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Assessment of Climate Change Impacts on Drought Return Periods Using Copula Functions

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  • Drought events are usually characterized by their duration, severity, and intensity which are calculated based on different indices for drought recognition. Streamflow Drought Index (SDI) used in hydrological droughts is applied in this study to calculate drought variables of historical events in Upper Klamath River basin in Oregon. Historical extreme events in this area necessitate studies on possible potentials of future droughts in the region. While the return period of drought variables are mostly studied by separate probability distributions modeling individual variables, this study employs Copula functions as multivariate probability distributions to model correlated drought variables altogether within a single function. The analysis follows by development of trivariate return periods and conditional probabilities to assess drought occurrence based on joint behavior of its variables. The trivariate return period is developed for two different cases: either 1) all the variables exceed particular values or 2) each variable does. Furthermore, the impacts of climate change are investigated by application of six GCMs and one emission scenario for the future time period of 2020-2090. The results indicate less severe droughts with smaller duration in future for Upper Klamath River basin comparing to historical events which generally implies wetter climate for the region. Maximum duration of 8 months for historical droughts shrinks to 6 months for future droughts, and the maximum severity is reduced from 12 to 8 for employed index. Moreover, the GCM IPSL-CM4 predicts the most water availability in the region among other applied GCMs.
  • Presented at The Oregon Water Conference, May 24-25, 2011, Corvallis, OR.
  • KEYWORDS: drought event, copula function, trivariate return period, climate change impacts
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  • Corvallis, Oregon, USA
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