The transboundary setting of California's water and hydropower systems: linkages between the Sierra Nevada, Columbia, and Colorado hydroclimates.

Climate fluctuations are an environmental stress that must be factored into our designs for water resources, power, and other societal and environmental concerns. Under California's Mediterranean setting, winter and summer climate fluctuations both have important consequences. Winter climatic conditions determine the rates of water delivery to the state, and summer conditions determine most demands for water and energy. Both are dictated by spatially and temporally structured climate patterns over the Pacific and North America. Winter climatic conditions have particularly strong impacts on hydropower production and on San Francisco Bay/Delta water quality. It is thus noteworthy that precipitation from winter storms in California is more variable than in neighboring regions. For example, annual discharge from the Sacramento-San Joaquin system has a coefficient of variation (standard deviation/mean) of 44% compared to 19% in the Columbia Basin and 33% in the Colorado Basin. Also, in California, multi-year droughts occur more often than would be expected by chance, but wet years do not exhibit such persistence. A crucial aspect of California's climate stresses is that they influence conditions over broad spatial scales. Climate patterns that cause the state's climatic fluctuations typically reach well beyond its boundaries. This breadth affects California because much of the energy and water used here is supplied by distant parts of the state as well as from the Northwest and Southwest. When dry winters occur in the Sierra Nevada, they also tend to occur in the Columbia and Colorado Basins. These regional scales, coupled with California's reliance on resources from an especially broad region, including power from the Columbia and Colorado Basins and water from the Colorado, make the state especially vulnerable to climate fluctuations. These vulnerabilities are likely to grow as the population and demands for resources in the region continue to grow.