Evaluating Potential Climate Change Impacts on Water Resource Systems Operations: Case Studies of Portland, Oregon and Central Valley, California

Since its initial report in 1990, the Intergovernmental Panel on Climate Change has consistently noted that climate is changing (IPCC, 1990; 1996; 2001). These reports, the studies upon which they are based, and other studies indicate that water resources are particularly susceptible to the impacts of climate change (Frederick and Major, 1997; Gleick et al., 2000). The most significant impacts of climate change on U.S. water resources are expected to occur in the midlatitudes of the West, where the runoff cycle is largely determined by snow accumulation and melt patterns (Cohen et al., 2000). It is well documented that the effects of warmer climates on the seasonality of runoff in these regions will likely shift a portion of spring and summer melt runoff earlier in the year (Smith and Tirpac, 1989; Piechota and Dracup, 1996; Piechota and Dracup, 1997; Lettenmaier et al., 1999; IPCC, 2001). Despite the high degree of regulation in many western U.S. water supply systems, the impact of these shifts on runoff seasonality is generally negative. This is due to the significant water storage in snowpack that, under normal climates, is relied upon to augement low streamflows during relatively dry summers (Hamlet and Lettenmaier, 1999; VanRheenen et al., in review).

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