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Show Conclusions (1) Tlie agriculture of the Central Valley de- pends on. an adequate supply of irrigation water. Present supplies of surface water need to be aug- mented in order to relieve the pressure on ground water resources and to make more efficient use of the land resources already developed. Projects which would provide supplemental supplies to exist- ing farming areas should, generally speaking, take priority over new land developments and should proceed rapidly. (2) Tlie large increase in population in and adjacent to the Central Valley has created heavy demands for land and farming opportunities. Regional needs for both farms and such foods as meat and dairy products justify further extension of irrigated land, possibly to the limit of the irrigable area in tbie Central Valley. 3. Ground Water Replenishment The Problem The extent to which problems associated with ground water replenishment affect water resources development in the Central Valley. The Situation Both gjround water and surface water supplies are used extensively in the basin. Approximately 40 percent of the land now irrigated is supplied by water pumped from subsurface supplies, some for their totaJ supply, and others for a supplemental supply when their rights to surface water are ex- hausted. This latter condition exists to some extent throughout the valley but diminishes toward the southern end of the San Joaquin Valley. Replenishment of the underground water in gen- eral has h»een through natural means-the sinking of winter rains into the ground, percolation from stream be=ds, canals, and ditches, and percolation by irrigation and waste water. During recent years there has been some "spreading" of water during the nonirjigation season to replenish the under- ground reservoir. This has been done especially in the Kings River area, where all of the winter water is stored underground. In this storage most evapo- ration losses are avoided, the water is recovered easily, ancd the demand on summer surface supplies by those farms using the underground reservoirs reduced. 132 During the period from 1921-39 the depth to the underground water surface in the Tulare Basin lowered by 25 or more feet over about one-half of the area. In some localities in the Tulare Basin the water surface was lowered more than 100 feet. In other parts of the basin water levels have been lowered as much as 10 feet per year. Water levels of 200 to 500 feet below the surface are not un- common, on the west side of the San Joaquin, and wells of 1,000 to 2,500 feet are needed. The total overdraft in the basin as a whole has been estimated to average 1.9 million acre-feet a year. Thus, over a period of 20 years an immense storage space, pos- sibly enough for more than 40 million acre-feet of water, has been made available. Ground water cannot be considered competitive with surface water. The two forms must supple- ment each other. The need for water is so great in the basin that full economical use must be made of both sources of supply, and the relative degree to which each should be used will depend on the neces- sity of the widest use of the limited water resource. Evaporation is definitely a limiting factor that must be considered in connection with surface storage in an area of high evaporation losses. Where reservoirs must hold water over from wet periods to dry as in the Central Valley, where the deficient period is seven or more years, evaporation loss becomes a major consideration. Even with an evaporation estimate of about 1 million acre-feet a year, during the 10-year period required to fill and hold the water to provide for periods of drought, the loss would be 10 million acre-feet or more, or a half year's supply for the whole valley. The capitalized value of the water lost by evapo- ration may well be large and indicates that serious thought should be given to any feasible means of preventing such loss. One means of reducing the loss is through use of underground reservoirs. The ground water storage capacity in the basin within economical pumping distance is at least equal to and may exceed the total potential surface storage capacity available in the basin. In those parts of the basin where good aquifers exist the un- derground reservoirs are the best storage because there are no evaporation losses from them. How- ever, water removed from the underground basins must be replaced. If it is not replaced the supply will eventually be depleted, or the ground water level will be lowered to such an extent that it is not economically feasible to continue pumping. In |
