cesses along by suggesting promising solutions, eliminating unfounded concerns, and forc ing a more rigorous and systematic view of problems and opportunities for policy makers.As in the case of Hetch Hetchy, mathemati cal models suggest insights that allow inter ested parties, media, and government officials to reformulate a classical water resource con troversy and envision potential new solutions.

Properties of optimal hedging for water supply releases from reservoirs are developed and discussed. The fundamental decision of how much water to release for beneficial use and retain for potential future use is examined analytically. Explicit correspondence is established between optimal hedging and the value of carryover storage. This more analytical view of hedging rules is useful for better understanding optimal hedging and simplifying numerical optimization of hedging operating rules. The derivations suggest the frequent optimality or near-optimality of two-point hedging policies for water supply operations.

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This paper presents results of a large-scale economic-engineering optimization model of California's water supply system. The results of this 4-year effort illustrate the value of optimization modeling for providing integrated information needed to manage a complex multipurpose water system. This information includes economic benefits of flexible operations, economic valuation of capacity expansion opportunities, estimating user willingness to pay for additional water, economic opportunity costs of environmental flows, and identification of promising conjunctive use and water transfer opportunities. The limitations of such modeling also are discussed. Overall, the results suggest improvements to system operation and water allocations with a statewide expected value potentially as high as $1.3 billion/year. Significant improvements in performance appear possible through water transfers and exchanges, conjunctive use, and various operational changes to increase flexibility. These changes also greatly reduce costs to agricultural and urban users of accommodating environmental requirements. Model results also suggest benefits for expanding selected conveyance and storage facilities.

The San Francisco Estuary and its upstream watershed have been highly altered by human development following the California Gold Rush in the mid-19th century. In this paper, we explore the inter-and intra-annual variability of freshwater flow to this estuary and the resulting salt intrusion under scenarios that represent pre-development and contemporary conditions. To place this comparison in context with the advent of systematic and accurate flow and salinity measurements in the estuary, we consider an additional "pre-project" scenario that represents early 20th-century water management (circa 1920), after major flood control and reclamation but before the introduction of large water storage, diversion, and export operations. We use an observed climate record that spans 82 years to compare freshwater flow associated with the scenarios' landscape and water use characteristics. Using published relationships between flow and salt intrusion length developed from three-dimensional hydrodynamic modeling, we evaluate the effect of these flow alterations as well as estuarine geometry modifications and historically observed sea level rise on salt intrusion. We conclude that the predevelopment estuary exhibited a more seasonally variable salinity regime, resulting from a more variable inflow regime from the upstream watershed.

Abstract.We evaluated the impact of landscape changes on the amount of delta outflow reaching San Francisco Bay. The natural landscape was reconstructed and water balances were used to estimate the long-term annual average delta outflow that would have occurred under natural landscape conditions if the climate from 1922 to 2009 were to repeat itself. These outflows are referred to as natural delta outflows and are the first published estimate of natural delta outflow. These natural delta outflows were then compared with current delta outflows for the same climate and existing landscape, including its re-engineered system of reservoirs, canals, aqueducts, and pumping plants.This analysis shows that the long-term, annual average delta outflow under current conditions is consistent with outflow under natural landscape conditions. The amount of water currently used by farms, cities, and others is about equal to the amount of water formerly used by native vegetation. Development of water resources in California's Central Valley transferred water formerly used by native vegetation to new beneficial uses without substantially reducing the longterm annual average supply to the San Francisco Bay-Delta estuary. Based on this finding, it is unlikely that observed declines in native freshwater aquatic species are the result of annual average delta outflow reductions.

Abstract. The San Francisco Estuary, composed of San Francisco Bay and the Sacramento–San Joaquin River Delta, is the largest estuary along the Pacific coast of the United States. The tributary watersheds of California's Central Valley are the principal sources of freshwater flow into the San Francisco Bay-Delta estuary. The Delta serves as one of the principal hubs of California's water system, which delivers 45% of the water used statewide to 25 million residents and 16 000 km2 of farmland. The development of California, from small-scale human settlements that co-existed with an environment rich in native vegetation to the eighth largest economy in the world was facilitated by reconfiguring the state's water resources to serve new uses: agriculture, industry, and a burgeoning population. The redistribution of water from native vegetation to other uses was accompanied by significant declines in native aquatic species that rely on the San Francisco Bay-Delta system. These declines have been attributed to a variety of causes, including reduction in the amount of freshwater reaching the San Francisco Bay-Delta watershed (Delta outflow); decreased sediment loads; increased nutrient loads; changes in nutrient stoichiometry; contaminants; introduced species; habitat degradation and loss; and shifts in the ocean–atmosphere system, among others. Among these stressors, only the volume of Delta outflow has been regulated in an effort to address the decline in aquatic species. As native species evolved under natural landscape conditions, prior to European settlement in the mid-18th century, we evaluated the impact of landscape changes on the amount of Delta outflow. We reconstructed the natural landscape and used water balances to estimate the long-term annual average Delta outflow that would have occurred under natural landscape conditions if the climate from 1922 to 2009 were to repeat. These outflows are referred to as "natural" Delta outflows and are the first reported estimate of natural Delta outflow. We then compared these "natural" Delta outflows with current Delta outflows for the same climate and the existing landscape, including its re-engineered system of reservoirs, canals, aqueducts and pumping plants. This analysis shows that the long-term, annual average Delta outflow under natural landscape conditions is equal to current Delta outflow because the amount of water currently used by farms, cities, and others is about equal to the amount of water formerly used by native vegetation. The development of water resources in California's Central Valley transferred water formerly used by native vegetation to new beneficial uses without reducing the long-term annual average supply to the San Francisco Bay-Delta estuary. Thus, it is unlikely that reductions in annual average Delta outflow have caused the decline in native freshwater aquatic species.

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