I. HYDROGEOLOGY OF ARID REGIONS
3
limestone may contain streams flowing in caverns, probably less than 1%
of the springs and wells in deserts derive their water from such sources.
Most underground water seeps slowly through small, interconnected openings in rocks and soil. Water-bearing materials may be confined locally
to narrow valleys but more commonly are widespread tabular zones which
underlie thousands of square kilometers of desert.
Modern research into the hydrogeology of arid regions is advancing rapidly along several lines such as hydrochemistry, exploration techniques,
and economics of development. Despite the high level of activity, many
questions remain. Our lack of knowledge stems in part from the difficulties
of studying remote and inhospitable regions, but it stems mostly from the
nature of the problems. Almost flat water tables and potentiometric gradients make determinations of directions of groundwater flow difficult. Lack
of rain during long periods makes direct measurements of the hydrologie
balance, including groundwater recharge, almost impossible. Excessive
depths to reach water make the collection of hydrogeologic data difficult
and expensive. Finally, in areas of artificial discharge of groundwater,
levels in wells drop rapidly and do not lend themselves to steady-state
analysis of groundwater movement.
II. Laws of Groundwater Motion
The laws of groundwater motion must be understood, at least in their
rudimentary forms, before the general aspects of desert hydrogeology can
be pieced together. Water in the subsurface will move in response to a
number of energy sources. Differences in heat, surface tension, electrical
potentials, pressure, gravity, and water chemistry will interact in a complicated manner to cause water to move. Normally, surface tension and gravity are most important where water and air occur together above the water
table. Below the water table, and in other zones fully saturated with water,
pressure and gravity are most important.
Movement of soil moisture owing to temperature gradients is a phenomenon observed commonly in desert regions. After a series of warm
days, rapid, radiant cooling of thin, flat rocks, dark highway pavements,
and other surficial objects will cause an accumulation of moisture on their
undersides. Near bodies of desert brine, chemical osmosis may be effective
through clays and silts. Theoretically, under special circumstances, chemical osmosis could be the dominant force causing water motion. Of the
energy sources listed above, only natural electrical potentials are considered negligible within desert environments.
Forces which arise from the surface tension of water are called capillary
3
limestone may contain streams flowing in caverns, probably less than 1%
of the springs and wells in deserts derive their water from such sources.
Most underground water seeps slowly through small, interconnected openings in rocks and soil. Water-bearing materials may be confined locally
to narrow valleys but more commonly are widespread tabular zones which
underlie thousands of square kilometers of desert.
Modern research into the hydrogeology of arid regions is advancing rapidly along several lines such as hydrochemistry, exploration techniques,
and economics of development. Despite the high level of activity, many
questions remain. Our lack of knowledge stems in part from the difficulties
of studying remote and inhospitable regions, but it stems mostly from the
nature of the problems. Almost flat water tables and potentiometric gradients make determinations of directions of groundwater flow difficult. Lack
of rain during long periods makes direct measurements of the hydrologie
balance, including groundwater recharge, almost impossible. Excessive
depths to reach water make the collection of hydrogeologic data difficult
and expensive. Finally, in areas of artificial discharge of groundwater,
levels in wells drop rapidly and do not lend themselves to steady-state
analysis of groundwater movement.
II. Laws of Groundwater Motion
The laws of groundwater motion must be understood, at least in their
rudimentary forms, before the general aspects of desert hydrogeology can
be pieced together. Water in the subsurface will move in response to a
number of energy sources. Differences in heat, surface tension, electrical
potentials, pressure, gravity, and water chemistry will interact in a complicated manner to cause water to move. Normally, surface tension and gravity are most important where water and air occur together above the water
table. Below the water table, and in other zones fully saturated with water,
pressure and gravity are most important.
Movement of soil moisture owing to temperature gradients is a phenomenon observed commonly in desert regions. After a series of warm
days, rapid, radiant cooling of thin, flat rocks, dark highway pavements,
and other surficial objects will cause an accumulation of moisture on their
undersides. Near bodies of desert brine, chemical osmosis may be effective
through clays and silts. Theoretically, under special circumstances, chemical osmosis could be the dominant force causing water motion. Of the
energy sources listed above, only natural electrical potentials are considered negligible within desert environments.
Forces which arise from the surface tension of water are called capillary