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california Water Today 73 populated deserts of Southern California but also the immense irrigated agri- cultural areas in the Tulare Basin and the Imperial Valley and rapidly grow- ing urban communities in the Palm Springs area. Most of urban Southern California also has little natural runoff. The large infrastructure projects of the mid-20th century, designed to import water from other regions, have allowed water use patterns to diverge starkly from the distribution of runoff. Net water use (Box 2.1) is twice as high as locally available supplies in the South Coast Gross and net water use: some water is reused Gross (or “applied”) water use is the water delivered to a home, business, or farm— not all of which is consumed. Some water—such as excess irrigation water and discharges from wastewater treatment plants—flows to streams, lakes, aquifers, or the sea (“return flow”). Some of this return flow (“recoverable flow”) is available for reuse, because it returns to freshwater streams, lakes, or canals or recharges groundwater basins. Net (or “consumptive”) water use is that part of gross water that is unavailable for reuse. Net use consists of (1) water consumed by people or plants, embodied in manufactured goods, or evaporated into the air (evapo - transpiration) and (2) water return flows discharged into saline or contaminated waters or groundwater basins (“nonrecoverable flow”). Once this water is used, it is generally not available for reuse within the watershed without prohibitive treatment cost. Gross water use (water delivered for a purpose) Evapotranspiration (crop use, evaporation from land and water) Return ﬂow (excess irrigation water, wastewater discharges) Nonrecoverable ﬂow (ﬂows into saline or polluted water bodies) Recoverable ﬂow (available for reuse) Net water use (unavailable for reuse) 2.1 (continued)