Drought, however it may be measured, is a deficit in available water for the specific purposes for which it is required (Palutikof et al., 1982). It can be defined from three points of view: meteorological, hydrological and agricultural. The first involves an arbitrary criterion of a shortfall in expected moisture (rainfall) resulting from a change in the region’s rain-producing mechanisms, some of which may be linked to shifts in the atmospheric circulation. The second definition is concerned with low streamflows and groundwater levels and, hence, inadequate water supply for irrigation, domestic and industrial uses. Agricultural drought considers supplies of water for crops and grasslands (Hounan et al., 1975).