculated for tens of years using the relationship (6), was equal to 20.7 m m / y e a r .
819
V.N. Chubarov
F r o m information on moisture exchange in deep (fig. 1) and upper (fig. 2) soil layers, the critical thickness of the zone of aeration can be found when groundwater evaporation and desuction are impossible.
It was shown (fig. 2,1 and III) that as a result of evaporation the soil moisture content at a depth of 0.8 m never decreases to a value of less than 2.2 percent (the critical moisture content is BPK). O n the equilibrium epure /?r (fig. 1), the height of 2.2 m corresponds to this moisture content. It is obvious that if the height is less than 2.2 m , the point with a moisture content of 2.2 percent is to the left of the equilibrium epure which indicates the possibility of groundwater loss as a result of evaporation. O n the contrary, if the height (on the equilibrium epure) is more than 2.2 m , upward movement of liquid moisture is ruled out. S u m m i n g up the heights of 2.2 m and 0.8 m (the lower boundary of the zone BPK), one obtains the unknown critical thickness of the zone of aeration for evaporation equalling 3 m .
In a similar way, the critical thickness for the process of groundwater discharge to feed plants is found. In this case, the critical moisture content, below which the process of desuction practically stops, m a y be considered to be the wilting point (fi3) which is taken to be equal to 1.5 percent. The latter was determined using Rode 's method (1960). Taking into account that roots of the majority of desert plants do not penetrate deeper than 6-8 m , one finds the critical thickness for desuction (12 m ) .
For any extrapolation it is necessary to have relationships of values of evaporation, infiltration and desuction and the thickness of the zone of aeration (in this case and further on, water balance elements enumerated pertain to the groundwater and not to the upper soil layer).
For evaporation and desuction, relationships m a y be found in the following way. F rom pallet curves data for evaporation, the relationship between the highest possible evaporation rate for a given type of soil and the thickness of the zone of aeration is found. It is possible to assert that this relationship in the range of evaporation values from 5 to 150 mm/yea r is very similar to the one to be determined. In fact, these evaporation values are always found in the desert where evaporativity is so high. O n the other hand, the excess of the evaporative capacity of the atmosphere over the possibility of liquid moisture flow in the soil to the evaporating surface leads to the formation of a mulch-layer which automatically decreases the evaporation rate to an amount equalling the moisture flow from the groundwater. It appears that the error m a d e in calculating the thickness of the zone of aeration does not exceed the value of the thickness of the mulch-layer (i.e., 0.2-0 4 m ) .
All this makes it possible to substantiate and suggest for sandy soils a classification of types of moisture exchange in the zone of aeration, and to present for the sandy desert the conditions (the thickness of the zone of aeration, in particular) under which the type distinguished is observed (table 4). This enables one to extrapolate the groundwater recharge data for an area.
Thus, in the sandy desert in barkhan sand areas with the zones of aeration 3 m thick, groundwater is recharged by atmospheric moisture with a rate of 10-20 mm/yea r . If the thickness of the zone of aeration is less than 1.0 m , groundwater evaporates very intensively and as a result the groundwater level in the desert becomes stable, as a rule, at depths of more than one metre. In the depth range of 1.0-3.0 m combined processes of fairly intensive evaporation and recharge of groundwater by precipitation occur, therefore, in the sandy desert, solonchaks (shors), under which groundwater has a high salinity, are confined to the sites with the depths mentioned.
In areas with vegetated sands and a zone of aeration more than 12 m thick groundwater discharge or recharge as a result of liquid moisture transfer is practically ruled out. In this case, the process of intrasoil evaporation not exceeding 0.5 m m / y e a r (Ogilvy, 1963) as a result of diffusion transfer of vapour associated with the temperature gradient takes place. Long duration of this process (absence of recharge taking into consideration)
820
The method of groundwater recharge evaluation
T A B L E 4. Types of moisture exchange between groundwater and the atmosphere (types of groundwater recharge through the zone of aeration)
Types of water exchange
Characteristics (determination) of recharge types or relation between values of
groundwater recharge elements
Infiltration
(/) Evaporation Desuction
Conditions under which a recharge type is observed or supposed in the sandy desert
Thickness of the zone of aeration, Character
of relief
Infiltrating (washing, periodically washing)
Evaporative (sweating)
Desuction (desuction)
Infiltrating-evaporative (periodically sweating)
Evaporative-infiltrating (periodically washing)
Infiltrating-desuction (periodically desuction)
> 0
0
0
> 0
I<E
I>E
KD
0
0
> 0
0
0
D>I Desuction-infiltrating (periodically washing)
Evaporative-desuction (periodically desuction)
Desuction-evaporative (periodically sweating)
Equilibrium (non-washing)
Mixed types, e. g. desuction-infiltrating-evaporative (peridiocally washing, periodically sweating, per. desuction)
0
0
0
E<D
E>D
0 0
E>I>D
Barkhan more than 3 sands
0 - 3 1
3-12 1
0-1
1-3
3-12
0 - 3 1
Solonchaks and barkhan sands
Vegetated sands
Solonchaks and
barkhan sands
Vegetated sands
Vegetated sands
Vegetated sands
more than Vegetated 12 sands
0-3 Vegetated sands
1. Only for dry years. Note: In brackets are the types of soil water regime according to Rode's classification (Rode, 1956).
leads to gradual salinization of groundwater, which is in full accordance with Kunin ' s work (Kunin, 1959). It appears that geobotanical methods in case of very thick zones of aeration cannot be used due to absence of moisture exchange at depths of m o r e than 12 m .
