I. HYDROGEOLOGY OF ARID REGIONS
15
Although the examples above are artificial, they are thought to represent
realistic figures for aquifers containing desert-derived recharge. From calculations similar to those above, it can be assumed that most water in widespread desert aquifers is moving at rates of less than 0.1 meter/day.
Many desert aquifers extend for more than 200 km. If water is traveling
at velocities of less than 0.1 meter/day, then water at a distance of 200 km
from a recharge area is at least 5500 years old. This is the line of reasoning
which suggested to early workers that water extracted from many desert
aquifers was recharged thousands of years ago when climatic conditions
were different than today. Recent studies of the
14
C content of desert
groundwater (Tamers, 1967; Degens, 1961; Thatcher et al, 1961) have
supported this conclusion.
Some water buried within deep alluvial basins may have been part of
the water that helped transport the sediments that fill the basin. If the
basins have little or no hydraulic gradients in their central parts, the presently retained water in the basin could be very old indeed, perhaps with
an age of more than one million years. Slow natural compaction of the
sediments forces some of the old water to the surface where it may emerge
in springs. In most places, this water probably mixes with younger water
as it migrates to the surface.
Water that does not originate in a desert but still serves as recharge to
desert aquifers may, after recharge because of copious quantities of water
and steep hydraulic gradients, travel quite rapidly. For example, the
Lambayeque Valley in the coastal desert of Peru receives recharge from
the Rio Chancay which flows seaward from the Andes (Schoff and Sayan,
1969). Gradients are generally between 0.002 and 0.005. Hydraulic conductivites are estimated to be between 20 and 35 meters/day. If the effective porosity is assumed to be 0.3, then average water velocities are from
0.13 to 0.58 meter/day, or roughly an order of magnitude greater than
velocities calculated above for average desert conditions.
VI. Groundwater Discharge
Discharge of groundwater in deserts will cause a striking change in the
appearance of the desert. Large oasislike patches of vegetation and extensive salt crusts usually mark the presence of springs or groundwater at
a shallow depth. Owing to the general scarcity of water, springs with large
discharges are not common in deserts. Notable exceptions occur in regions of carbonate or volcanic rocks which can form extensive aquifers
capable of collecting water from a large area and, because of the high
permeabilities of these rocks, of discharging the water in local concentrated
15
Although the examples above are artificial, they are thought to represent
realistic figures for aquifers containing desert-derived recharge. From calculations similar to those above, it can be assumed that most water in widespread desert aquifers is moving at rates of less than 0.1 meter/day.
Many desert aquifers extend for more than 200 km. If water is traveling
at velocities of less than 0.1 meter/day, then water at a distance of 200 km
from a recharge area is at least 5500 years old. This is the line of reasoning
which suggested to early workers that water extracted from many desert
aquifers was recharged thousands of years ago when climatic conditions
were different than today. Recent studies of the
14
C content of desert
groundwater (Tamers, 1967; Degens, 1961; Thatcher et al, 1961) have
supported this conclusion.
Some water buried within deep alluvial basins may have been part of
the water that helped transport the sediments that fill the basin. If the
basins have little or no hydraulic gradients in their central parts, the presently retained water in the basin could be very old indeed, perhaps with
an age of more than one million years. Slow natural compaction of the
sediments forces some of the old water to the surface where it may emerge
in springs. In most places, this water probably mixes with younger water
as it migrates to the surface.
Water that does not originate in a desert but still serves as recharge to
desert aquifers may, after recharge because of copious quantities of water
and steep hydraulic gradients, travel quite rapidly. For example, the
Lambayeque Valley in the coastal desert of Peru receives recharge from
the Rio Chancay which flows seaward from the Andes (Schoff and Sayan,
1969). Gradients are generally between 0.002 and 0.005. Hydraulic conductivites are estimated to be between 20 and 35 meters/day. If the effective porosity is assumed to be 0.3, then average water velocities are from
0.13 to 0.58 meter/day, or roughly an order of magnitude greater than
velocities calculated above for average desert conditions.
VI. Groundwater Discharge
Discharge of groundwater in deserts will cause a striking change in the
appearance of the desert. Large oasislike patches of vegetation and extensive salt crusts usually mark the presence of springs or groundwater at
a shallow depth. Owing to the general scarcity of water, springs with large
discharges are not common in deserts. Notable exceptions occur in regions of carbonate or volcanic rocks which can form extensive aquifers
capable of collecting water from a large area and, because of the high
permeabilities of these rocks, of discharging the water in local concentrated