San Francisco Estuary and Watershed Science

Water budgets integrate and summarize the water inputs and outputs that are essential for effective water resources management. Using water data collected from different sources, we constructed three water budgets (a 12-year annual average, a wet year, and a critically dry year) for the Sacramento–San Joaquin Delta (Delta), the Sacramento River (SR) watershed, and the San Joaquin River (SJR) watershed. Although multiple water budgets for the Delta exist, the water budgets presented here are the first to provide all three of the following: (1) water budgets for the entire Delta watershed, divided into management-relevant components, (2) comparisons between wet and dry years and between different regions of the watershed, and (3) discussion of major gaps and uncertainties in the available water data to guide and inform future data collection and water management. Results show that, from 1998 to 2009, the Delta received 24.2 million acre feet (maf) of water each year on average, which primarily exited the Delta as river outflow (71%), water exports (22%), and evapotranspiration (ET; 6%). The SR watershed received 56.9 maf of water (95% as precipitation). The major outputs from the SR watershed were ET (63%) and flows to the Delta (34%). In the SJR watershed, total water input was 28.7 maf composed of precipitation (74%), water imported from the Delta (18%), and storage depletion (7%). The major outputs from the SJR watershed were ET (65%), water exports (19%), and flows to the Delta (14%). Most values varied greatly from year to year. Although streamflows, water exports, and valley precipitation are relatively well measured and estimated, uncertainties are higher for groundwater storage change as well as for ET and precipitation in montane regions. Improvement in data collection and synthesis in these components is necessary to build a more detailed and accurate water budget.