A* essay answer for the exam question "Describe and explain the physical and human reasons for the water stress in South West USA".
Written by an A* A level student who scored full UMS in their Geography exams as well as having several of their essays chosen as model answers by the exam board i...
Describe and explain the physical and human reasons for the water stress in South West
USA
Water stress is defined as being where water availability is a constraint on human
activity and is often specified as being less than 1700m3 per person per year. The South
West USA—consisting of the four states of Nevada, Utah, Arizona and California—is
experiencing severe water stress as a result of a number of physical and human factors.
Being the main source of water in the South West and providing water for 40 million
people in seven states in addition to irrigating 4 million acres of farmland, the Colorado
River Basin has been called ‘The Lifeline of the South West’ but studies have shown that
this vital resource has lost nearly twice as much water as Lake Mead, Nevada—the
country’s largest reservoir’ in nine years.
There are a number of human factors behind the water crisis that the South West is
suffering from, which can be categorised according to their contribution either to
increasing demand or reducing supply. Globally, the twentieth century saw massive
increases in water consumption of 600% due to population growth, lifestyle changes
and industrialisation. The South West follows this wider trend, having increasing
domestic and industrial water footprints as rapid urbanisation and population growth
occurs, particularly in the desert sunbelt. In 1970 the total South West population
equalled 23 million but has grown by 110% to over 48 million by 2008. Population
increases of this scale require increased food production and consequently irrigation for
agricultural production but additionally, the nature of most of this growth as migration
to a better climate—for example, at times 3000 people a month were moving to Phoenix
— and location resulted in increased water demands due to lifestyle requirements. The
sunbelt lifestyle centres on luxury and recreation and therefore, despite being located in
a desert with minimal precipitation, swimming pools, spas and irrigated golf courses
and gardens are all expected. Furthermore, almost all arable agriculture requires
irrigation because of the lack of precipitation and as a consequence, California alone
accounts for over 20% of all irrigation water use in the USA. All of these factors result in
an increased water demand, but humans have also had an impact in reducing water
supplies by over-abstraction from aquifers, pollution—often resulting from farm
drainage of pesticides such as in the Salton Sea, California—leading to some water
sources being unusable and finally inefficient or damaged water supply infrastructure
that results in waste.
However, human factors that have resulted in an increased demand and a reduced
supply are not alone in causing the South West’s water stress. The physical geography of
this region means that precipitation is seasonal and predictable. For example, while
some 2,500 mm of snow may fall in the Rockies every year, less than 250mm of rain falls
further downstream due to being in the rainshadow of the Sierra Nevada mountain
range. The Colorado river basin also experienced high evaporation—because of
temperatures reaching 45°C alongside having a wide basin due to lateral erosion,
meaning a greater surface area of water is exposed— resulting in the loss of up to 95%
of rainfall. Moreover, most of the major rivers in California are fed by snowmelt from the
Sierra Nevada Mountains, but as this snowpack is reducing, there is a reduction in the
spring meltwater. In California in particular, only 22% of the 200-500mm annual
precipitation is available as runoff for human use to major loss as evapotranspiration
and a small percentage that flows out to the sea. The seasonality of rainfall—50% of
California’s rain falls between November and March—means seasonal shortages occur
and this unpredictability is expected to increase with climate change. As a result of these
factors, the South West is in general an arid region, experiencing little precipitation.
Further to this, due to high levels of evapotranspiration, much of the precipitation that
falls is lost, therefore contributing to water stress in the South West.
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