The definition of drought is not very simple, and the question "What is drought?" continues to pose a problem (Sivakumar, 1991). This is because drought could mean different things to different people, and there are probably as many definitions of drought as there are users of water. In general, a drought is when a shortfall in precipitation creates a shortage of water, whether it is for crops, utilities, municipal water supply, recreation, wildlife, or other purposes. According to a WMO definition (Bogardi et al., 1994), "drought is a sustained, extended deficiency in precipitation."
Operational definitions of drought vary from place to place and are crucial to identify the beginning and intensity of drought. There are three main types of drought: meteorological, agricultural, and hydrological (National Drought Mitigation Center, 1996c).
1. Meteorological drought is an expression of rainfall departure from normal over some period of time. Meteorological drought definitions are usually region specific and are based on a thorough understanding of the climatology of the region.
2. Agricultural drought occurs when there is not enough soil moisture to meet the needs of crops at a particular time.
3. Hydrological drought refers to deficiencies in surface and subsurface water supplies. It is measured as stream flow and as lake, reservoir, and groundwater levels.
Some economists look at drought in socioeconomic terms. According to them, a socioeconomic drought is when physical water shortages start to affect supply and demand of goods.
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