Irrigation in Arid Regions

Wayne D. Criddle and Howard R. Haise.

A farmer has to bear six factors in mind when he plans to irrigate his land. They are : The adequacy, reliability, and quality of the water supply; the control and conveyance of water; water requirements, including consumptive use, effective rainfall, net irrigation requirement, and irrigation efficiency; application of water; drainage for removal of both surface and subsurface excess water; and institutional arrangements.

Existing rights and uses must be respected. The fact that water flows past a man's farm does not necessarily give him the right to divert the waters and use them for irrigation. Downstream users may have established rights to the waters in accordance with State laws. If the flow is insufficient to meet all needs, the first appropriators may be entitled to receive their water first. Domestic and municipal uses also may have first priority. Irrigation, power, industry, navigation, wildlife, and recreation may have lower priorities.

The source of irrigation water usually is the rain and snow that falls on the watershed above. It may flow down to the valley in surface streams that can be stored or diverted for use, or it may travel underground and have to be pumped up. Occasionally the source is the ground water reservoir under the cropped land. Seldom in the and West does the surface water supply fully coincide with the irrigation needs. Because peak streamflows from melting snows usually occur in the spring or early summer, storage works must be developed to control the flow of the streams and make the water available as needed.

Only a relatively few holdings can be served with individually developed water supplies. Most farmers must work out agreements with their neighbors as to the use of their water rights and the development of facilities to divert, convey, and distribute the water. Such arrangements usually are needed to insure that each farmer contributes his proportion of the costs of development and operation of irrigation and drainage systems. Each, furthermore, must receive his share of water at a set time and in a way that he can use it.

The quality of the water is highly important. Foreign material, whether dissolved or in solid form, may be objectionable. Excess silt, moss, sand, brush, small insects, or other debris in the water may make special treatment necessary. The treatment may consist of simple screening or may require the use of expensive sand or debris traps, depending on the kind and amount of material the water carries and the method of irrigation.

Heavy concentrations of dissolved salts are objectionable. Their total amount and kinds and the characteristics of the crops, soils, climate, and irrigation practices must all be considered in appraising water quality.

Various physical controls and structures are needed after a suitable water supply for irrigation is found. A way to control the supply and deliver it economically from the point of origin to the point of use on the farms must be made. It may include storage and diversion dams on the streams; canals and laterals to carry water to the farms; farm distribution systems; and other structures as canal linings, pumping plants, headgates, drops to control erosion, measuring devices, pipelines, siphons, check dams, and spites.

This part of irrigation is largely engineering. It is not specifically related to the crops that are grown, except that the capacity must meet their needs. It is related to the soils only as the soils affect the stability of structures, seepage losses, and so on. Surface and underground storage reservoirs must be developed to make the natural flow of the streams correspond directly with the irrigation needs of the crops.

Preparation of irrigated land is necessary for efficient and uniform application of water regardless of the method of irrigation.

Sprinkler irrigation usually requires the least preparation, although considerable leveling is sometimes done. For good surface and subsurface irrigation, land leveling is essential. Greater efficiencies of application, more uniform distribution, ease in farming, increased production, and savings of water are among the advantages.

LAND LEVELING usually is done by tractor-drawn equipment. Some farmers prefer to do their own leveling with scrapers and levelers that can be pulled with medium-sized farm tractors, even though that takes more time than leveling by contractors who use heavy equipment.

The carryall is an efficient machine that scrapes, spreads, and transports soil. Bulldozers, graders, terracers, and maintainers are sometimes used for rough grading, but the cost of using them is higher than when a carryall is used, and only skilled operators can use them efficiently.

After the fields are rough leveled, smoothing equipment such as levelers, floats, or land planes are used to establish the final grade and finished surface. Land planes usually are more than 60 feet long and are supported at the corners by swivel wheels. A combination scraper-bucket in the center of the frame can scrape high spots and carry enough dirt to fill in low spots. Smaller levelers, usually about 30 feet long, are pulled by medium-sized farm tractors and can be used for additional smoothing operations.

Because a certain amount of soil settling always occurs on newly leveled land, it is advisable to grow a small grain or similar crop with border irrigation or a row crop with furrow irrigation after the first leveling. After harvest, the high and low spots can be releveled before a perennial crop, like alfalfa, is established.

