I. HYDROGEOLOGY OF ARID REGIONS
13
Many aquifers in deserts receive their recharge from water not related
to the hydrology of the desert itself. Examples are ( 1 ) aquifers near large
rivers, such as the Nile, which are sustained by high precipitation outside
of the desert; (2) aquifers recharged by surplus irrigation water such as
those in the Imperial Valley, California; and (3) aquifers which have recharge areas outside of deserts such as extensive carbonate aquifers which
crop out in the south flanks of the Atlas Mountains along the northwestern
border of the Sahara Desert.
Few reliable data on recharge rates in deserts are available. Water-budget calculations for the semiarid portions of the Basin and Range region
of North America suggest rates of 0.1-5 mm/year. These values are, of
course, derived from averages of large areas. Actual maximum recharge
rates in stream channels could exceed 3 meters/day for short periods (see
data by Worts, 1951; Burkham, 1970). Minimum recharge rates through
soil-covered surfaces in semiarid regions are close to zero unless annual
precipitation exceeds at least 200 mm. If the period of rain coincides with
the period of maximum plant growth, then the annual precipitation must
exceed at least 400 mm before significant recharge takes place through normal soil cover. Exact local control on recharge depends, of course, on a
complex set of variables such as antecedent soil moisture, intensity of precipitation, duration of precipitation, permeability characteristics of the soil,
slope of the surface, temperature, etc.
V. Circulation of Groundwater
Water moves in the subsurface from higher areas of recharge to low
areas of discharge. The path taken by the water will not be a straight line,
but may be rather long and indirect (Fig. 9 ) . The actual path taken by
the groundwater is a function of the distribution of permeability and the
boundary conditions in the saturated part of the medium. Some important
aspects of the boundary conditions are the location and magnitude of water
transfer into or out of the medium under consideration. The mistaken belief that topography necessarily influences the position of the water table
is nowhere better refuted than in desert areas (see Fig. 9).
On a regional scale, nevertheless, topography influences the water table
because low areas are commonly areas of groundwater discharge into rivers
or lakes. Also, orographie precipitation may make more water available
in highland areas. The water table is thus somewhat higher where recharge
is more abundant.
Owing to the almost negligible recharge in vast parts of most deserts,
groundwater gradients (Ah/As) need to be only from 0.001 to 0.0001 in
13
Many aquifers in deserts receive their recharge from water not related
to the hydrology of the desert itself. Examples are ( 1 ) aquifers near large
rivers, such as the Nile, which are sustained by high precipitation outside
of the desert; (2) aquifers recharged by surplus irrigation water such as
those in the Imperial Valley, California; and (3) aquifers which have recharge areas outside of deserts such as extensive carbonate aquifers which
crop out in the south flanks of the Atlas Mountains along the northwestern
border of the Sahara Desert.
Few reliable data on recharge rates in deserts are available. Water-budget calculations for the semiarid portions of the Basin and Range region
of North America suggest rates of 0.1-5 mm/year. These values are, of
course, derived from averages of large areas. Actual maximum recharge
rates in stream channels could exceed 3 meters/day for short periods (see
data by Worts, 1951; Burkham, 1970). Minimum recharge rates through
soil-covered surfaces in semiarid regions are close to zero unless annual
precipitation exceeds at least 200 mm. If the period of rain coincides with
the period of maximum plant growth, then the annual precipitation must
exceed at least 400 mm before significant recharge takes place through normal soil cover. Exact local control on recharge depends, of course, on a
complex set of variables such as antecedent soil moisture, intensity of precipitation, duration of precipitation, permeability characteristics of the soil,
slope of the surface, temperature, etc.
V. Circulation of Groundwater
Water moves in the subsurface from higher areas of recharge to low
areas of discharge. The path taken by the water will not be a straight line,
but may be rather long and indirect (Fig. 9 ) . The actual path taken by
the groundwater is a function of the distribution of permeability and the
boundary conditions in the saturated part of the medium. Some important
aspects of the boundary conditions are the location and magnitude of water
transfer into or out of the medium under consideration. The mistaken belief that topography necessarily influences the position of the water table
is nowhere better refuted than in desert areas (see Fig. 9).
On a regional scale, nevertheless, topography influences the water table
because low areas are commonly areas of groundwater discharge into rivers
or lakes. Also, orographie precipitation may make more water available
in highland areas. The water table is thus somewhat higher where recharge
is more abundant.
Owing to the almost negligible recharge in vast parts of most deserts,
groundwater gradients (Ah/As) need to be only from 0.001 to 0.0001 in