Articles | Volume 19, issue 10
Research article
22 Oct 2015
Research article |  | 22 Oct 2015

Reconstructing the natural hydrology of the San Francisco Bay–Delta watershed

P. Fox, P. H. Hutton, D. J. Howes, A. J. Draper, and L. Sears

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 long-term 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.

Short summary
The development of California was facilitated by redistributing water from the natural landscape to other uses. This development was accompanied by declines in native aquatic species, which have been attributed to reductions in Delta outflow. By reconstructing the natural landscape and using water balances to estimate natural Delta outflow, this flow is shown to be consistent with current outflow on a long-term annual average basis.