Geoscience Reference
In-Depth Information
developed and developing countries,
for lack of
understanding or management.
Ground water
Aquifers
. The other major source of water in dryland
areas is from ground water. In some parts of the world
huge aquifers lie under the ground surface. These are
water-bearing strata all or part of which is saturated with
water and is able to yield significant quantities. Originally
the water pressure in some aquifers was such that drilling
for water would result in a free flow of water at the surface.
Such artesian wells used to be common in Australia, where
the Great Artesian Basin under much of eastern Australia
allowed ready access to ground water. Water falling on the
Eastern Highlands of Australia sank into the water table
and helped to sustain groundwater levels.
Aquifers can be found in many parts of the world at
different levels below the surface (
Figure 26.9
).
They
achieve their greatest significance in dry areas where alter-
native sources are limited. Some deep aquifers are regarded
as 'fossil', since they are no longer being significantly
recharged. Much of the ground water under the Sahara
and the Middle East is fossil. The rate of exploitation of
this non-renewable resource means that water table lev-
els will decline rapidly. At the rates of exploitation by Saudi
Arabia in the 1990s, and assuming that 80 per cent of the
ground water can be extracted, the supply will be
exhausted in about fifty years. Much of this water is
pumped to the surface, using abundant and cheap local
fuel supplies, and used for irrigating crops of wheat. By
heavily subsidizing land, equipment and the irrigation
water, and by buying the wheat at several times the world
price, the Saudi government encouraged large-scale wheat
farming in the desert based on fossil ground water. This
policy has since changed and subsidies (and production)
are now much less.
Gravels - porous
Marls, slates - impermeable
Limestone - porous
Crystalline - impermeable
Water table
Rainfall
recharge
C
Deep well
non-flowing
A
Deep well
artesian
flowing at surface
B
Shallow well
non-flowing
Spring
s
Perched water table
in clay lens (unconfined)
Confined aquifer
Figure 26.9
Groundwater levels in an artesian basin.
