On sloping lands, irrigated by the usual furrow and border methods, it is impractical to apply just enough water to wet the root zone uniformly over the entire length of run. To insure proper application at the end of the run, some water usually is wasted, and provisions must be made to collect and remove the runoff. The usual practice is to intercept the water with shallow ditches and discharge it into natural channels or deep drains. In areas where water supplies are limited, waste water is sometimes collected at the lower end of the field and pumped back to the upper end of the field for reuse. Surface drains are not needed then.

The level border method of irrigation is an effective way to attain high irrigation efficiencies on flat land. Water is impounded and allowed to penetrate the soil uniformly in long, narrow, level basins. If rainfall is heavy, surface drainage is necessary if this system is to succeed. If provision is not made to drain the basins, untimely rains might flood crops.

IN SUBSURFACE DRAINAGE, water moves into and through the soil and is discharged through natural drainage channels or through deep open or closed drains, which convey the water away from the field. The planning and design of subsurface drainage systems require knowledge of the drainage requirements of crops and the water-transmitting properties of the soil.

Surface drains are designed to remove ponded water and thus reduce the amount of water that enters the soil profile, but usually they are installed to maintain ground water tables at a sufficient depth to prevent waterlogging of the root zone. Adequate control of the water table in and regions also prevents the accumulation of salts.

Tile drains, one of the five major types of subsurface drains, are hollow cylinders that are 4, 6, 8, or more inches in diameter and have a wall thickness of about one-twelfth their inside diameter. Usually they are made of burned clay or concrete. Clay tiles are commonly made in 1 -foot lengths; 2-foot concrete tiles often are used. Water enters the tile drain through the space or joint between each tile. The tile line is surrounded with a gravel filter in arid regions to prevent the inflow of soil sediments.

Mole drains are a cylindrical channel formed in the soil by pulling a ball or bullet-shaped device through the soil at the bottom of a narrow blade.

Mole drains are similar to the tile drains in shape and function, but are unlined and unstabilized. They depend on the stability of the soil to maintain an open channel. They often have a dual role; they remove excess water during wet periods and are a conveyance system for subirrigation in dry periods. The life expectancy of mole drains is related directly to the stability of the soil in which they have been formed.

Perforated pipe or tubing long sections of perforated conduits of metal, plastic, or bituminous-fiber may be placed in a trench and covered or pulled into a mole channel behind a mole plow a technique that eliminates the need for digging a trench. This system is effective in draining or crossing quicksand pockets that do not provide adequate stability for conventional tile systems.

Deep open drains or channels, which are dug to depths of 5 to 12 feet or more for the purpose of controlling the water table, actually function as a subsurface drain. Frequently they serve as main outlet channels for other drains.

THE TYPE OF SUBSURFACE system used depends on the nature of the drainage problem. In treating localized drainage problems, such as small depressions and waterways, and in intercepting wet-weather springs and seeps, subsurface drains are placed where needed without regard for a uniform pattern. In draining extensive uniform areas that are essentially flat, however, a definite pattern usually is established, with laterals at more or less uniform spacings discharging into main outlets.

These patterns are sometimes called gridiron or herringbone layouts.

In any system, careful attention must be given to the design of the outlet drain to assure that it is of adequate capacity, especially if the outlet is a closed drain. If a free-gravity outfall cannot be obtained, a pump outlet may be used.

DRAINAGE PRACTICES in and regions resemble those in humid areas in many respects. We mention two important differences. Because of the salinity factor, the water-table depth that is required for favorable conditions for plants is considerably greater in and soils than in soils in humid regions. The drainage needs in and regions are closely related to irrigation practices, which are subject to control, but in humid areas drainage needs depend largely on natural rainfall.

To maintain water tables at depths sufficient to control salt accumulation in and regions, drains are commonly installed to depths of 6 feet or more. Depths of 2.5 to 4 feet are considered adequate in many humid areas. If salinity is not involved, a shallow water table may be advantageous in that it serves as a source of water for crop use.

In the Netherlands a water table held at a constant depth of about 24 inches is desirable, and many crops are grown under constant water-table conditions. The natural rainfall there is in excess of 20 inches, approximately the minimum total required to keep the soils leached and at the same time to provide sufficient water for crop use.

Good drainage practices must be accompanied by sound irrigation and soil-management practices if maximum benefits are to be obtained.

It is essential that management of excess water be integrated with other soil-management practices in order that the overall farm operational program may be successful. In addition to water management, such practices as tillage, replenishment of organic matter, proper fertility practices, insect and disease control, and good irrigation practices are essential. Improved irrigation practices and other farming procedures which control the use of excess water will considerably lessen in most cases the need for costly drainage installations.