Geoscience Reference
In-Depth Information
Drought
HUMAN IMPACT
In a world of increasing population and development the demand for water is continuously rising. Fresh water can
be obtained direct from rivers, by pumping from ground water or from storage reservoirs. As demand increases, so
it is possible, up to certain levels, to increase extraction from each of these systems. Unfortunately, precipitation is
not so regular that similar annual totals are received every year. When an area experiences a prolonged period of
below-average rainfall, drought conditions may eventually prevail.
Drought is not easy to define. We can think of it as a meteorological drought, and in Britain a drought used to be
defined as a period of fourteen consecutive days without rain. A climatological drought would be the result of a
longer period with little or no rainfall at a time when rainfall is expected. We may not consider the summer dry period
of the Mediterranean to be a climatological drought because rain is not expected, but dry weather in winter or spring
could produce a drought. We may also experience a hydrological drought where water levels in rivers and aquifers
are well below what would be expected, or an agricultural drought where the impact of reduced precipitation results
in crop failure.
Some climatic regimes do have less predictable rainfall, with high variability from year to year. As a result agricultural
or industrial planning, and even the supply of domestic water, become more difficult. With increasing demand the
balance with supply may become a problem during periods of below-average rainfall, or even during average
conditions. In recent periods many parts of the globe have been affected by drought. Perhaps the best known case
is that of the Sahel of West Africa, where rainfall between 1968 and the present has been generally well below
previous levels (
Figure 9.7
).
In addition it is now realized that the periodic changes in sea surface temperatures in
the South Pacific, known as El Niño or the El Niño-Southern Oscillation (ENSO), can have major effects on rainfall
levels in Australasia, Indonesia and even parts of the western United States. Most of the severe droughts in these
areas occur during phases of El Niño. See additional case study 'Australian droughts' on the support website at
Even areas which normally experience reliable rainfall can occasionally have prolonged periods of below-expected
values. In the 1960s much of the north-eastern United States had a dry period, with water levels in reservoirs falling
to record lows. In western Europe there have been a number of dry summers which have caused water supply
problems and in some cases a reduction in agricultural production. In 2003 this was associated with very high
temperatures that worsened the situation. The worst case for temperate latitudes is when two dry summers are
linked by a dry winter and there is no major recharge of the reservoirs or aquifers. This happened in 1975-76 in much
of north-west Europe. Over the sixteen-month period from May 1975 to August 1976 less than 50 per cent of average
precipitation fell in some areas.
In a developed society, resources can be used to increase the supply of water. Reservoirs can be enlarged, if this is
politically acceptable; increased water can be extracted from rivers, if it is environmentally acceptable; an improved
distribution system can be achieved by reducing leaks in the pipeline network or by linking water supplies in different
parts of the country, if that is economically acceptable. It is assumed that it is less likely that all areas of the country
will be suffering drought uniformly. In most cases such measures cost money and take time. In dry areas where
energy is cheap, desalination plants can be used to extract fresh water from the sea, as in Saudi Arabia.
In developing areas resources are less readily available. Steps to improve water supplies may be taken only through
international action during severe droughts, as in Ethiopia. Short-term measures may be taken to pump ground water
where it is available. This leads to a concentration of population and, in many cases, grazing animals around the new
supply, which may cause more problems than it solves. Some countries have tried to increase water storage by
constructing large dams to compensate for low river flows during drought. Lake Kariba on the Zambezi and Lake
Nasser on the Nile are good examples but they give rise to major environmental problems (see
Chapter 26).
Drought is something that affects all parts of the world but its impact varies according to the level of development
and the duration of the drought.
