Ground water is an important resource in California and Nevada and accounted for nearly 40 percent of all freshwater used in the two States during 1985 (fig. 13). Fresh ground-water withdrawals in California during this period were about 16 times as much as those in Nevada. In Segment 1, irrigated agriculture accounts for the greatest amount of ground-water use, followed by withdrawals for public supplies. More than 25 million people, or about 66 percent of the population of the two-State area, depend on publicly supplied ground water.
Total withdrawals of fresh ground water during 1985, by county, are shown in figure 14. Counties with the largest withdrawals are those where vast areas are irrigated, such as the Central Valley of California, or counties having large population centers. The large withdrawals shown for central Nevada and some southern California counties are somewhat misleading. These areas are largely desert, and because of the extremely large size of some of the counties, small withdrawals in scattered pumping centers plot as unrealistically large withdrawals when totalled for counties that contain hundreds of thousands of square miles.
The Central Valley aquifer system had the largest ground-water withdrawal in Segment 1 during 1985 (fig. 15). Approximately 9,000 million gallons per day (about 10 million acre-feet per year) was withdrawn from the Central Valley aquifer system. Of that amount, approximately 8,000 million gallons per day, or 8.9 million acre-feet per year, was withdrawn for irrigation and accounted for about 11.5 percent of all ground-water withdrawals in the United States. One acre-foot, or 43,560 cubic feet of water, is the volume of water that will cover an area of 1 acre to a depth of 1 foot.
The Coastal Basins aquifers supply the largest population centers in Segment 1 and are second only to the Central Valley aquifer system in total ground-water withdrawals (fig. 15). Much ground water is withdrawn for agricultural use in these coastal basins, but public supply accounted for about 54 percent of the approximately 4,370 million gallons per day (about 4.9 million acre-feet per year) withdrawn during 1985; this is due primarily to the large population in the coastal cities of southern California that depend heavily on ground water for public supply.
Irrigated agriculture is the largest user of fresh ground water withdrawn from the Basin and Range aquifers. Although ground water is a significant source of water for public supplies, large population centers, such as Las Vegas, the Carson City area, and the Reno--Sparks area, depend heavily on surface water for their public supplies. The desert basins receive little precipitation during the year, and surface and ground water are scarce, which limits population growth in the region.
The northern California volcanic-rock aquifers and the northern California basin-fill aquifers together supplied only 5 percent of the total fresh ground water withdrawn in Segment 1 during 1985. These aquifers compose only a small part of the segment, and the demand for ground water in northern California is not great.