Environmental Engineering Reference
In-Depth Information
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 about 1995 was generally well below the 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 a
major effect on rainfall levels in Australia, Indonesia and even parts of the western
United States. Most of the severe droughts in these areas occur during phases of El Niño.
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 in the 1980s and 1990s there were a number of dry summers which
caused water supply problems and in some cases a reduction in agricultural production.
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
1975-6 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 this 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 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 27).
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.
worthwhile to grow the crop; if it is only 20 per cent, it is unlikely to be worth the risk.
Similarly, it is possible to determine in the same way how often, on average, it will be
necessary to irrigate crops.
Rainfall variability may also be expressed statistically by the coefficient of variation
(CV). This is calculated from the formula:
where is the average rainfall and
s
is the standard deviation. This defines the variability
relative to the mean. With a standard deviation of 200 mm and a mean annual