چکیده:
Drought is a sustained and regionally extensive occurrence of below average natural water availability. It is mainly caused by low precipitation and high evaporation rates, but in regions with a cold climate, temperatures below zero can also give rise to a winter drought. Drought can be characterized as a deviation from normal conditions in the physical system (climate and hydrology), which is reflected in variables such as precipitation, soil water, groundwater and streamflow. Drought is a recurring and worldwide phenomenon having spatial and temporal characteristics that vary significantly from one region to another (Tallaksen & van Lanen, 2004). Drought should not be confused with aridity, which is a long-term average feature of a dry climate, or with water scarcity, which reflects conditions of long-term imbalances between available water resources and demands (Tallaksen & van Lanen, 2004; Working Group on Water Scarcity and Drought, 2006). It is important, however, to note that the most severe human consequences of drought are often found in arid or semi-arid regions where water availability is already low under normal conditions (aridity), demand is close to, or exceeds, natural availability and society seldom lacks the capacity to mitigate or adapt to drought.