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72 Part i california Water Water Availability California’s water supplies are variable and diverse, with most water originating as precipitation. This is then supplemented with imported water, artificial and natural water reuse, and overdraft of groundwater. Geographic, Seasonal, and Interannual Disparities On average, roughly 200 million acre-feet (maf) of precipitation fall annually on California. Most of this water evaporates, particularly in the hottest and driest areas of the state. The remainder, known as “unimpaired runoff” (averaging about 75 maf/year) flows downhill into streams and groundwater basins, and becomes available for management and use (Table 2.1). The geographic disparities in natural water availability are particularly stark: About two-thirds of annual runoff comes from about one-fifth of California’s land area, primarily mountainous areas in the northern half of the state (Figure A). In contrast, the driest one-third of the state contributes only about 0.1 per- cent of total water availability. These driest areas include not only the sparsely Table 2.1 Regional average annual water availability, storage, and use, 1998–2005 (maf) Hydrologic region Precipitation Unimpaired water availability Storage capacity Water use Surface Ground Gross Net North Coast 53.0 26.0 3.8 11.0 22.0 22.0 San Francisco Bay 6.9 2.3 1.0 3.6 1.9 1.7 Central Coast 13.0 3.7 1.2 45.0 1.5 1.0 South Coast 11.0 2.2 3.1 140.0 5.0 4.2 Sacramento River 57.0 22.0 16.0 91.0 23.0 15.0 San Joaquin River 23.0 8.0 11.0 270.0 11.0 7.3 Tulare Lake 14.0 3.6 2.0 510.0 13.0 8.0 North Lahontan 6.9 2.2 1.2 8.0 0.9 0.5 South Lahontan 11.0 0.8 1.0 210.0 0.7 0.5 Colorado River 5.7 0.2 1.0 170.0 4.6 4.1 California 200.0 71.0 41.0 1,458.6 83.0 64.0 sOURcEs: Authors’ calculations using regional portfolio data from the california department of Water Resources (dWR) (2009); data on unimpaired water availability were calculated by J. Viers. NOTEs: The table shows average annual values in millions of acre-feet. see Table 2.2 for more details on water use, Figure 2.1 for a map of hydrologic regions, and box 2.1 on the distinction between gross and net use. Overall hydrologic region water availability estimates vary across sources and calculation methods. Unimpaired water availability includes surface runoff and groundwater infiltration; total volumes estimated by dWR were distributed across regions by Geographic information system modeling.