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the set. Topics covered include water rights, ground- water, water recycling and reuse, agricultural drainage, water conservation, the Sacramento- San Joaquin Delta, San Francisco Bay, the Colorado River and flood management. THE RESOURCE California's climate is considered "Mediterranean," characterized by warm, dry summers and mild winters. While the state's generally sunny and pleasant weather is one of its great attractions, it also can be unpredictable with flooding followed by drought and few years of "normal" precipitation. Precipitation averages about 193 million acre-feet per year. (An acre-foot, the common measurement for water, equals 325.851 gallons, or enough water to cover a football field to a depth of one foot.) About 121 million acre-feet are lost to evaporation, perco- lation into the ground and transpiration by native plants. This leaves about 71 million acre-feet in average annual runoff, much of which eventually flows into California's two great river systems: the Sacramento and San Joaquin rivers. Both these rivers flow through the Central Valley and meet in the Delta. Runoff and precipitation in California can be quite variable. During the 1987-1992 drought, annual runoff was about half the average amount - approxi- mately 35 million acre-feet - whereas runoff following the 1995 flooding was about 130 million acre-feet. In 1999. runoff was estimated at nearly 80 million acre-feet. Precipitation is heaviest on the north coast and can reach more than 100 inches per year but decreases as one travels southward. Conversely, California's inland deserts bordering Mexico can receive less than 2 inches of rain annually. The state's mountain ranges also affect precipitation. The Coastal Range prevents moisture from reaching the dry Central Valley and the Sierra Nevada catches clouds before they reach Nevada. As clouds rise and cool they drop their moisture and feed the streams that flow down the mountain's western slope. The Sierra Nevada snowpack melts in the warmth of spring and runoff fills reservoirs where it is stored for use in the dry summer. In addition to the state's runoff, California annually receives about 1.4 million acre-feet in runoff from Oregon and 4.4 million acre-feet from the Colorado River. In a normal precipitation year, about half of the state's available surface water - 35 million acre- feet - is collected In over 1,300 local. state and federal reservoirs. This water is called "developed water" because it is managed, stored, diverted from rivers or otherwise developed for human or environ- mental use. A worker is dwwfed by the jl\'e huge penstocks and spifhray of Shasta Dam on the lippeI' Sacramento Ril'er. Roughly one-third of the state's water supply in a normal year comes from groundwater. Its usage can increase to 60 percent or more during drought years. California leads the nation in groundwater with- drawals, pumping about 16.6 million acre-feet annually, according to DWR. The state has substan- tial groundwater reserves that lie beneath about 40 percent of the state's land area. This water doesn't exist in underground lakes but in the pores and spaces between alluvial materials (sand. gravel. silt or clay) in water-bearing formations called aquifers. Of an estimated 850 million acre-feet of water stored in California's underground aquifers, only about 250 million acre-feet can be economically used. However, this is six times the 44 million acre-feet capacity of the state's surface water reservoirs. Eighty percent of the state's pumped groundwater goes towards agricultural irrigation. At the heart of California - and of most discussions about water - is the Sacramento-San Joaquin Delta. This 1,153 square mile maze of islands and interconnected waterways is located where the Sacramento and San Joaquin rivers converge and flow into San Francisco Bay. About 42 percent of the sfate's annual runoff flows through the Delta. Two-thirds of Californians gel all or part of their drinking water from the Delta by virtue of local, state or federal water projects that export water to the San Francisco Bay area and central and southern California. The Delta also is the largest estuary on the West Coast boasting hundreds of species of birds that travel along the Pacific Flyway. It is home to 28 native and 28 non-native fish populations in addition to the salmon and steel head species that migrate through the Delta on their journey to and from the ocean. 3