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. As with other types of accounting, this task is harder and costlier when information is not well organized.
Here we present a 30-year set of water balances for the San Joaquin Valley, California’s largest agricultural region and home to more than half of the state’s irrigated acreage. The valley has multiple sources of surface water and is the largest user of groundwater in California. Of particular interest in this region is understanding the extent of long-term depletion of water stored in aquifers (overdraft). This practice will need to be curbed as water users implement the Sustainable Groundwater Management Act (SGMA). Ending overdraft can be achieved by augmenting other water supplies and reducing net water use.
Annual estimates for water use and availability are available from California’s Department of Water Resources (DWR) and the U.S. Bureau of Reclamation (USBR). However, this information is often difficult to navigate and piece together, and has some important gaps. State water balances (e.g., DWR Bulletin 160 – California Water Plan Updates) are sometimes hindered by missing or inadequate data. They also are produced with significant lags; the state’s last published water balances are for 2010, and do not include any of the latest drought years.
To develop a high-level, up-to-date picture of water supplies and uses in the valley, we combined available public data to develop estimates of annual regional water balances for the years 1986 to 2015. Similar exercises should be done at the sub-basin and hydrologic region scales to enable local water users to develop and implement plans for bringing their basins into long-term balance under SGMA, using these balance data and information as planning tools.
The downloadable Excel file contains annual data, calculations, and sources (a detailed description of data and methods is provided in Technical Appendix A of PPIC, 2017). Changes in groundwater storage are calculated as the residual in the water balance—the difference between other water supplies and net water use. 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. In drier years, stored surface water and groundwater pumping supplement annual inflows;
- Variance in Delta imports from the State Water Project (SWP) and the Central Valley Project (CVP) is largely independent of other valley conditions. 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.
- Differences between annual water supplies and net water use result in changes in surface and groundwater storage. Wet years tend to increase storage and dry years tend to draw more water from reservoirs and aquifers.
In most years, consumptive water use exceeds local surface and groundwater inflows, leading to overdraft of groundwater and concerns for long-term water use sustainability. Valley-wide, just a few wet years saw net groundwater recharge. The 30-year average annual groundwater overdraft is roughly 1.8 million acre-feet per year (MAF/yr). It averaged 2.2 MAF/yr from 2001-2015—the driest 15-year period since the 1920s. Local watershed inflows average about 55 percent of total inflow; Bay-Delta inflows from SWP and CVP imports average about 25 percent of supplies, and direct diversions from the Delta by north-valley water users about 6 percent. Average shares of water sources in the San Joaquin Valley are in the charts below.
Our recent report, Water Stress and a Changing San Joaquin Valley, describes a range of approaches for bringing the valley’s water accounts into long-term balance. A variety of factors, including SGMA, water market opportunities, water rights, and other regulatory and management decisions, will lead water managers to rely increasingly on water accounting at the basin and sub-basin levels.
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. Better water data collection and management—that is both more timely and transparent—is an important government role that will require stakeholders’ support. Improved accounting methods and data analysis across state and local agencies—as is common in several other western states—can facilitate better water management in this important region.
Excel File Notes:
- Remember to ‘Enable Macros’ if prompted.
- Sheets and workbook are protected to avoid accidental changes. As such, source data sheets are shown but formulas cannot be edited.
Alvar Escriva-Bou, Henry McCann, Ellen Hanak, Jay Lund, Brian Gray (2016). Accounting for California’s Water. 28 pp. Public Policy Institute of California, San Francisco, CA.
Alvar Escriva-Bou, Henry McCann, Elisa Blanco, Brian Gray, Ellen Hanak, Jay Lund, Bonnie Magnuson-Skeels, and Andrew Tweet (2016). Accounting for Water in Dry Regions: A Comparative Review. 177 pp. Public Policy Institute of California, San Francisco, CA.
Ellen Hanak, Jay Lund, Brad Arnold, Alvar Escriva-Bou, Brian Gray, Sarge Green, Thomas Harter, Richard Howitt, Duncan MacEwan, Josué Medellín-Azuara, Peter Moyle, and Nathaniel Seavy (2017). Water Stress and a Changing San Joaquin Valley. 48 pp. Public Policy Institute of California, San Francisco, CA.
Jay Lund (2016). “Better Accounting Begets Better Water Management.” California WaterBlog.
Jay Lund (2016). “How Much Water was Pumped from the Delta’s Banks Pumping Plant? A Mystery.” California WaterBlog.
California Department of Water Resources (2013). Bulletin 160: California Water Plan Update 2013, Volume 2: Regional Reports – San Joaquin River Hydrologic Region. Division of Statewide Integrated Water Management: Strategic Water Planning.
California Department of Water Resources (2013). Bulletin 160: California Water Plan Update 2013, Volume 2: Regional Reports – Tulare Lake Hydrologic Region. Division of Statewide Integrated Water Management: Strategic Water Planning.