Drought indices for drought risk assessment in a changing climate

As common as drought occurrences are around the world, a standard definition for drought isn’t recognized. Even among the experts who study droughts, a single standard definition is not easy to agree upon. In the simplest of meanings, drought can be identified as a deficit of precipitation from an expected average over an established time frame. To better define drought, one needs to establish the context in which the phenomenon and its associated impacts are being described. Wilhite and Glantz (1985) identified more than 150 published definitions of drought; from these, drought was classified into 4 types: meteorological, agricultural, hydrological, and socioeconomic. As their names imply, these diverse drought types impact different sectors, but in most instances the impacts related to each overlap both temporally and spatially (Figure 12.1).