Geoscience Reference
In-Depth Information
75 per cent in parts of south-east England. The
water is extracted from a chalk aquifer that by and
large receives a significant recharge during the
winter months. Apart from very dry periods (e.g.
the early 1990s) there is normally enough recharge
to sustain withdrawals. Not all groundwater is
recharged so readily. Many aquifers have built up
their water reserves over millions of years and receive
very little infiltrating rainfall on a year by year basis.
Much of the Saudi peninsula in the Middle East
is underlain by such an aquifer. The use of this water
at high rates may lead to groundwater depletion, a
serious long-term problem for water management.
The Ogallala aquifer Case Study introduces ground-
water depletion problems in the High Plains region
of the USA.
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Case study
OGALLALA AQUIFER DEPLETION
The Ogallala aquifer (also called the High Plains
aquifer) is a huge groundwater reserve underlying
an area of approximately 583,000 km
2
in the
Great Plains region of the USA. It stretches from
South Dakota to Texas and also underlies parts of
Nebraska, Wyoming, Colorado, Kansas and New
Mexico (see Figure 8.9).
The aquifer formed through erosion from the
Rocky Mountains to its west. The porous material
deposited from this erosion was filled with water
from rivers draining the mountains and crossing
the alluvial plains. This has created a water reserve
that in places is 300 m deep. A major problem is
that now the aquifer is isolated from the Rocky
Mountains as a recharge source and has to rely on
natural replenishment from local rainfall and
infiltration. This is a region that receives around
380-500mm of rainfall per annum and has very
high evaporation rates during the summer. The
climate is classified as semi-arid.
Ever since Europeans first settled on the Great
Plains the Ogallala aquifer has been an important
water source for irrigation and drinking water
supply. Since the 1940s there has been rapid
expansion in the amount of irrigated land in the
region (see Figure 8.10) so that in 1990 as much
as 95 per cent of water drawn from the aquifer was
used for irrigating agricultural land (McGuire and
Fischer, 1999). Improving technology has meant
South Dakota
Nebraska
OGALLALA
AQUIFER
Kansas
Oklahoma
D
allas
Texas
MEXICO
Gulf of Mexico
Figure 8.9
Location of the Ogallala aquifer in the
Midwest of the USA.
that the windmill driven irrigation that was
predominant in the 1940s and 1950s has been
replaced with pumps capable of extracting vast
amounts of water at a rapid rate. The result of this
