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00257~ A desalting plant near San Diego has been in operation since March 1962. It is the first large multistage flash distillation plant in the United States. It was constructed by the United States Government and the State of California for the purpose of demonstrating the technical and economical aspects of the distilla- tion process for desalting sea water. The plant produces a million gallons of fresh water per day, or about 1,100 acre-feet per year, having 10 to 15 p.p.m. total dissolved solids. Water Use--Estimated present agricultural, municipal and indus- trial water requirements for the southern California area are pre- sented in table 10. The southern California Indian areas may be divided into two groups: those in the desert areas and those in the coastal areas. Present irrigation diversions on the desert reservations are esti- mated to be 2,600 acre-feet. Diversions on the reservations in the coastal area are estimated to be 2,100 acre-feet. Multiple-purpose reservoirs provide 46,000 surface acres of fishing water in southern California. In addition, California has nine fishing lakes covering 333 acres. Two State fish hatcheries circulate 22 c.f.s. from wells. Present Water Supply-Demand Relationship The present total water supply available to the Pacific Southwest, on a sustained-yield basis, is estimated to be about 16.4 million acre-feet per year. The total annual use of water in the area, including the 1.5 million acre-feet per year which must be guaranteed for delivery to Mexico, is estimated at about 18.0 million acre-feet. The difference between these two figures, 1.6 million acre-feet per year, represents the apparent overdraft of the ground- water basins under long-term water supply conditions. The actual ground-water overdraft is estimated to be almost 3 million acre-feet annually, the difference being surplus water which was available in past years and was used for power production, then allowed to flow across the International Boundary. It is estimated that with com- pletion of the Senator Wash regulating reservoir now under construc- tion, and with tighter scheduling and control of river operations which will be required in the future, such flows can be reduced to about 5,000 acre-feet per year. The ground-water supplies of the Pacific Southwest are a legacy from past ages and are being depleted at a rapid rate. At the present time nearly 300,000 acres of developed land are lying idle in Arizona alone for lack of an adequate water supply. Much of this idle land has resulted from the depletion of ground-water storage. Unless additional waters are made available, the retrenchment of the agri- cultural economy of the area is inevitable. Tables 11 and 12 present a summary of the present regional water supply-demand relationship. 111-9