drainage basin: see catchment area.

drainage, in agriculture, the removal of excess water from the soil, either by a system of surface ditches, or by underground conduits if required by soil conditions and land contour. Diesel or centrifugal pumps are sometimes used to drain large areas. Drainage was practiced in the Nile basin c.400 B.C. and in ancient Rome. Today drain pipes of clay, concrete, or plastic, laid several feet underground, are much used in the United States, where c.110 million farm acres (44.5 million hectares) were artificially drained in 1987. Proper drainage improves soil structure; increases efficiency of phosphorus fertilizer; conserves soil nitrogen; and controls waterlogging, leaching, and salinization of soils caused by irrigation.

drainage, in mining, removal of water seeping into shafts and other underground mine workings from the surrounding ground. Unless seeping water is removed continually, it may endanger haulage and mining equipment, weaken supporting structures, and, in some instances, flood the mine completely. Water in a mine is drained into sumps, or reservoirs, usually excavated below the lowest working level of the mine, and is then removed by pumps. Methods to minimize seepage include the sealing of visible fissures through which water enters and the injection of concrete into the ground surrounding the shafts.