Accounting for Water in the San Joaquin Valley.
by Brad Arnold1, Alvar Escriva-Bou1,2, Jay Lund1, and Ellen Hanak2 University of California – Davis, Center for Watershed Sciences Public Policy Institute of California Accounting for water supplies and uses is fundamental to good water management, but it is often difficult and controversial to implement.
Total net water supply— from local and imported inflows, precipitation, and changes in storage (including groundwater overdraft or recharge)—must equal the sum of net water used or stored within the valley (in surface reservoirs and aquifers) plus exports and outflows.
These annual data show: Local inflows from Sierra Nevada watersheds vary wildly between years, and drive regional groundwater pumping; Net or “consumptive” water use—the water consumed by people or plants, evaporated into the air, or discharged into saline water bodies or groundwater basins—is fairly constant across these years.
These imports are affected by water conditions in the Sacramento Valley, Delta pumping regulations, and water demand in other importing regions (especially Southern California); San Joaquin River outflows also vary significantly, reflecting variable inflows from the Sierra Nevada watershed, as well as changes over time in environmental and water quality regulations on valley outflows.
The 30-year average annual groundwater overdraft is roughly 1.8 million acre-feet per year (MAF/yr).
Average shares of water sources in the San Joaquin Valley are in the charts below.
The valley’s overall dryness and the high variability between drought and wet years require better long-term water planning and more robust water accounting.
Accounting for California’s Water.
Bulletin 160: California Water Plan Update 2013, Volume 2: Regional Reports – San Joaquin River Hydrologic Region.