I. HYDROGEOLOGY OF ARID REGIONS
17
concentration, in turn, will inhibit plant growth. If the salinity of the
groundwater is known, then the rate of salt-crust accumulation can be related directly to the rate of groundwater evaporation (Feth and Brown,
1962). Under optimum conditions, the rate of evaporation from a moist
soil surface will be about the same as from a free water surface. For hot
dry deserts, this may be from 250 to 320 cm/year. As the salinity of the
water increases, however, the rate of evaporation is reduced drastically
(Fig. 10). Under certain conditions, moreover, condensation rather than
evaporation can probably take place on the surface of the concentrated
brine (Turk, 1970). Evaporation of groundwater from a playa surface
with brine saturating a salt crust is therefore but a small fraction, perhaps
less than one-tenth, of the rate of evaporation from a freshwater lake in
the same region.
Considerable speculation exists in the literature concerning the loss of
groundwater through capillary movement from great depths to the surface.
Several lines of reasoning tend to argue against a large loss of water by
this mechanism. First, larger pore spaces caused by strata of coarse-grained
material can produce discontinuities in the capillary system. Second, only
fine-grained material will have such a large capillary rise; however, the
same material will be of such small grain size that the hydraulic conductivity will be exceedingly low (on the order of 10~
5
-10~
7
meters/day). Even
with an unusually high hydraulic gradient of about 0.3 and an effective
porosity of 0.3, only about 0.003-0.3 mm/year would be available for
evaporation. Third, salinity of surface brines would slow the evaporation
process.
■2?
Ι 0 ° Ί
o
"
. —_
-c 90-1
^ ^ ^ ^ ^
ί
80 Ί
^ \ v
o
]
^v
-
70
1
^ \
_.
6c
H
\
σ
\
I 5oH
\
c
\
i H
\
3
\
-
2
°j
\
o
\
o
IO-I
\
o
~ I
.
QOi
1
1
1
1
1
1
1
§
I.00
I.05
IIO
I.I5
I.20
I.25
I.30
135
LU
Specific gravity of brine
Fig. 10. Relation between density of brine and evaporation.
17
concentration, in turn, will inhibit plant growth. If the salinity of the
groundwater is known, then the rate of salt-crust accumulation can be related directly to the rate of groundwater evaporation (Feth and Brown,
1962). Under optimum conditions, the rate of evaporation from a moist
soil surface will be about the same as from a free water surface. For hot
dry deserts, this may be from 250 to 320 cm/year. As the salinity of the
water increases, however, the rate of evaporation is reduced drastically
(Fig. 10). Under certain conditions, moreover, condensation rather than
evaporation can probably take place on the surface of the concentrated
brine (Turk, 1970). Evaporation of groundwater from a playa surface
with brine saturating a salt crust is therefore but a small fraction, perhaps
less than one-tenth, of the rate of evaporation from a freshwater lake in
the same region.
Considerable speculation exists in the literature concerning the loss of
groundwater through capillary movement from great depths to the surface.
Several lines of reasoning tend to argue against a large loss of water by
this mechanism. First, larger pore spaces caused by strata of coarse-grained
material can produce discontinuities in the capillary system. Second, only
fine-grained material will have such a large capillary rise; however, the
same material will be of such small grain size that the hydraulic conductivity will be exceedingly low (on the order of 10~
5
-10~
7
meters/day). Even
with an unusually high hydraulic gradient of about 0.3 and an effective
porosity of 0.3, only about 0.003-0.3 mm/year would be available for
evaporation. Third, salinity of surface brines would slow the evaporation
process.
■2?
Ι 0 ° Ί
o
"
. —_
-c 90-1
^ ^ ^ ^ ^
ί
80 Ί
^ \ v
o
]
^v
-
70
1
^ \
_.
6c
H
\
σ
\
I 5oH
\
c
\
i H
\
3
\
-
2
°j
\
o
\
o
IO-I
\
o
~ I
.
QOi
1
1
1
1
1
1
1
§
I.00
I.05
IIO
I.I5
I.20
I.25
I.30
135
LU
Specific gravity of brine
Fig. 10. Relation between density of brine and evaporation.