California Institute for Energy and Environment (CIEE)

A series of drought simulations were performed using the California Department of Water Resources codes and historical datasets representing a range of droughts from mild to severe for time periods lasting up to 60 years. Land use, agricultural cropping patterns, and water demand were held fixed at the 1973-2003 mean and water supply decreased by effective amounts ranging between 25 and 50 percent for the Central Valley, representing light to severe drought types. An examination of the impacts include four sub-basins, the Sacramento Basin, the San Joaquin Basin, the Tulare Basin, and the Eastside Drainage. Model output results suggest the greatest impacts are at the San Joaquin and Tulare Basins, regions that are heavily irrigated. Surface diversions decrease by as much as 42 percent in these regions. Stream-to-aquifer flows reversed and aquifer storage dropped. Most significant was the decline in groundwater head for the severe drought cases, where results suggest the water table is unlikely to recovery within the foreseeable future. However, the overall response to such droughts is not as severe as anticipated and the northern Central Valley may act as groundwater insurance to sustain California during extended dry periods.