The relation between precipitation and evapotranspiration is a major factor in water availability. Generally, if annual precipitation exceeds annual potential evapotranspiration, then there is a net surplus of water and streamflow is perennial. However, annual potential evapotranspiration can exceed annual precipitation, which causes a net deficit of water. A net annual moisture deficit is present almost everywhere in California and Nevada (fig. 6). The only areas with an annual moisture surplus are the northern California coast, which receives considerable rainfall from winter storms, and the mountainous regions of northern and east-central California, where condensation of water vapor in rising, moist air masses results in abundant rain or snow. Water is available to recharge aquifers only at times when precipitation or snowmelt is greater than actual evapotranspiration. Thus, not all areas mapped as having a net water surplus in figure 6 are recharge areas.
In most of Nevada and in southern California, nearly all streams that head in the mountains are ephemeral and lose flow to alluvial aquifers within a short distance of where the streams leave the mountains and emerge onto the valley floors. In much of northern California and in the Humboldt, the Truckee, the Carson, and the Walker River drainages of Nevada, however, runoff is sufficient to support perennial streams. The Colorado River is supplied primarily by runoff from the Rocky Mountains. Before the inception of agriculture, the largest rivers in the vast Central Valley of California overflowed their banks during periods of peak winter flows and formed extensive marshlands. An elaborate flood control system and the lowering of the water table by withdrawals for irrigation now keep these rivers within their banks.
The geographical distribution of moisture in Segment 1 greatly influences patterns of agricultural and urban development. Much of Nevada receives little precipitation, and, consequently, ground- and surface-water supplies are limited. This limitation has severely restricted urban development and has put constraints on agricultural development. Las Vegas, the largest urban area in the State, obtains most of its water from the Colorado River, which is many miles away.
California receives relatively abundant precipitation. However, the precipitation is concentrated in areas of the State remote from most of the large urban centers and major agricultural areas. A further complication is the unpredictability of precipitation on an annual basis, which can often make surface-water supplies undependable. To provide a dependable, year-round supply of water to areas where it is most needed is a full-time, massive undertaking, and is accomplished by careful water management and an extensive water-transportation network (fig. 7).
