Page 92 - The World About Us

No soil moisture
1.1.1
1.1.1
2.2.32.3.5

What are the causes and effects of drought?

Drought is not an easy thing to
define as it varies across the world
and at different mes of the year.
However, it is best to think of droughts
occurring when a region receives
significantly below average rainfall.
So drought is not just defined by a
period without rain but also by the
normal or expected rainfall in an area.
In some parts of western Ireland, one
or two weeks without rain could be
considered a drought.
However, in other parts of the
world, especially ones with already
low annual rainfall, drought may be
harder to iden fy. It may take months
or even years for a drought to be fully
recognised and its effects become
apparent. Fig.246 Drought in Somaliland, 2012, had a devasta ng impact on the region.
In recent years there have been
severe droughts across many parts of high density, sedentary herds results moisture and allows salt water to
the world. These include regions of in higher surface temperatures and travel upstream from river deltas.
Australia, China, parts of India, Brazil loss of soil moisture. When the rains Wasteful irriga on techniques can
and the USA, as well as in the Sahel do arrive, hard, baked soil surfaces increase salt levels in soils as well as
and Horn of Africa. reduce infiltra on. This results in the deplete precious water resources.
loss of the water as run off.
Some of these droughts have ­ Over-use of ground water: As river
lasted many years impac ng ­ Urban heat islands: large ci es flow is reduced, ground water is used
significantly on the people, especially create warmer zones reducing the in increasing amounts. This can lower
in agricultural produc on. Where ability of clouds to form close by. the water table and reduce soil
ecosystems are already under stress, ­ Dams and irriga on schemes: The moisture. Over-abstrac on means
drought can also have a severe increased number of dams reduces ground water levels may not recover
impact on the resilience of wildlife. the ability of rivers to flood down- fully between droughts making the
stream. This reduces the available soil next one worse.
Many of the droughts have been
associated with the El Niño Southern
Oscilla on (ENSO, see page 96). There Fig.247 The falling waters of Lake Mead. Lake Mead is the USA’s largest reservoir and sits behind
is an ongoing debate about whether the Hoover Dam on the Colorado River, south-east of Las Vegas. Years of drought, and over-use of the
there is a connec on between climate Colorado’s water, meant Lake Mead was just 40% full in August 2017; 19km³ short of full capacity.
change, El Niño events and what
appear to be increased incidences of
drought.
Whatever the role of climate
change, it is clear that human
ac vi es can make the impact of Former water level
droughts more severe and long- in Lake Mead.
las ng.
These impacts include:
­ Deforesta on: reduced transpira on
from forests reduces the total amount
of water available locally for precipita-
on.
­ Over-grazing: reduced vegeta on
cover caused by the intense grazing of

8792

The global circulation of the atmosphere controls weather and climate. Extreme weather conditions cause different natural weather hazards.