Land leveling sometimes lowers the productivity of soils. Moving heavy equipment across fields compacts soils, especially if they are wet, and often creates a condition that is not easily remedied.

Rebuilding structure damaged by leveling operations is difficult in places where freezing and thawing do not occur. Deep-rooted legumes and grasses have been used in the Lower Rio Grande Valley of Texas to loosen surface soils and increase intake rates after fields were leveled. Rough or minimum tillage operations with alternate wetting and drying and applications of organic materials also help improve the intake rate of soil whose structure has deteriorated.

Land leveling may remove topsoil and expose subsoil. The problem is worse in the sections of higher rainfall (where soil profile development has progressed more) than in the and regions (where cutting 1, 2, or even 5 feet into deep alluvial soils often is considered feasible). Soils with a marked development of profile often present a physical or chemical problem, or both, when the organic layer is removed in the leveling process.

When newly irrigated lands were leveled for irrigation in the Columbia River Basin, deficiencies of nitrogen, phosphorus, and zinc occurred when subsoils were exposed. Symptoms of zinc deficiency were noted first on corn and beans, but the deficiencies were found later to affect 14 other crops. On areas where surface soils were re-moved, fertilizers containing nitrogen, phosphorus, and zinc were applied, and production jumped from 280 to 2,280 pounds of field beans an acre.

Symptoms of a deficiency of zinc have been noted in North Dakota on corn growing on Gardena subsoils that were exposed by leveling operations. Additions of 180 pounds of nitrogen and 100 pounds of P₂O₅ with and without 15 pounds of zinc sulfate an acre increased corn forage yields 5,390 to 6,420 pounds, respectively. When no fertilizers were added, yields were 1,480 pounds an acre, compared to 7,940 pounds when those rates of nitrogen, phosphorus, and zinc, and 20 tons of manure were applied.

In a long-time experiment in Colorado, topsoil was removed to varying degrees. Liberal applications of nitrogen and phosphorus restored the productivity of Fort Collins subsoil equal to that of the original surface soil. Yields of sugar beets on check plots averaged about 7 tons an acre, compared to 20 tons on fields that got nitrogen and phosphorus. The ease with which stands were established indicated also that within a relatively short time the physical condition of the soil was as good as that of the natural surface soil.

In some localities of shallow topsoil underlain by sand or gravel, farmers have found it feasible to remove and save the topsoil, which is spread over the field again after the subsoil is leveled. This practice takes money and work, but the farmers believe many dollars can be spent to make poor land (which may nevertheless sell for 500 dollars an acre if it can be irrigated) into good land.

Leveling of land to be irrigated is not something that can be done once and then forgotten. Normal farming operations, erosion by wind and water, silting from some water supplies, and other factors tend to get the land out of level. For efficient irrigation, the grade must be uniform. Therefore some periodic floating or smoothing usually is desirable and should be considered in planning the farm operations and equipment needs.

Because a smooth land surface is desirable for spreading water over the field, some farmers tend to overtill their fields. Soils high in silt soon lose their structure under continuous tillage. The density of soil is increased by running tractors over it. Although maximum compaction might be obtained by working the soils while wet, working some soils when dry forms a dust, which lowers the intake rate. Most irrigated soils therefore should be tilled no more than absolutely necessary to prepare the seedbed, control weeds and insects, and ready it for irrigation. All other tillage merely costs the farmer money, makes it harder to get water into the soil, and lowers production because it impedes movement of soil water and air.

THE THREE GENERAL METHODS of applying water to cropland are surface irrigation, sprinkler or overhead irrigation, and subsurface irrigation.

Whatever the method, the aim is to apply adequate water to the soil uniformly over the field with no harm to the crop or the soil and with a minimum of water and labor.

In surface irrigation, the water may completely cover the surface (commonly called flood irrigation) or it may flow in furrows or small ditches. Either method may be used on sloping or relatively flat lands.

Flood irrigation includes border irrigation (running water downslope between dikes), and basin irrigation (quickly ponding water on a level area), and flooding downslope from the contour ditches.

For border irrigation, the slope of the land down the strip generally should not be less than 0.1 percent and not more than 2 percent, although slopes exceeding 6 percent have been used for narrow border strips planted to grass or dense cover crops. If crops are to be established on steeper slopes, sprinkler irrigation should be used to prevent erosion unless rainfall is adequate to start the crop. Cross slope within the strip is not permitted.