Irrigation in the West

by GEORGE D. CLYDE

MORE THAN 20 million acres of land west of the ninety-seventh meridian can be used successfully for farming and livestock production only because it is irrigated. Private enterprise irrigates about four-fifths of this total, the Federal Government about one-fifth.

In planning and operating irrigation systems, one has to know how much water the crops will need and how much more will be lost, through evaporation and seepage, in conveying and applying the needed amount. The need of applying just the right amount is recognized by courts and engineers responsible for allocating limited water supplies and by farmers among whom the supplies are divided. Irrigation should be planned so as to give the crops enough moisture but not more than enough. If more water is applied at any time than the soil of the root zone can hold, the excess will percolate out of the root zone and go to waste or the soil will become waterlogged.

Farm crops differ in the amounts of water they require. Any one crop needs different amounts in different soils, at different stages of growth, and in different kinds of weather. Plants use more water in warm weather than when it is cool. Winds, especially warm winds, increase the transpiration of plants and the evaporation from soils. For best growth, soil moisture should be easily available to plants from the time the seed is deposited in the soil until the crop matures. The quantity of irrigation water required depends chiefly on the rate at which the crop uses the water and the length of the growing season of the crop.

Water for irrigation comes chiefly from surface streams. A little is pumped from natural reservoirs under the ground.

Over the great part of the West, precipitation during the wet season occurs chiefly as snowfall. Snow accumulates on high mountains; in melting it feeds the surface streams. The streams fed by snow melt reach their highest stages in the spring not in the season when irrigation water is most needed. Part of their flow, therefore, must be stored artificially, in surface or subsurface reservoirs. Frequently it is possible to divert a stream over a spreading ground so that its waters will percolate into an underground water basin, and thus to increase the supply of water available for pumping.

A farmer or stockman who depends on irrigation needs to know at the beginning of the season about how much irrigation water will be available to him, so that he can adjust his plans to his water resources. In 12 Western States having mountainous areas on which snow accumulates, Federal and State investigators have developed methods of surveying snow and forecasting streamflow that are now widely used in this country and in Canada.

The value of water-supply forecasts is illustrated by one of our experiences with drought in Utah. In 1934, snow surveys on April 1 indicated that the Utah supply of irrigation water for that year would be only about 35 percent of normal. Governor Blood called a meeting of all users of water. Two emergency programs were put into operation one to develop supplemental irrigation supplies before the crops needed them and one to conserve water drastically so as to save perennial plantings.

Under the first program, 400,000 acre-feet of water was added to the available supply in 6 weeks by pumping from wells, lowering outlets of reservoirs, draining lakes, and developing springs. Under the second program irrigation streams were united, acreages to be irrigated were reduced, and long, small irrigation ditches were abandoned. Strict water-conserving practices were put into effect among the water users. The result was that more than $5,000,000 worth of crops matured that might otherwise have been lost.

Out of 125,000 miles of irrigation canals and laterals in use in 17 Western States in 1939, all but about 5,000 miles, or 4 percent, were unlined, untreated earth channels. Such channels are the cheapest to construct, but they are expensive to maintain, and they permit seepage losses often amounting to 70 percent or more.

A lining of clay 4 inches thick, we have found, effectively reduces seepage losses from earth canals, and if such a lining is covered with a blanket of gravel it becomes highly resistant to erosion. It is, however, subject to weed growth. Bentonite with and without a mixture of soil has been used experimentally as a canal lining. Straight bentonite has a permeability of only about 0.001 foot per year, and if even as little as 2 to 10 percent of bentonite is mixed with soil, the mixture is apparently just as impermeable.

Treating channel surfaces with light oil reduces seepage, stabilizes the channel, and eliminates weed growth. The procedure is to spray 16- or 22-gravity oil under high pressure on the sides of the channel and sluice additional oil on the bottom. Three applications are needed, with enough time between them to let the oil sink in. The oil should penetrate to a depth of 8 to 10 inches. This treatment takes 1 1/2 gallons of oil to the square yard of surface.