English

Abstract

JUNE 2015 Storage in California’s Reservoirs and Snowpack in This Time of Drought Michael D. Dettinger* and Michael L. Anderson 1 Volume 13, Issue 2 | June 2015 * Corresponding author: mddettin@usgs.gov; U.S. Geological Survey, La Jolla, CA 92093 USA and Scripps Institution of Oceanography, La Jolla, CA 92093 USA 1 Division of Flood Management, California Dept. of Water Resources, Sacramento, CA 95814 USA doi: http://dx.doi.org/10.15447/sfews.2015v13iss2art1 INTRODUCTION The San Francisco Bay and Sacramento–San Joaquin Delta (Delta) are the recipients of inflows from a watershed that spans much of California and that has ties to nearly the entire state. Historically, California has buffered its water supplies and flood risks both within—and beyond—the Delta’s catchment by devel- oping many reservoirs, large and small, high and low. Most of these reservoirs carry water from wet winter seasons—when water demands are low and flood risks are high—to dry, warm seasons (and years) when demands are high and little precipitation falls. Many reservoirs are also used to catch and delay (or spread in time) flood flows that otherwise might cause damage to communities and floodplains. This essay describes the status of surface-water and snow- pack storage conditions in California in spring 2015, providing context for better understanding where the state’s water stores stand as we enter summer 2015. RESERVOIR STORAGE About a dozen major reservoirs (listed in Figure 1) operated by state, local, or federal agencies, hold about half of the water stored in California’s reser- voirs, on average. Hundreds of other, mostly smaller reservoirs are scattered around the state and together store amounts of water roughly equal (on average) to the storage in the dozen major reservoirs. In early spring snowpack in the state’s mountains also con- tains about 70% as much water, on average, as the long-term average combination of the major and “other” reservoirs. Figure 1 shows the history of res- ervoir storage 1 in the dozen major reservoirs and in another 148 reservoirs across California (including two in the Klamath River basin just across the border in Oregon) during the past 45 years. As expected, in dry periods such as 1976–77, the late 1980s to early 1990s, the end of the 2000s and, again, during the present drought (2012–2015), the amount of water stored in California’s reservoirs declines and, in wet years such as 1978, 1983, 1998, 2005–2006 and 2011, storage in reservoirs recovers. The dozen major surface reservoirs listed in Figure 1 are used aggressively to ameliorate droughts and floods. As a consequence, their storage fluctuates more from winters to summers, and from year to year, than does the total of storage in the other res- ervoirs. For example, between April 2011 and April 2015, total storage in the major reservoirs declined by 50% while storage in the other reservoirs declined by 38%. Indeed, over the long term illustrated in Figure 1, the variance of monthly storage in the 1 Monthly reservoir-storage, precipitation, and statewide April 1 snow water content estimates were obtained from http://www.cdec.water. ca.gov, an open-access California Department of Water Resources data archive.