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Show areas where this has occurred, as in several localities of the San Joaquin, irrigated farming has failed. The maintenance of ground water as a firm source of supply depends on obtaining, over a pe- riod of years, as much replenishment as there is withdrawal. Various means of replenishment are available. Water can be stored temporarily in sur- face reservoirs, and then later released to the stream channels for underground storage where the ground will absorb it. For example, water stored in Friant Reservoir is now being released from the Madera Canal into various stream channels and sloughs for ground water replenishment. In this way, these channels, which previously served as sources of recharge only during the winter months when natural flow was available, are now made to function continuously, using water stored in Friant Reservoir. It is the practice of the Kern County Land Co., which controls the use of a major portion of the Kern River supply, to use surface water in con- junction with ground water supplies. If it finds that in one area ground water is becoming relatively high compared to another area, a greater portion of the surface supply is diverted to the latter area, in order to build up the ground water storage. This shifting of water may become desirable in other areas. The use of artificial percolation areas is common in some areas, particularly in southern California. Such areas have been used to a certain extent in the Central Valley, even though the opportunities are not so favorable. The method is being given further consideration. The technical problems associated with ground water replenishment are important to the future of the Central Valley. Study of the problem is now under way, but the program should be expanded substantially. There are very few data on the full capacity of the underground reservoir, its depth, or any barriers that might divide the basin into sub- basins. Little is known about the possibilities in the Sacramento or on west side San Joaquin. The whole underground situation needs study, as the full utilization of these storage possibilities may have a profound influence on the conservation of the water resources and the best and maximum utilization of the land. There have been differences of opinion as to the relative degree to which ground water and surface supplies should be used. In one case (Solano Proj- ect) some local interests were of the view that the proposed reservoir was too large and that it should be reduced in size and greater use made of ground water. In another case (Cache Creek) local inter- ests are in disagreement as to the extent to which surface reservoirs should be operated for surface irrigation as opposed to ground water replenish- ment. These are local problems and must be solved individually on the basis of local conditions which vary widely throughout the basin. To meet the ultimate water needs of the valley and the San Francisco Bay area, maximum use must be made of both surface and ground water. Some deep pumping has been developed, taking water from great depths, especially in the western part of the basin south of Bakersfield. This deep pumping has tapped some aquifers which either are trapped or originate in distant areas. The impor- tance of this supply and the manner of its replenish- ment are not now known. Conclusions Ground water resources are used independently and in conjunction with surface water supplies in the Central Valley to a much greater extent than in many other regions. Both ground water and surface water resources must be developed and utilized to the maximum feasible extent in order to meet the probable ultimate water needs of the basin. The use of the storage capacity of underground reservoirs is one of the important conservation pos- sibilities in the valley. Potential reservoir evapora- tion adds a cogent reason for as complete use of underground storage as is compatible with compre- hensive multiple-purpose development. The possi- bility of replenishment should be explored more fully as soon as possible, and full knowledge should be obtained as to all the characteristics and limita- tions of underground storage possibilities. Such knowledge should be available in planning for the full use and conservation of water resources. These studies also should include the possibilities of assuring the movement of water from the basin to remove toxic salts. A study should also be made of the value of the deep aquifers as a water supply. All of these studies are proposed by the Geologi- cal Survey, and they should receive every assistance to the end that the studies can be completed within a few vears. 133 |