The Role of Snowpack, Rainfall, and Reservoirs in Buffering California Against Drought Impacts

California's vast reservoir system, fed by annual snow- and rainfall, plays an important part in providing water to the State's human and wildlife population. There are almost 1,300 reservoirs throughout the State, but only approximately 200 of them are considered storage reservoirs, and many of the larger ones are critical components of the Central Valley and State Water Project facilities. Storage reservoirs capture winter precipitation for use in California's dry summer months. In addition to engineered reservoir storage, California also depends on water "stored" in the statewide snowpack to significantly augment the State's water supply as it melts slowly over the course of the summer.

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The Role of Storage Reservoirs

Of the State's almost 200 storage reservoirs, about a dozen major reservoirs hold about half of the water stored in California's reservoirs (Dettinger and Anderson, 2015). Each of these major reservoirs is multipurpose, and operated to meet environmental mandates, including providing flows and cold water storage for anadromous fish. Nine of these 12 major reservoirs either impound storage in, or provide water supply to, the Sacramento River and San Joaquin River watersheds. These reservoirs include Shasta, Oroville, Trinity, New Melones, Don Pedro, Exchequer, Pine Flat, Folsom, and Millerton. The other three major reservoirs - San Luis, Castaic, and Perris - are off-stream storage reservoirs built to help optimize the operations of either the Federal Central Valley Project, or the California State Water Project. Many of the smaller reservoirs are operated primarily for hydropower production, and supply millions of megawatts of water-powered electricity to the State's population. These reservoirs - large and small - all play an important role in keeping the California community thriving. Despite having a cumulative storage capacity of almost 45 million cubic acre-feet, these reservoirs currently hold much less than their full capacity for a variety of reasons.

Snow & Reservoirs

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Snowpack plays an important role in keeping California's reservoirs full. Winter and spring snowpack typically melt gradually throughout the year, flowing into and refilling reservoirs. Snowpack accounts for the bulk of California's water source and storage, as early spring snowpack "contains about 70% as much water, on average, as the long-term average combination of the major and 'other' reservoirs" (Dettinger and Anderson, 2015).

Recently, Michael Dettinger, a U.S. Geological Survey Research Hydrologist, collaborated with Michael Anderson, California State climatologist, to investigate California's reservoirs' long-term water storage. They looked as far back as 1970 to determine the reservoir's involvement in California's water landscape, specifically studying how annual snowpack has historically impacted the water supply within the 12 major reservoirs and 148 of the smaller reservoirs. They then analyzed the recorded snow water content to determine the total water in the snowpack for every April 1 since 1970. By examining the correlation between these two sets of data, Dettinger and Anderson concluded the 12 major reservoirs have, historically, been operated aggressively to alleviate the impacts of drought and flood. They determined that the inter- and intra-seasonal managed fluctuations of the 148 smaller reservoirs were not as significantly pronounced.

Dettinger and Anderson also used this understanding of the historical relationship between snowpack and reservoir usage to investigate how the current California drought affected reservoir storage. In April 2015, the California Department of Water Resources measured the snow water content as essentially zero. Because the April snow water content helps recharge surface reservoir storage during the spring and summer months, a snow water deficit results in storage reservoirs - depleted throughout the year - to go without crucial refilling. Dettinger and Anderson determined that reservoir replenishment in winter 2015 was only about 9% of normal. Thus in 2015, California's major reservoirs - which are important tools to manage water supply through drought conditions - did not receive the snowpack runoff necessary to refill them after three years of drought. The authors state, "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." Unfortunately, due to the expected consequences of climate change, the lack of snow storage experienced in the current drought could become more the norm than the exception in years to come.

Rain, Soils, & Reservoirs

Rainfall is another essential water source. It replenishes water in soils, groundwater, streams, lakes, and reservoirs alike. During droughts, dry soils absorb and store a substantial amount of water in the watersheds above reservoirs, preventing rainfall from flowing into and refilling reservoirs.

USGS scientists Alan Flint and Lorraine Flint recently analyzed the current drought conditions for two of California watersheds - the Feather and Tuolumne. The Feather and Tuolumne watersheds flow into the Oroville and Don Pedro Reservoirs, respectively. They examined the dry soil in the watersheds, seeking to estimate the rainfall needed to recover them from the drought. Although intense rain could cause enough runoff to be generated without the soil moisture deficit being corrected, Flint and Flint determined that significant rainfall would first be needed to fill the current soil moisture deficit. While intense rainfall will generate runoff into reservoirs, much of the precipitation would go to amend the soil moisture deficit before becoming the greatly-needed inflow to the reservoirs. Flint and Flint predict that - when considering the soil moisture deficit and the volume of the reservoirs - large storms would be needed to alleviate storage deficit conditions in Oroville and Don Pedro Reservoirs caused by the drought. They suggest the Oroville Reservoir would need a storm totaling 14.8 inches to fill completely, while the Don Pedro Reservoir would need a storm totaling 15.9 inches to fill. These values are an optimistic estimate as they assume all the rainfall happens at once, with no drainage or evapotranspiration. These storms are sizable when considering the average annual precipitation of the 8-Station Index (i.e. eight monitoring stations in the Northern Sierra area) is 50 inches, and of the 5-Station Index (i.e. five stations in the San Joaquin area) is 40.8 inches.

One concept scientists use to understand soil water deficit is the climatic water deficit (CWD). This indicator is calculated as potential evapotransipiration (the amount of water used by plants), minus actual evapotranspiration, and reflects the loss of water in the soil, as well as the soil-water deficit accumulated throughout the year as a result of landscape vegetation demands. Water-year 2015 accumulated between 125% and 400% greater CWD than baseline over about 90% of the state. In the Central Valley that has deep soils, and for the Modoc Plateau that had July precipitation the CWD was normal for water-year 2015. High CWD indicates high landscape stress, which in California has lead to massive forest die-off and extreme wildfire conditions in 2015, as well as very poor forage conditions on rangelands.

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The studies conducted by Dettinger and Anderson, and Flint and Flint, are telling of the impact both snow and rain have upon our vital California Reservoirs. Without a doubt, California needs precipitation to help alleviate drought conditions, but studies such as these can help managers plan for new water storage options, and to optimally manage current reservoirs so that Californians can be better prepared for future drought.