Joint behavior of drought characteristics under climate change is evaluated using copula method which has recently attained popularity in analysis of complex hydrologic systems with correlated variables. Trivariate copulas are applied in this study to analyze the major drought variables; duration, severity, and intensity in the Upper Klamath River basin in Oregon. Results show that, among the variables, duration-severity is the most correlated pair whereas duration-intensity is the least correlated one. The impact of climate change on future droughts is evaluated using five Global Climate Models (GCMs) under one emission scenario. Comparing to the historical events, an overall decrease in drought duration and severity is estimated for the time period of 2020-2090 and the maximum duration is shown a decrease from 8 months to 5 months. Among the five GCMs employed in this study, GFDL-CM2.1 and CSIRO-MK3.0 are recognized as the wettest and driest projections, respectively. High uncertainty associated with GCM products is demonstrated in the analysis of return period by means of bivariate copulas; however, all projections result in larger return periods; i.e., less frequent droughts comparing to historical droughts during the reference period.
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