On even, gentle slopes, border dikes may be used for irrigation. The Water is confined between low earthen dikes spaced 20 to 100 feet apart according to slope, size of stream, and length of run. The longitudinal slope of the border strip is kept low, and its cross slope is usually zero. The border method permits quick application and uniform distribution of water.

Row crops, such as corn, beets, and potatoes, are usually irrigated by the furrow method. In furrow irrigation the water flows to the field through one or more head ditches and is conducted to the rows through any one of several types of delivery structures. The head ditch is provided with checks small metal or canvas dams to raise the water level. From the head ditch the flow is carried to a regulation bay, or secondary ditch. From the regulation bay, water is supplied to each row by means of a spile. Raising or lowering the water in the regulation bay will control the flow to the rows so as to get uniform distribution and good penetration without erosion.

The use of plastic siphon tubes in place of lath boxes or other spiles has been perfected by the Nebraska Agricultural Experiment Station. The siphon tube has many advantages over the common type of spile. It can be placed over the ditch bank without disturbing the bank and can be used without a regulation bay. The tubes are light, strong, and durable. They are 1 to 2 inches in diameter and 30 to 40 inches long. A popular size is the 1 1/2-inch diameter, which will discharge 13 gallons a minute with a head of 2 inches. By raising or lowering the discharge end of the tube the rate of flow can be controlled. The use of siphon tubes is saving farmers money in labor and reducing soil losses and water cost through better control.

On steep or uneven ground, portable pipes serve as a convenient means of irrigation. Sprinkler systems, stationary or portable, are recommended for irrigation use where the land is too rough for surface irrigation and cannot be leveled because the soil is too shallow; where the soil is too porous to hold sufficient quantities of water from surface irrigation in the root zone; where the stream of irrigation water is too small for reasonably rapid application by surface methods; where land slopes steeply and soils are either highly erodible or relatively impermeable; and where water is expensive or labor scarce.

Wherever irrigation is practiced, drainage problems are likely to arise. As the higher lands in a valley are irrigated, the lower lands become wet and sometimes alkaline. Both ancient and modern history offer examples of the abandonment of once fertile, productive areas because of water-logging or the accumulation of alkali. Research has shown how these conditions can be avoided or controlled by applying irrigation water properly and by removing excess water and alkali.

Methods of locating the source of water that is causing a drainage problem and charting its flow pattern have been improved greatly in the past few years.

Among various types of hydraulic probe, available for charting lines of flow in the field, is the piezometer, developed through cooperative research in the Imperial Valley of California and in Delta, Utah. This device is a 1/4 or 3/8-inch galvanized iron pipe, which is quickly and easily driven with a jackhammer into the ground to depths as great as 300 feet. After being flushed out and primed, it forms an observation well that reflects accurately the hydrostatic pressure within the soil at its termination point. When a series of piezometers are set in the field so as to terminate above, below, and out from either side of a tile line, they reflect a pattern of hydrostatic pressure around the tile line from which streamlines can be drawn. From such drawings it is possible to trace the source- and direction of seepage from canals and to chart the effectiveness of tile systems, open drains, and interception lines; to determine the presence of artesian wells; and, with supplementary information on soil permeability, to predict the effect of pumping from relief wells.

THE AUTHOR

George D. Clyde is chief of the Division of Irrigation, Soil Conservation Service. He started learning about irrigation during his boyhood on his father's farm in Utah. He was graduated in agricultural engineering from the Utah State Agricultural College, and taught civil engineering there after graduate studies at the University of California. In 1945, after 22 years of work at the Utah State College, Mr. Clyde resigned as dean of the school of engineering and director of research work in the engineering experiment station to join the Soil Conservation Service.