Severity of Droughts in Arid Regions

The increasing water shortage in Jordan threatens the environment and badly affects most of the scoi-economic activities. In central Jordan, where major agricultural and water storage projects are located, the October ? May rainy season precipitation for two gauging sites; namely Amman Airport and Madaba, is used in the present study to investigate the occurrence of extreme droughts. The extreme regional dry year occurs when the precipitation in that year falls under the threshold of 1 Standard Deviation below the long-term mean at site. Historical precipitation data, 1938 ? 2005, are used to simulate 50000 precipitation data using multivariate stochastic simulation model of order 1, MAR(1). Drought analysis using the historical precipitation shows that extreme droughts fall out as individual years, while using generated precipitation droughts is distributed as 1 year events mostly, and as 2 and 3 years duration although less. This study also presents a theoretical model to estimate the return period of extreme droughts. The estimated return period of the 1 year extreme drought in central Jordan is around 10 years, whereas it is 160 years or more for droughts of longer durations. The probability that an extreme drought occurs at least once in a planning period of 25 years, defined as the risk, is found to be more than 90% for drought of 1 year duration, while it is found to be 15% or less for events of 2 years or more.