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Describe and explain the physical and human reasons for the water stresses in
the Southwest of the USA.
South West USA is made up of 4 states which have a combined population of 48.5 million
with the majority of people
concentrated along a strip between San
Francisco and San Diego in California. This
area is the world's 7th largest economy
with a combined GDP of $2,300 billion in
2008; however it is still an area suffering
from major water stress (see map.) The
best way to describe the area's water
supply is that it is low and uncertain;
these issues are caused due to both
physical and human factors.
The physical geography of the area is a
major contributor to why it has water problems annual precipitation levels in the region are
typically below 750mm with extensive desert areas having an average below 250mm per
year. The rainfall is seasonal (50% of California's rainfall falls between November and March)
and highly unpredictable, leading to great variations in the flow of the river. 65% of the
precipitation is lost through evaporation and transpiration (it can reach 40 degrees in some
areas), while 13% flows out to sea leaving only 22% as run off for human use.
One of the greatest contrasts in precipitation are found between the western slopes of the
Sierras in California and the valleys just to the east of this range; as the warm moist air from
the Pacific Ocean ascends up the western slopes of the Sierra Range, the air cools,
condensation takes place and most of the moisture falls as precipitation. As the air descends
down the eastern slope, it is warmed by compression, and very little precipitation occurs.
The effects of this mountain barrier are felt not only in the west but throughout the State,
with some areas left completely parched.
The low precipitation levels have resulted in prolonged droughts e.g. in 1987-1992 and
2005-2008; these could possibly become more frequent and severe in the future as a result
of climate change. Changes in global temperatures would also lead to less and more irregular
There are many human factors behind the water crisis; these factors result in an increase of
the demand of water and reduction in the supply of water. The main reason why demand for
water has increased is due to the exponential growth of the population California's
population has grown from only 2 million in 1900, to 10 million by 1950, and 37.7 million by
2007. It is likely to reach 45-50 million by 2025. There is also an increasing water footprint for
industry and agriculture; rapid urbanisation in the desert Sunbelt e.g. Phoenix and Las Vegas
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The increasing demand for water exceeds the natural
It is not just the overall size and rate of California's population that is creating the issues, but
also its spatial imbalance. 75% of the demand for water comes from the south whereas 75%
of precipitation falls in the north.
Human factors reducing supply include the over-abstraction from aquifers and the increase in
pollution which are making some water sources unusable.…read more
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What other future solutions are there to the water shortages in the Southwest of the USA?…read more