Vijendra K Boken

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. A hydrological drought develops when the meteorological drought is prolonged and causes shortage of surface and groundwater in the region. An agricultural drought sets in due to soil moisture stress that leads to significant decline in crop yields (production per unit area), and a socioeconomic drought is a manifestation of continued drought of severe intensity that shatters the economy and sociopolitical situation in a region or country.

Meteorological drought is just an indicator of deficiency in precipitation, whereas hydrological and agricultural droughts are physical manifestations of meteorological drought. This book focuses on agricultural drought, which is the most complex category of drought. One can detect a hydrological drought by observing water levels in ponds, reservoirs, lakes, and rivers. However, such an instantaneous observation is not possible in the case of agricultural drought. Unlike a hydrological drought, an agricultural drought occurs over a large area, and its impact is not accurately assessed until after crops are harvested—a few months after the symptoms of agricultural drought begin to appear. The symptoms may be deficiency in precipitation, failure of rains/monsoon systems, and poor crop conditions.

Precipitation is one of several components of the hydrological cycle that is driven by atmospheric and oceanic circulations, together with gravity (Yevjevich et al., 1978). At times, the cycle experiences an abnormal change in its pattern due to changes in the patterns of atmospheric and ocean circulations. Such an atmospheric activity is called El Niño and Southern Oscillation (ENSO). Chapter 3 describes how this activity is linked to the occurrence of agricultural droughts. An agricultural drought at a global scale can cause severe food shortage or even famine in some countries, causing loss of both the human and livestock population. Chapters 21 and 22 focus on methods to help move livestock to greener pastures at times of droughts in Africa.

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