If the depth to the water is less than 12 m , plant roots can be fed with groundwater. This feeding approximately estimated at depths of 6-8 m amounts to no more than 20 m m / y e a r . H o w e v e r , with depth it sharply increases. This fact (that was not taken into account earlier to the best of our knowledge) m a y be responsible for the c o m m o n occurrence of groundwater at depths of more than 10-12 m in vegetated sand areas in the desert excluding solonchak sites.
If the depths to the water are less than 3 m , evaporation occurs together with desuction, water salinity increases and salt-loving plants appear.
821
D.M. Katz
REFERENCES AôpaiaoBa M . M . , A . <I>. EojibHiaKOB, H . C . OpeuiKHHa and A . A . Po«e. 1956. Hcna-
peHHe noABeuieHHOH BJiara. IIoiBOBeAeHHe, N» 2. KyHHH, B . H . 1959. MecTHtie BOABI nycTbiHH H Bonpocbi H X Hcnojib30BaHHH.
H3fl-B0 A H C C C P , M . HepnHH, C . B . 1955. PaBHOBecne KOHAeHCHpoBaHHoñ H napoo6pa3H0H BJiarH B
noiBax H rpyHTax. Tpyflbi JleHHHrpaflCKoro HHCTHTyTa BOAHoro TpaHcnopTa, Bbtn. 22.
HepnHH, C . B . 1962. BjiaronpoBOAHOCTb CTpyKTypHbix noiB. C6. TpyAOB no arpoHo-MHqeCKOH (J)H3HKe, Bbin. 10.
HepnHH, C . B . , M . K . MeJibHHKOBa. 1957. PaBHOBecne KOHAeHCHpoBaHHoñ H napo-o6pa3HOH BJiara B no^Bax H rpyHTax. Bonpocbi arpoHOMH^ecKoft (J>H3HKH, JI.
OrHJibBH H . A . 1963. BHyTpnrpyHTOBaH KOHAeHcanHH H HcnapeHHe B nycn.msx H H X rHAporeojiorniecKoe 3Ha*ieHHe. BMJIJI. M O H I I , N° 2.
OrHJibBH H . A . , B . H . MyôapoB. 1963. H3yneHHe A H H ^ M H I Í H BJiarH H npon,eccoB ee KOHAeHcainiH B 30He aapanHH. B KHHre: « JlHH3bi npecHbix B O A nycTbiHH». H3A-B0 A H C C C P , M .
PoAe A . A . 1956. B O A H B I H peJKHM TIOHB H ero THnbi. LToHBOBeAeHHe, N° 9. PoAe A . A . 1960. MeTodbi H3yieHHH BOAHoro peJKHiua no^B. H3A-B0 A H C C C P , M . Pone A . A . 1962. O paGoTe MescAyHapoAHoro KOMHTeTa no TepuomoJioniH B o6jiacTn
(J)H3HKH noiB. noiBOBeAeHHe, N° 12. HySapoB, B . H . 1963. BJiarooSMen B 30He aapauHH KaK dpaKTop (bopMHpoBaHHH
npecHbix rpynTOBbix B O A B nycTbiHe. BKJJIJI. M O H T I , N° 2.
The regularities of groundwater evaporation in irrigation lands of the arid zone
D . M . Katz All-Union Scientific Research Institute of Hydrogeology and Engineering
Geology, Moscow, U . S . S . R .
A B S T R A C T : The report deals with the lysimeter data on groundwater discharge as a result of evaporation and transpiration from different types of ground surface (both land without vegetation and with cotton, alfalfa, maize and other irrigated crops). The investigations cover soils of a various texture under semi-desert and desert climatic conditions, the water table being at a depth of 0.5 to 3 meters. The results of mathematical treatment of the observation data are presented and pertinent regularities are established.
RÉsuMé : La communication se rapporte à des données de lysimètres relatives au débit des eaux souterraines en tenant compte de l'évaporation et de la transpiration de diverses surfaces de sols (sols sans végétation, sols avec coton, avec luzerne, avec maïs, avec d'autres cultures irriguées). Les recherches portent sur des sols de textures variées dans les conditions du désert ou du semi-désert, la nappe phréatique étant à une profondeur de 0,5 à 3 mètres. Les résultats du traitement mathématique des résultats d'observation sont présentés et des règles pertinentes sont établies.
T h e problems of groundwater evaporation in the arid zone are of supreme concern to investigators studying the water-salt regime of the zone of aeration. T h e interest s h o w n in these problems is fully understood, if one considers the great practical importance
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