It is the area drained by a river and all its tributaries. It is also known as catchment area. It is an area of land where surface water from rain, melting snow, or ice converges to a single point at a lower elevation, usually the exit of the basin, where the waters join another water body, such as a river, lake, reservoir, estuary, wetland, sea, or ocean. Drainage in agriculture is 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.