Agricultural Drought and Its Monitoring and Prediction: Some Concepts

Droughts develop largely due to below-average precipitation over a land area, and they adversely affect various economic sectors in a region. Some of these adverse effects include reductions in agricultural production, hydropower generation, urban and rural water supplies, and industrial outputs. These effects lead to other consequences, secondary and tertiary, that further impact an economy. For instance, when agricultural production declines, food and other commodities tend to cost more and cause economic inflation in a society. Chain effects of persistent droughts can shatter an economy and even cause famine and sociopolitical upheaval in some countries. How does one define a drought? Usually, either precipitation or a form of drought impact is used to define a drought. Because precipitation and drought impacts vary spatially, there is a geographical dimension to definitions of drought. In Saudi Arabia or Libya, droughts are recognized after two to three years without significant rainfall, whereas in Bali (Indonesia), any period of six days or more without rain is considered drought (Dracup et al., 1980; Sen, 1990). In Egypt, any year in which the Nile does not flood is considered a drought year. More than 150 definitions of drought are available in the literature (Gibbs, 1975; Krishnan, 1979; Dracup et al., 1980; Wilhite and Glantz, 1987). For example, a drought can be characterized as climatological, meteorological, water management, socioeconomic, absolute, partial, dry spell, serious, severe, multiyear, design, critical, point, or regional (Palmer, 1965; Herbst et al., 1966; Joseph, 1970a, 1970b; Askew et al., 1971; Beard and Kubik, 1972; Karl, 1983; Santos, 1983; Alley, 1984; Chang, 1990). Often, the difference between an estimated water demand and an expected water supply in a region becomes the basis to define a drought for that region (Kumar and Panu, 1997). A few of the chapters in this book provide a brief description of drought definitions that have been adopted in some countries. Despite the variation in drought definitions, a drought is broadly categorized as meteorological, hydrological, agricultural, or socioeconomic. A meteorological drought is said to occur when seasonal or annual precipitation falls below its long-term average.