What is drought?
Drought is a recurrent feature of the climate that results from a shortfall in precipitation over an extended period of time, its inadequate timing compared to the needs of the vegetation cover, or a negative water balance due to an increased potential evapo-transpiration caused by high temperatures. These conditions may be exacerbated by strong winds, atmospheric blocking patterns and antecedent conditions in soil moisture, reservoirs and aquifers, for example. If this situation leads to an unusual and temporary deficit in water availability, it is termed a drought. Droughts are to be distinguished from aridity
, a permanent climatic feature, and from water scarcity
, a situation where the climatologically available water resources are insufficient to satisfy long-term average water requirements.
The exact definition of a drought depends on several factors, such as the prevailing effects on the hydrological system, the economic, environmental, or social sector analysed, and the related processes and impacts. Droughts are commonly grouped into three basic types. A meteorological drought
is generally defined as a period with an abnormal precipitation deficit, in relation to the long-term average conditions for a region. When a meteorological drought leads to a soil moisture deficit that limits water availability for natural vegetation and crops, the result is a soil moisture or agricultural drought
. A hydrological drought
is associated with the effects of periods of shortfalls of precipitation (including snowfall), on surface or sub-surface water supply (i.e. streamflow, reservoir and lake levels, and groundwater). A fourth type of drought – socioeconomic drought
– is sometimes defined, when the demand for some economic goods and services (e.g. water, animal fodder, food grains, fish, hydro-electric power) exceeds supply as a result of a weather-related shortfall in water availability.
Due to the different types of drought, drought monitoring is based on the analysis of a series of drought indicators
, representing different components of the hydrological cycle (e.g. precipitation, soil moisture, reservoir levels, river flow, groundwater levels) or impacts (e.g. vegetation water stress). Usually indicators represent statistical anomalies of the current situation with respect to the long-term climatology at a given location and period of time. As such they are a measure of the probabilistic severity of a given event.
In order to ease the interpretation and to provide information for decision-making, individual indicators can be logically combined into high-level indicators that highlight different warning levels with respect to a given economic sector or the environment. An example is the Combined Drought Indicator (CDI)
, for agricultural and ecosystem drought, as shown in EDO.