In general, the term drought describes an extended period of abnormal dry weather with a virtual absence of precipitation, causing considerable hydrological inbalance, i.e. drought occurs, when evaporation and transpiration exceed precipitation of rain. When prolonged it causes damage to agriculture, depletion of ground and soil water, and limits water available for drinking, sanitation and industry in the area concerned. Streams and lakes dry up, and water-tables fall.
br>A general definiton of drought is when an area experiences at least three weeks with less than one-third of normal precipitation for the time of the year. However, because of the great variation in climate and moisture requirements, both between plants (biomes and agriculture) and human activities (e.g. domestic or industrial supply) no duration or definition is universally applicable.
In the UK an absolute drought is defined as a period of at least 15 consecutive days on none of which is there more than 0.2 mm of rainfall. Whereas in Libya, droughts are recognized only after two years without rain. These arbitrary definitions give no indication of the impact of drought. Droughts in densely populated areas reliant on agriculture, such as India and China, can have disastrous effects. Persistent periods of drought, such as occurred south of the Sahara in the 1970s and in central Australia in the early 1980s, lead to speculations about climatic change.
A major cause of drought in Britain is the persistence of warm anticyclones, and the displacement of mid-latitude depressions by blocking anticyclones. Droughts in Africa, for example in the Sahel in the 1970s or in Zimbabwe in the early 1990s, result from the failure of the inter-tropical convergence zone to move sufficiently far from the equator.
From a gardeners point of view there also is physiological drought , a condition of soil water being sufficient, but temporarily unavailable, as when the water is frozen, or when the rate of evapotranspiration exceeds the rate of uptake of water by a plant. In the latter case, the plant will wilt in the daytime, but recover overnight, when evapotranspiration ceases.