By Roger Bales
Rising global temperatures affect runoff from the Sierra Nevada, which provides over 60% of California’s developed water supply .
Over 80% of the runoff in the central and southern Sierra Nevada comes from the snow zone. In the wetter but lower-elevation northern Sierra, rainfall contributes over one-third of the annual runoff .
The average snowline, the elevation above which most precipitation is snow, goes from about 5,000 feet elevation in the north to 7,000 feet in the south. On average, each 1.8 degrees Fahrenheit (1 Celsius) of warming could push the snowline another 500 feet higher, reducing the snow total.
Shifts from snow to rain and earlier runoff also mean that more of the capacity behind existing dams will be allocated to flood control, further reducing their capacity for seasonal water-supply storage.
A wealth of research has established that the Sierra Nevada could see low- to no-snow winters for years at a time by the late 2040s if greenhouse-gas emissions don’t decline, with conditions worsening beyond that possible.
Warming will also increase water demand from forests as growing seasons lengthen and drive both drought stress leading to tree mortality and increased risk of high-severity wildfires .
Sustainability in a warming climate
Water storage is central to California’s water security.
Communities and farms can pump more groundwater when supplies are low, but the state has been pumping out more water than is replenished in wet years. Parts of the state rely on water from the Colorado River, whose dams provide for several years of water storage, but the basin lacks the runoff to fill the dams .
Public opposition has made it difficult to build new dams, so better use of groundwater for both seasonal and multiyear storage is crucial.
The state’s Sustainable Groundwater Management Act requires local agencies to develop sustainability plans. That provides some hope that groundwater pumping and replenishment can be brought into balance, most likely by leaving some cropland unplanted. Managed aquifer recharge south of the Sacramento-San Joaquin Delta is gradually expanding, and much more can be done.
If the state doesn’t do more, including tactics such as applying desalination technology to make saltwater usable, urban areas can expect the 25% cuts in water use put in place during the 2012-15 drought to be more common and potentially even deeper.
California’s water resources can provide for a healthy environment, robust economy and sustainable agricultural use. Achieving this will require upgrading both natural infrastructure—headwaters forests, floodplains and groundwater recharge in agricultural areas—and built infrastructure, such as canals, spillways and levees. The information is available ; officials now have to follow through.
Roger Bales is a distinguished professor of engineering at the University of California, Merced.