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DEFINITIONS OF DROUGHTS

Many drought definitions are adopted in different fields (meteorology, hydrology, economy of water resources), with reference to various hydrometeorologic variables.

Meteorological drought:

Meteorological definitions of drought are the most prevalent. They often define drought solely on the basis of the degree of dryness and the duration of the dry period. Thus, meteorological drought has been defined as a period of more than a particular number of days with precipitation less than some specified small amount.

Hydrologic Drought:

Definitions of hydrologic drought are concerned with the effects of dry spells on surface or subsurface hydrology, rather than with the meteorological explanation of the event. The frequency and severity of hydrologic drought is often defined on the basis of its influence on river basins. Hydrologic droughts are often out of phase with both meteorological and agricultural drought.

Agricultural Drought

Agricultural drought occurs when soil moisture is depleted to the extent that crop and pasture yields are significantly affected. Agricultural drought definitions link various characteristics of meteorological drought to agricultural impacts, focusing, for example, on precipitation shortages, departures from normal, or numerous meteorological factors such as evapotranspiration.

Socio-economic Drought

Definitions which express features of the socioeconomic effects of drought can also incorporate features of meteorological, agricultural, and hydrological drought (Wilhite and Glantz, 1985). They are usually associated with the supply and demand of some economic good. YevJevich (1967) has suggested that the time and space processes of supply and demand are the two basic processes that should be considered for an objective definition of drought.

3. MANAGEMENT USE OF DROUGHT INDICES

To water managers, drought means problems in meeting demand. In that sense, drought means not having sufficient water to meet demands because supplies fall belowexpected levels. The “expected levels” are socioeconomic, because expectations can beadjusted. Because of this link with socioeconomics, a drought index that will be useful to management must incorporate aspects of demand—that is, how adequate are supplies to meet demand? To design an index for a particular situation, the following approach might be used:Index available water supplies expected or mean water supplies.