20
STANLEY N. DAVIS
As a general rule, groundwater is more saline in deserts than in other
regions. This is true for the following reasons.
1. Evapotranspiration exceeds precipitation, so soluble salts originally
in rain will tend to be concentrated by evaporation at the land surface.
Occasional heavy rain will redissolve these salts, and runoff will carry the
water and salts to points of groundwater recharge.
2. Dry fallout, which is almost ubiquitous in deserts, contains easily
soluble salts. Precipitation which eventually becomes groundwater recharge
will dissolve significant amounts of these salts.
3. Deserts favor the preservation of natural beds of gypsum, halite,
and other evaporite deposits which will contribute dissolved material
to water from infrequent storms.
4. Slow water movement in deserts will mean that flushing of connate
saline water will also be very slow. Thus, some of the salinity in desert
aquifers may be related to ancient saline water originally deposited with
the sediments.
Brackish and saline groundwater from deserts generally have unusually
large amounts of sodium, chloride, and, less commonly, sulfate (Table I,
sample 8). If water is concentrated to a brine, calcium sulfate and some
sodium chloride will be precipitated, leaving a brine enriched in potassium,
magnesium, and chloride (Table I, sample 9).
Despite factors leading to high dissolved solids in deserts, fresh water
can be found in some localities (Table I, samples 1, 3, 5, and 6; Feth,
1966). Conditions favorable for low dissolved solids in deserts are recharge water derived from outside the desert or aquifers that accept recharge from barren surfaces of dense rock which are generally devoid of
dust and crusts of salt. Also, the origins of much of the fresh groundwater
found in deserts of today probably antedates the formation of the deserts.
VIII. Large-Scale Groundwater Circulation
Many deserts have aquifers or groups of aquifers which allow a regional
groundwater circulation to develop. Three of the largest and most important of these desert systems are discussed in this section. Other large regions, notably the southwestern Sahara, the northern Gobi, the western
Australian, parts of the Kalahari, and the southwestern Arabian deserts,
are underlain at a shallow depth by dense crystalline rocks which are
capable of yielding only limited amounts of water except in the zone of
weathering or in the basal parts of large accumulations of dune sand.
STANLEY N. DAVIS
As a general rule, groundwater is more saline in deserts than in other
regions. This is true for the following reasons.
1. Evapotranspiration exceeds precipitation, so soluble salts originally
in rain will tend to be concentrated by evaporation at the land surface.
Occasional heavy rain will redissolve these salts, and runoff will carry the
water and salts to points of groundwater recharge.
2. Dry fallout, which is almost ubiquitous in deserts, contains easily
soluble salts. Precipitation which eventually becomes groundwater recharge
will dissolve significant amounts of these salts.
3. Deserts favor the preservation of natural beds of gypsum, halite,
and other evaporite deposits which will contribute dissolved material
to water from infrequent storms.
4. Slow water movement in deserts will mean that flushing of connate
saline water will also be very slow. Thus, some of the salinity in desert
aquifers may be related to ancient saline water originally deposited with
the sediments.
Brackish and saline groundwater from deserts generally have unusually
large amounts of sodium, chloride, and, less commonly, sulfate (Table I,
sample 8). If water is concentrated to a brine, calcium sulfate and some
sodium chloride will be precipitated, leaving a brine enriched in potassium,
magnesium, and chloride (Table I, sample 9).
Despite factors leading to high dissolved solids in deserts, fresh water
can be found in some localities (Table I, samples 1, 3, 5, and 6; Feth,
1966). Conditions favorable for low dissolved solids in deserts are recharge water derived from outside the desert or aquifers that accept recharge from barren surfaces of dense rock which are generally devoid of
dust and crusts of salt. Also, the origins of much of the fresh groundwater
found in deserts of today probably antedates the formation of the deserts.
VIII. Large-Scale Groundwater Circulation
Many deserts have aquifers or groups of aquifers which allow a regional
groundwater circulation to develop. Three of the largest and most important of these desert systems are discussed in this section. Other large regions, notably the southwestern Sahara, the northern Gobi, the western
Australian, parts of the Kalahari, and the southwestern Arabian deserts,
are underlain at a shallow depth by dense crystalline rocks which are
capable of yielding only limited amounts of water except in the zone of
weathering or in the basal parts of large accumulations of dune sand.