The Central Valley is one of the most important agricultural areas in the world. No single region of comparable size in the United States produces more fruits, vegetables, and nuts. More than 7 million acres are currently (1995) under irrigation. During 1985, crop irrigation accounted for 96 percent of the surface water and 89 percent of the ground water withdrawn in the Central Valley.
Discovery of gold in the Sierra Nevada and the subsequent proliferation of hydraulic mining operations provided the impetus for the construction of a surface-water diversion system that consisted of hundreds of miles of canals used to transport water to where it was needed for gold-washing operations. This was the beginning of the valley's modern-day aqueduct system, which has become vital to the agricultural economy.
Fertile soil, favorable climate, abundant water, and rapid population growth in the Central Valley encouraged the development of agriculture, which soon became one of the major industries of California. Surface water satisfied most irrigation needs until the late 19th century, when a rapid increase in irrigated acreage produced a demand for water that exceeded the surface-water supply, and ground-water supplementation became necessary; the drought of 1880 was a major stimulus for ground-water development. Wells were used to supplement less dependable surface-water supplies and to provide water where surface-water diversion canals had not been constructed. Shallow ground water was obtained easily in 1880, and artesian pressure was sufficient to produce flowing wells in much of the valley. After 1900, ground water gradually became a more significant part of the total irrigation supply and, eventually, the large number of wells reduced artesian pressure to such an extent that it became necessary to install pumps in order to obtain water.
The invention of the deep-well turbine pump around 1930 allowed withdrawals from greater depths, which encouraged further development of ground-water resources for irrigation. Withdrawals increased sharply during the 1940's and 1950's, and averaged about 11.5 million acre-feet per year by the 1960's and 1970's, which was approximately 20 percent of the total irrigation withdrawals for the United States at that time. Withdrawals reached a maximum of 15 million acre-feet per year during 1977, a drought year. During the 1960's and 1970's, withdrawals greatly exceeded recharge, and water levels declined precipitously, as much as 400 feet in places. The declines caused a major reduction in the amount of ground-water in storage and resulted in widespread land subsidence, mainly in the western and southern parts of the San Joaquin Valley.
Increased rainfall and construction of additional surface-water delivery systems halted most of the serious water-level declines after 1977, and water levels recovered to pre-1960 levels. The network of aqueducts in the Central Valley is currently (1995) sufficient to provide one-half or more of the water needed for irrigation in years of average or above-average precipitation. In dry years, however, reliance on ground-water supplies is greater, and aquifers might again be subject to withdrawals in excess of recharge during a severe drought, such as that of 1976--77.
The Central Valley is bounded on the west by the Coast Ranges and on the east by the Cascade Range and the Sierra Nevada. The valley has only one surface-water outlet, the Carquinez Strait east of San Francisco Bay. Much of the valley is surrounded by dissected uplands formed by erosion of coalesced alluvial fans at the base of the mountains (fig. 72) where the terrain ranges from hilly to slightly rolling. The valley floor, which consists primarily of alluvial deposits and flood-plain deposits of the major rivers, is relatively flat to gently rolling and is generally below an altitude of 500 feet. Lake beds in the southern end of the valley become partially to completely flooded in wet years. A prominent feature, Sutter Buttes, which is the remnant of a volcanic plug, rises nearly 1,500 feet above the valley floor in the central Sacramento Valley.
The Sacramento River drains the northern end of the Central Valley, and the San Joaquin River drains much of the middle third. The two rivers join in the Sacramento--San Joaquin Delta and empty into the upper end of San Francisco Bay. The southern end of the valley is occupied by the Tulare Basin, in which drainage is completely internal and the inflowing water is removed by evapotranspiration.
The climate of the Central Valley is Mediterranean and Steppe, characterized by hot summers and mild winters, thus allowing for a year-around growing season; at least one crop is under cultivation at all times. About 85 percent of the precipitation falls from November to April. Most of the precipitation that falls on the valley floor evaporates before it can infiltrate downward to become recharge. Much of the moisture that moves inland from the Pacific Ocean is intercepted by the Coast Ranges, so that annual precipitation in the valley is relatively low. Annual precipitation decreases from north to south, with an average of about 23 inches in the northern part of the Sacramento Valley, to about 6 inches in the southern part of the San Joaquin Valley. Rainfall amounts vary greatly from year to year. Annual precipitation is exceeded by potential evapotranspiration throughout the entire valley, which causes a net annual moisture deficit.
In contrast, the mountains that surround the Central Valley intercept moisture from eastward-moving weather systems and have an annual surplus of moisture in the form of rain and snow. Precipitation can exceed 80 inches annually in the Sierra Nevada. Annual runoff from rainfall and snowmelt is approximately 32 million acre-feet; most of the runoff originates in the Cascade Range and the northern Sierra Nevada (fig. 73). This water flows to the valley in perennial streams and provides nearly all the average annual 12 inches of recharge the valley aquifer system receives. Runoff from the Coast Ranges is principally on the western slopes to the Pacific Ocean.
