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78 Part I California Water and one-fifth of surface water).'Small,but locally important,amounts of water are derived from other sources, including recycled wastewater and brackish water desalination. The state's primary imported water source is the Colorado River,which now provides 4.4 maf/year,California's allotment under the federal law that appor- tions Colorado River water among Arizona, California, and Nevada. These supplies have diminished from a high of 5.1 maf/year in the late 1990s and early 2000s as other states'demands have grown,limiting California's ability to draw on their allotments.2 Although supplies on the Colorado are also variable (and expected to diminish over time),'California's Colorado River entitlement is stable.Other interstate flows are relatively small and affect only local basins in the eastern Sierra Nevada and upper Klamath Basin. Much of California's runoff flows into the groundwater basins that underlie most of California's land area,where it often becomes a major source of water supply. Over the eight-year period shown in Table 2.1,groundwater pumps withdrew an average of 15 maf/year and accounted for 28 to 42 percent of gross agricultural and urban water use.Groundwater is more important in dry years and is particularly important for agricultural and urban uses in several regions(Figure 2.5).Most of this water is regularly replenished with irrigation water,artificial recharge (from managed recharge basins),seepage from stream flow,and precipitation. However, in some regions more water is pumped out of basins than is replenished over many years;this is known as overdraft. Chronic overdraft— essentially groundwater mining—could be as high as 2 maf/year on average state- wide(California Department of Water Resources 2009).As much as 1.4 maf/year of overdraft occurs from agricultural uses in the Tulare Basin(Kern,Tulare,and Kings Counties)(U.S.Geological Survey 2009).In the Central Coast,the Salinas Basin also suffers from chronic groundwater overdraft(about 19 taf/year[thou- sand acre-feet per year]),largely from agricultural pumping(Monterey County Water Resources Agency 2001; California Department of Water Resources 1995a).Although groundwater mining can help meet demands during droughts, it is an ultimately unsustainable water source(Harou and Lund 2008). 1. Over the 1998 to 2005 period,surface water reuse ranged from 8 to 15 maf/year and aquifer recharge ranged from 5 to 7 maf/year. 2. As discussed in Chapters 4 and 6,a variety of conservation and water transfer arrangements,known collectively as the Quantification Settlement Agreement of 2003,were developed to help wean California off these surplus water supplies from the Colorado River. 3. On projected declines in Colorado River supplies,see Barnett et al.(2008)and Rajagopalan et al.(2009).Although there is general agreement that supplies are likely to diminish with climate change,there is debate about the likely timing and the extent to which improved water management can forestall extreme shortages of supplies.