Snowpack, Rain and Reservoirs: How California Weathers Drought

California's reservoir system, fed by snow and rain, supplies water to people and wildlife. The state has almost 1,300 reservoirs, but only about 200 are storage reservoirs — many of the big ones part of the federal Central Valley Project and the California State Water Project. They catch winter precipitation for the dry summer. The state also relies on water "stored" in its snowpack, which melts slowly through summer.

California's twelve major reservoirs. USGS.

The storage reservoirs

A dozen major reservoirs hold about half the water in California's storage reservoirs. All are multipurpose, and most must meet environmental mandates, such as flows and cold water for anadromous fish.

• Nine store water in, or supply, the Sacramento and San Joaquin watersheds: Shasta, Oroville, Trinity, New Melones, Don Pedro, McClure (behind Exchequer Dam), Pine Flat, Folsom and Millerton.
• Three — San Luis, Castaic and Perris — are off-stream reservoirs that help run the two big water projects.
• Many smaller reservoirs mainly generate hydropower.

Together they can hold almost 45 million acre-feet, but they hold less, for many reasons.

Snow refills the reservoirs

Winter and spring snow melts through the year and refills reservoirs. Early-spring snowpack "contains about 70 percent as much water, on average, as the long-term average combination of the major and 'other' reservoirs."

USGS research hydrologist Michael Dettinger and California State Climatologist Michael Anderson compared reservoir storage with April 1 snowpack every year since 1970, for the 12 major and 148 smaller reservoirs. The major reservoirs, they found, have been run aggressively to ease drought and floods; the smaller ones haven't.

Water stored in 173 California reservoirs — 12 major, 161 others — and in snowpack (dashed line), January 2005 to April 2016. Modified from Dettinger and Anderson (2015). USGS.

The 2015 drought:

• The governor proclaimed a drought emergency on January 17, 2014.
• By April 2015, snowpack water was essentially zero.
• Winter 2015 refill was only about 9 percent of normal, so after 3 years of drought the major reservoirs didn't get the runoff they needed.
• "The current challenge to statewide water managers is less the lack of water in the reservoirs and much more the lack of water in snowpack that normally would be expected to melt soon and replenish our reservoirs," the authors wrote. With climate change, snowless years like 2015 may become the norm.

Dry soils drink the rain

Rain replenishes soil, groundwater, streams and reservoirs. But in drought, dry soil absorbs much of it before any reaches a reservoir.

USGS scientists Alan Flint and Lorraine Flint estimated how much rain the Feather and Tuolumne watersheds — above Oroville and Don Pedro — would need to recover:

That's optimistic — it assumes all the rain falls at once with no drainage or evapotranspiration — and huge, next to average annual precipitation of 50 inches at the northern Sierra's eight-station index and 40.8 inches at the San Joaquin's five-station index.

Climatic water deficit

The climatic water deficit (CWD) is potential minus actual evapotranspiration: how much water plants would use if they could, minus what they got. In water-year 2015, CWD ran 125 to 400 percent above baseline across about 90 percent of the state. High CWD means a stressed landscape: in 2015 it brought forest die-off, extreme wildfire conditions and poor rangeland forage.

Climatic water deficit through water-year 2015, as a percentage of the 1951–1980 baseline. Updated from Flint and others (2013). USGS.

Studies like these help managers plan new storage and run existing reservoirs optimally.

Sources

Based on The Role of Snowpack, Rainfall, and Reservoirs in Buffering California Against Drought Effects, U.S. Geological Survey Fact Sheet 2016–3062 (publication page), quoting Dettinger and Anderson (2015). Figures restored from the fact sheet's PDF. A work of the United States government in the public domain.