Drainage is the natural or artificial removal of surface and sub-surface water from an area. Many agricultural soils need drainage to improve production or to manage water supplies.
OmegaWiki Dictionary
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drainage 1) Removal of groundwater or surface water, or of water from structures, by gravity or pumping.\n2) The discharge of water from a soil by percolation (the process by which surface water moves downwards through cracks, joints and pores in soil and rocks).\n(Source: MGH / WHIT)