California’s Agricultural and Urban Water Supply Reliability and the Sacramento–San Joaquin Delta

Much of the water supplied in California for agriculture and cities is taken directly from the Sacramento–San Joaquin Delta (Delta) or indirectly from surface and groundwater diversions upstream. These water supplies have great economic and social value, and considerable ecosystem effects. Long thought of as the major source of water for economic growth in California, the reliability of water supplied from the Delta is threatened by drought, flood, climate change, earthquakes, growing water demands, and deteriorating conditions for endangered species and native ecosystems. Research in recent years has improved understanding of how management of the Delta ties together the quantity and quality of water available statewide. These ties run from the Sierra mountains and coastal streams, through the Central Valley, to the San Francisco Bay Area, and over the Tehachapi Mountains to southern California. For decades, Californians counted on reducing Delta outflows to supply water for growing water demands in its watershed and in water importing areas. With greater competition for water, concern for environmental effects, and a changing climate, the reliability of such supplies is now diminishing. This must lead to tighter accounting and modeling of water supplies in the Delta and throughout its watershed. This paper reviews issues about Delta water supplies, operations, regulations, and reliability; the economic value of supply; costs of unreliability in quantity and quality; and several directions for further scientific and technical work on water supply reliability.