Definitions of Drought
Drought is defined in many ways, but in general, drought means there is insufficient water (from precipitation, storage and conveyance) to meet needs over an extended period of time, usually a season or more. Definitions vary depending on geographic location, management methods, and water demand. According to The National Drought Mitigation Center, there are four general ways to describe drought: meteorological, agricultural, hydrological, and socioeconomic.
Meteorological Drought - is defined usually by the degree of dryness (in comparison to some “normal” or average amount) and the duration of the dry period. (Generally, when there is a prolonged period of below average precipitation).
Agricultural Drought - is determined by the availability of moisture in soil. Normally, an agricultural drought occurs during dry, hot periods of low precipitation, but it can also occur during periods of average precipitation when soil conditions require extra water. Drought effects are different in irrigated and non-irrigated agriculture. In regions using irrigation, the impacts of agricultural droughts are usually lower than in regions where crops are not irrigated.
Hydrological Drought - is when water reserves in lakes, reservoirs, and aquifers fall below an established statistical average-usually after a meteorological drought. However, a hydrological drought can happen even during times of average precipitation, if demand for water is high and increased usage has lowered water reserves.
Socioeconomic Drought - is brought about by the combination of meteorological, agricultural, and hydrological drought. Socioeconomic drought occurs when the demand exceeds supply as a result of a weather-related shortfall in water supply.
In recent years, California policy makers have coined the term, regulatory drought. While not officially defined, a regulatory drought can occur in response to a policy decision. For example, Delta water exports have been curtailed due to litigation on endangered species in the Delta. Regulatory restrictions can affect hydrological drought by limiting the ability to move water to storage or distribution areas when needed.
The definition of drought being used depends on the speaker and context. For instance, when the governor issues a drought declaration, he is referring to a hydrological drought; however, when a climatologist speaks of drought, he/she is referring to meteorological drought. Perception of drought impacts is linked to these various definitions.