A naturally occurring phenomenon that exists when precipitation has been significantly below normal recorded levels, causing serious hydrological imbalances that adversely affect land resource production systems.
Meteorological drought is defined solely on the basis of the degree of dryness (often in comparison to some normal or average amount) and the duration of the dry period and must be region-specific.
Agricultural drought focuses on factors such as differences between actual and potential evapo-transpiration and soil-water deficits, are crop-specific and depend heavily on the timing of rain and dry periods relative to crop-cycles. Agricultural droughts can therefore occur in the absence of meteorological drought, and vice versa.
Hydrological drought is defined by deficiencies in surface and sub-surface water supplies, which lead to a lack of water availability to meet normal and specific water demands. Hydrological or water supply drought occurs less frequently than agricultural drought because considerable time elapses between precipitation deficiencies and declines in ground water and reservoir levels. Likewise, these components of the hydrologic system are usually the last to recover from longer term droughts.