Drought Definition, Meteorological Drought, Agricultural Drought, Hydrological Drought

Although many erroneously consider it a rare and random event, drought is a normal, recurrent feature of climate. It can occur in virtually all climatic zones, with its characteristics varying significantly from one region to another. Drought is a temporary aberration and differs from aridity since the latter is restricted to low rainfall regions and is a permanent feature of climate.

Drought is an insidious hazard of nature. Although it has scores of definitions, it originates from a deficiency of precipitation over an extended period of time, usually a season or more. Drought should be considered relative to the long-term average or normal balance between precipitation and evapo-transpiration (evaporation + transpiration) in a particular area. It is also related to the timing and the effectiveness of precipitation. Other climatic factors such as high temperatures, high wind, and low relative humidity are often associated with drought in many regions, including the Pacific basin.

Drought should not be viewed as merely a physical phenomenon or natural event. Its impacts on society result from the interplay between a natural event and the demand people place on water supply. Human beings often exacerbate the impact of drought. Recent droughts in both developing and developed countries, and the resulting economic and environmental impacts and personal hardships, have underscored the vulnerability of all societies to this "natural" hazard.

TYPES OF DROUGHT

Meteorological Drought: Usually defined on the basis of the degree of dryness (in comparison to some "normal" or average amount) and the duration of the dry spell.

Agricultural Drought: Links various meteorological characteristics to agricultural impacts, focusing on precipitation shortages, differences between actual and potential evapo-transpiration, soil water deficits, reduced ground water and/or reservoir levels. Agriculture is usually the first economic sector to be affected by drought.

Hydrological Drought: Associates the effect of periods of precipitation shortfalls on surface or subsurface water supply. The frequency and severity of hydrological drought is often defined on a watershed basin scale. Although climate is a primary contributor to hydrological drought, other factors such as changes in land use, land degradation, and the construction of dams all affect the hydrological characteristics of the basin.