Meteorological Drought

Meteorological drought is measured on the basis of the degree of dryness in comparison to average amounts of rainfall for a particular area and the duration of the dry period. The common practice to date has been to use the percentage of normal rainfall as an indicator of drought. Less than 75% of normal rainfall is regarded as a severe meteorological drought but a shortfall of 80% of normal will cause crop and water shortages which will ultimately affect social and economic factors. Normal rainfall for a particular place is calculated over a 30 year period using rainfall figures for at least 30 years.

Agricultural Drought

Agricultural drought links the lack of precipitation or other sources of water supply such as river flow or groundwater to agricultural impacts. In more general terms, agricultural drought exists when root-zone soil moisture is insufficient to sustain crops between rainfall events. In this context, the status of soil water is deficit in the top meter of a soil profile may be used as a drought measure.

Hydrological Droughts

This concept is used to refer to the impacts that the lack of precipitation has on ground and surface water systems.

Different types of droughts are really different stages of the same natural and recurring process, all triggered by the lack of rainfall. It becomes more likely that other types of droughts will follow when a meteorological drought occurs for a long period of time, and the amount of land it affects is extensive. During every extended dry period, soil storage gets emptied first. If a lack of precipitation continues, then other "storages" are also affected. Therefore a hydrological drought usually comes after an agricultural drought as it takes longer for lack of precipitation to affect the river flow or groundwater level.

When drought ends and precipitation returns to normal or exceeds it, this sequence is repeated. Soil storage is replenished first, followed by flow increase in surface water, the replenishment of reservoirs and finally the recharge of ground water. The length of the recovery period is a function of the drought intensity and duration, and the amounts of subsequent precipitation.

Adapted from the International Water Management Institute: Drought Information Centre< and the National Drought Mitigation Centre