Drought

Drought Definitions

There is no universally accepted definition of drought because many disciplinary perspectives of drought exist. Each discipline incorporates different physical, biological, and/or socioeconomic factors in its definition. A broad definition of drought is a deficiency of precipitation over an extended period of time, usually a season or more, which results in a water shortage for some activity, group, or economic sector. Droughts are classified as meteorological, agricultural, hydrological, or socioeconomic.

Meteorological drought is usually defined by a certain precipitation deficiency over a period of time. The thresholds chosen, such as 50 percent of normal precipitation over a six-month time period, vary by location and according to applications. Hence, most meteorological drought definitions relate actual precipitation departures to long-term average amounts. Meteorological droughts are a normal, recurring feature of climate and occur in virtually all climatic regimes. Meteorological drought definitions solely look at the water supply side.

Hydrological drought is defined by deficiencies in surface and subsurface water bodies, relative to average conditions. There is no direct relationship between precipitation amounts and the status of surface and subsurface water supplies in streams, lakes, reservoirs, and aquifers, because the hydrological system components often are used for irrigation, water supply, hydroelectric power production, canals, and flood control.

Agricultural drought links various characteristics of meteorological drought to agricultural impacts, focusing on precipitation shortages, differences between actual and potential evapotranspiration, and soil water deficits. A plant's demand for water is dependent on prevailing weather conditions, biological characteristics of the specific plant, its stage of growth, and the physical and biological properties of the soil. Agricultural drought is defined by the lack of availability of [Page 301]soil water to support crop and forage growth. This lack is also influenced by infiltration rates and the water-holding capacity of soil. During drought onset, agriculture is usually the first sector to experience drought because soil moisture will normally be the first component of the hydrological system to be affected. Drought events can disrupt food production systems and can be a significant natural trigger for famine.

Socioeconomic drought differs from the other types of drought in that it reflects the relationship between the supply and demand for some commodity or economic good that is dependent on precipitation, such as agricultural goods, livestock forage, drinking water, or hydroelectric power.

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