PROBLEMS FROM DEVELOPMENT 43 



part of the valley are too fine or other conditions are unfavor- 

 able for movement of water across the valley, and the "dry" 

 side thus receives negligible recharge. This is the situation in 

 San Joaquin Valley, Calif., where the pumping for irrigation 

 on the west slope is almost entirely from storage, and water 

 levels in wells have declined markedly as a result. 



West Slope of San Joaquin Valley, Calif. 6 Practically all 

 the replenishment to the ground-water reservoir of San 

 Joaquin Valley comes from the numerous streams that 

 drain the Sierra Nevada and thus enters the eastern part of 

 the ground-water reservoir. The ground waters along this 

 eastern slope are of different chemical character from those 

 under the west slope. The streams of the west slope are small 

 and dry a good part of each year; several carry enough ob- 

 noxious chemical constituents to make the water undesira- 

 ble for irrigation. The ground-water reservoir under the 

 west slope receives little water from these streams and local 

 precipitation. The contribution from the more abundant 

 supplies of the east slope is considered nil. 



In comparison with the recharge, the pumpage from wells 

 on the west slope is very large. It is estimated that about 

 1 million acre-feet, or one-seventh of the total in the valley, 

 was pumped from west-side wells in 1949. Water levels are 

 falling at a rate of 10 to 20 feet a year, and pumping levels 

 are 300 to 400 feet below ground surface in most of the ir- 

 rigated area. 



Commonly there are three zones of water-bearing forma- 

 tions under the west slope, of which the upper and lower 

 are generally strongly saline. The usable middle zone, which 



6 References: Ewing, F. A., "Irrigation and Drainage in San Joaquin Valley, 

 California," U.S. Soil Cons. Serv., Mimeo. rept., 1946, 80 pp. 



Forbes, Hyde, Geology of the San Joaquin Valley as Related 

 to the Sources and Occurrence of Ground-water Supply, 

 Trans. Am. Geophys. Union, vol. 22, part I, pp. 8-20, 1941. 



Ingerson, I. M., "Ground-water Conditions in California," Pre- 

 sented at Conference of Agricultural Extension Service, Uni- 

 versity of California, February 1949. 



