applying water to the surface of the soil.
There are three methods of surface watering: furrow irrigation, border irrigation, and flooding. In furrow irrigation the water is run in small furrows and is absorbed into the soil primarily by capillary action (through the sides and bottom of the furrows). Border irrigation and flooding involve running water over the field. The water enters the soil by gravity.
Furrow irrigation is used for row crops (cotton, sugar beets, corn, and vegetables) and, sometimes, for grain plantings, orchards, and vineyards. The irrigation is done along trough furrows (depth, 8-25 cm), spur furrows (25-30 cm), and large furrows (35–40 cm). The furrows are made by a furrowing machine. The distance between furrows corresponds to the width of the interrow spaces (45-70 cm); for purposes of water supply and for preplanting waterings the distance between furrows is 90-140 cm. The length of a furrow (100–400 m) depends on the dip (most favorable is 0.002:0.01) of the plot and the water permeability of the soil. The flow is 1–4 1/sec.
Border irrigation is used for narrow-row grass and grain fields, orchards, and vineyards. Earthen ridges 1—25 cm high are used to break the watering area into strips with widths equal to or multiples of the operating width of the planter. The length of the strips is 150–400 m, and the desirable dip is 0.002–0.01.
Flooding, an ancient method of irrigation, is used for flushing salinized soils, for irrigating meadows, for raising rice, and, sometimes, for raising corn and other crops in the rice rotation. In rice cultivation the area being irrigated is broken up by temporary earth ridges (the longitudinal ridges with gentle slopes are made permanent) into plots of 1–4 hectares (ha) or of 12-16 ha. Careful leveling of the surface of the rice fields makes it possible to reduce the depth of flooding and the rate of irrigation.
K. K. SHUBLADZE