Irrigation water use includes all water artificially applied to farm and horticultural crops as well as water used to irrigate public and private golf courses. Irrigation water can be self supplied or supplied by irrigation companies or districts. However, all irrigation withdrawals in this report are identified as self-supplied withdrawals.
Irrigation of crops developed concurrently with the settlement of the arid West, where natural precipitation was insufficient to raise many crops. In the humid eastern States, irrigation has been used to supplement natural precipitation to increase the number of plantings per year or the yields of crops, and to reduce the risk of crop failures during droughts.
Information about the number of acres irrigated and the quantity of water withdrawn was obtained from a variety of sources such as State agencies responsible for permitting or allocating the withdrawal of water, the U.S. Soil Conservation Service, U.S. Bureau of Reclamation, county Cooperative Extension Service, individual farmers, agricultural research stations, and the U.S. Bureau of the Census, Agricultural Census, and Farm and Ranch Survey. Total acres irrigated are reported in two classes---spray irrigation (includes center pivot, travelling gun, trickle, and drip) and flood irrigation (includes flooding, furrow, and ditch).
Methods of estimating withdrawals for irrigation varied greatly. In some instances, they were based on theoretical estimates of water required to raise a given crop in an area. In other instances, accurate records of water application rates were available. Fairly accurate estimates of water withdrawn for irrigation can be made if the acreage irrigated, water application rates, and conveyance losses are known. It usually is difficult to obtain reliable estimates for consumptive use and for conveyance loss. Thus, some of the estimates of consumptive use and conveyance loss may be only rough approximations of actual conditions. In most States, consumptive use was based on coefficients ranging from 40 to 100 percent of withdrawals, or on theoretical crop requirements. In a few States, consumptive use was calculated as the difference between reported withdrawals and reported return flows.
The quantity of water withdrawn for irrigation during 1990 was an estimated 137,000 Mgal/d or 153 million acre-feet. (See table 15: water-resources regions and table 16: States.) Irrigation withdrawals as well as acres irrigated during 1990 were about the same as during 1985. Irrigation use represents 40 percent of total freshwater use for all offstream categories.
The source and disposition of water for irrigation are shown in the
pie charts below (or as a
GIF file ).
Surface water was the source for about 63 percent of irrigation withdrawals, and,
except for a small fraction of 1 percent that was reclaimed wastewater, ground
water furnished the remainder. Surface-water withdrawals for irrigation during
1990 were about 6 percent less than during 1985, and ground-water withdrawals for
irrigation during 1990 were about 12 percent more than during 1985. Of the 137,000
Mgal/d withdrawn for irrigation, 20 percent was lost in conveyance, 56 percent was
consumptive use, and 24 percent was returned to surface- or ground-water supplies.
Irrigation is by far the largest water use in the West. The nine western water-resources
regions (excluding Alaska and Hawaii), led by the Pacific Northwest region, accounted
for 90 percent of the total water withdrawn for irrigation during 1990
(figure 16;
table 15.)
In the eastern regions, most of the water withdrawn for irrigation was in the
Lower Mississippi and South Atlantic-Gulf regions, which together had about 2,400
Mgal/d more water withdrawn during 1990 than during 1985. By State, California and
Idaho were by far the largest users of irrigation water
(figure 17a |
figure 17b: surface-water withdrawals |
figure 17c: ground-water withdrawals)
and, together, accounted for 34 percent of the national total
(table 16.)
Florida withdrew the most water for irrigation in the East although it ranked
thirteenth nationwide.
