Environmental Engineering Reference
In-Depth Information
Figure 5.8
Cumulative percentage frequency graphs of
annual precipitation at Sheffield, 1883-1999 and Timimoun,
Algeria, 1912-60.
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 the ground 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
