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74 Part I California Water Very little indoor water use is net use,unless the resulting wastewater is discharged to the sea.Most(but not all)landscape and agricultural irrigation becomes net water use,as it evapotranspires to the atmosphere. Net use can never exceed gross use.But because recoverable flow is often reused, total gross water use usually exceeds total flow into a region.This can be seen by comparing average statewide gross water use(about 83 maf/year)with the total available supplies over the same period(71 maf/year)(Table 2.1). Conservation actions often target reductions in gross water use.But only net water savings provide more water(Ward and Pulido-Velazquez 2008;Clemmens, Allen,and Burt 2008;Huffaker 2008;Hanak et al.2010;CALFED 2006;Scheierling, Young,and Cardon 2006).In agriculture,achieving significant net water savings generally requires switching to crops that consume less water or reducing irrigated land area.By contrast,irrigation efficiency investments may reduce gross water use per acre but increase net water use on farms by making it easier for farmers to stretch their gross supplies across additional acres of cropland.Reductions in net water use by agriculture usually imply reductions in agricultural production(Perry et al.2009). Even when they do not result in lower net use,reductions in water withdrawals from streams and groundwater basins can have environmental benefits,including improved stream flow;reduced pollution runoff into rivers,streams,and beaches (Noble et al.2003);and reduced energy use and costs for acquiring and treating water(California Energy Commission 2005).For example,a major means of manag- ing soil and aquifer salinization in the southern Central Valley has been to improve irrigation efficiencies,so that less salt-laden water from the Delta is applied to fields. Even though these irrigation improvements make little net water available for use, the resulting runoff is of better quality. and Tulare Basin, and 20 times as high as local runoff in the and Colorado River region(Figure 2.1). Water availability also varies by season and between years. California's Mediterranean climate has wet winters and very dry summers,reflected in the monthly variations in the Sacramento River's natural stream flow(Figure 2.2), the state's largest river. The historical record also shows both very wet years, often with substantial floods, and long multiyear droughts (Figure 2.3). The geologic record of the past 2,000 years shows even larger and longer droughts (Stine 1994).