Water-use trends in the desert southwest: 1950-2000

Abstract

The population of the Desert Southwest is among the fastest growing in the country. In this area, ground-water supplies have been developed, surface-water resources have been fully appropriated, and conservation and conjunctive water-use measures are being used to meet water-resource needs. Complex networks of water-distribution systems have been developed to deliver surface-water supplies, and interstate agreements, such as the Colorado River Compact of 1922, help manage the distribution of water among many States in the Western United States, including Arizona, California, Nevada, New Mexico, and Utah. The Colorado River, which lies on the borders of Arizona, California, and Nevada, plays an important role in supplying water to the Southwest. Water from the Colorado River is used to irrigate extensive farmland in the southern California deserts and is delivered to southern and central Arizona through the Central Arizona Project canal for domestic and agricultural uses. It is also the source of much of the water used for domestic purposes in southern Nevada. Estimated water-withdrawal and related data were compiled from various sources to identify trends in Arizona, California, Nevada, New Mexico, and Utah. More water is used for agriculture than domestic and industrial use in these five States. From 1950 to 2000, however, the percentage increase in withdrawal for domestic water use exceeded that for agricultural use. The estimated amount of water withdrawn for domestic, agricultural, and industrial purposes in Arizona, California, Nevada, New Mexico, and Utah increased 58 percent, from 39.6 to 62.8 million acre-feet, from 1950 to 2000. During this period withdrawals for domestic water use, which included self-supplied domestic and public supply (all deliveries to residential, commercial, and some industrial users), increased 410 percent from 2.0 million to 10.2 million acre-feet and the population in these five Southwestern States increased 250 percent. From 1965 to 2000, water withdrawals for agriculture, which were primarily for irrigation of crops and livestock uses, increased 14 percent in the five States, from 44.0 to 50.2 million acre-feet, while irrigated acreage increased 12 percent from 12.6 to 14.1 million acres. Water-use trends in the Southwest are dominated by water use in California where crop acreage is more than twice as large as the combined crop acreages in Arizona, Nevada, New Mexico, and Utah, and the population in 2000 was more than three times larger than the combined population of these States. Withdrawals for agriculture in California accounted for 62 percent of the water withdrawals for agriculture in the five States in 1950 and 68 percent in 2000. Water withdrawals for domestic-water use in California declined from 82 percent of the total domestic-water withdrawals in all five States in 1950 to 70 percent in 2000, indicating that the need for domestic withdrawals increased more in Arizona, Nevada, New Mexico, and Utah combined than in California. The population of California is larger than the combined population of Arizona, New Mexico, Nevada, and Utah, but the combined population of these smaller States grew faster than the population of California. From 1950 to 2000 the California population increased 220 percent, but the combined population of the four other States increased 390 percent. From 1960 to 2000, public supply per-capita use increased in Arizona, New Mexico, and California, and decreased in Nevada and Utah. Crop-application rates (water withdrawal for irrigation of crops divided by the irrigated crop acreage) from 1965 to 2000 ranged from 2.32 acre-feet per acre in Utah in 1975 to 6.21 acre-feet per acre in Arizona in 2000. More water is used per acre of irrigated land in Arizona than in the other four States. This could be due to several reasons, including differences in climate, conveyance losses, length of growing season, and type of crops grown. Trends in water withdrawals for industrial use are difficult to identify because of differences in data reporting from year to year. From 1950 to 2000, withdrawal for industrial use in the five States was generally less than 6 percent of the total withdrawal. Withdrawal for industrial use was less than 4 percent of the total withdrawal in Arizona, except in 1990; industrial withdrawal was less than 6 percent in California and less than 7 percent in Nevada, New Mexico, and Utah, except in 1980 when in Utah it was almost 12 percent of the total withdrawal. From 1950 to 2000, ground-water withdrawals increased 324 percent in Nevada, 147 percent in New Mexico, 208 percent in Utah, and 52 percent in California. Ground-water withdrawal decreased 15 percent in Arizona. For all five States, ground-water withdrawals increased 62 percent.