Oases are interesting examples of situations where the water table is close to or
at the desert surface (seeDeserts). Oases have played important roles in human
geography in diverse countries such as Egypt, Saudi Arabia, Niger, Peru, Libya,
and the United States. In the midst of aridity, these contrasting locales of plentiful
water have provided people with shade, water and forage for animals, and irriga-
tion for crops since prehistoric times.
Groundwater depletion is a major problem in places dependent on groundwater
for water supplies. It is clear that many groundwater withdrawals represent “over-
draft” situations in which use is many times that of replenishment. The world
groundwater overdraft is estimated at 200 billion cubic meters per year such that
the current rate of use cannot be considered sustainable over the long term.
In places like Long Island and Florida, some places proximal to the oceans have
had withdrawal of freshwater allowing lenses of salty groundwater from under-
neath the ocean bottom to supplant the freshwater and make the well water too
salty for use. Land subsidence is an issue in areas with substantial withdrawals
of groundwater. The groundwater within fine-grained sedimentary rocks and loose
sediments forms part of the crustal mass maintaining the level of the surface.
Withdrawal of the mass represented by the water allows the compaction of the
water-bearing strata by the mass of the materials overhead. Subsidence has been
noticeable and damaging in places suchas the Central Valley of California,166 Groundwater
Groundwater is an important source of water for drinking and irrigation in many world regions.
Even deserts contain groundwater, although it may be located at considerable depth. (U.S.
Geological Survey)