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Perennial gravel streams are usually dominated by a bed material stratified in terms of grain size, with a coarse surface layer overlying a finer subsurface (Figure 1a). However, field observations from some ephemeral gravel streams in the Middle East and other locations exhibit the opposite stratification (Figure 1b). Recent studies have found that under the same range of flow strengths, such an ephemeral stream has a much higher transport efficiency of bed load sediment (the movement of particles by rolling and sliding on the bed, or by making short hops of a few grain diameters in distance), which suggests a completely different behavior from perennial streams [e.g., Laronne and Reid, 1993; Reid and Laronne, 1995].

Implementation of adequate measures to assess and monitor droughts is recognized as a major matter challenging researchers involved in water resources management. The objective of this study is to assess the hydrologic drought characteristics from the historical rainfall records of Kuwait with arid environment by employing the criterion of Standardized Precipitation Index (SPI). A wide range of monthly total precipitation data from January 1967 to December 2009 is used for the assessment. The computation of the SPI series is performed for intermediate- and long-time scales of 3, 6, 12, and 24 months. The drought severity and duration are also estimated. The bivariate probability distribution for these two drought characteristics is constructed by using Clayton copula. It has been shown that the drought SPI series for the time scales examined have no systematic trend component but a seasonal pattern related to rainfall data. The results are used to perform univariate and bivariate frequency analyses for the drought events. The study will help evaluating the risk of future droughts in the region, assessing their consequences on economy, environment, and society, and adopting measures for mitigating the effect of droughts.