Drainage, in agriculture, the artificial removal of water from land. Drainage is employed in the reclamation of wetlands, in the prevention of erosion, and as a concomitant of irrigation in the agriculture of arid regions. Drainage can remove excess water from an area before it enters the soil, and thus it prevents leaching of nutrients and standing pools of water on the surface and can permit early spring planting. After excess water enters the soil, its removal is an expensive and specialized undertaking that is not directly connected with irrigation, although it sometimes may be necessary for irrigated land.