Agricultural drainage is used throughout the North America and in Wisconsin to improve crop production by removing excess surface (flooding) and subsurface (root zone) water from fields. This discussion focuses on the operating principals and design considerations of subsurface (tile) drain systems. In addition, a basic framework is introduced to evaluate the cost and benefit for drain tile installation. Crop production on certain soil types and landscapes is significantly enhanced by subsurface drainage. This includes areas with low permeability soils, isolated low pockets and lands with low slope gradients. Only water draining freely from the soil profile by gravity is removed by drain tiles. Tile drains are intended to function at atmospheric pressure as gravity flow systems. Flow occurs as a result of differences in the water surface elevation (e.g., the water table and tile elevations), thus making a positive (free flowing or pumped) outlet critical to their operation. The initial flow collector in the tile drain system is the perforated lateral. The depth to which tile laterals will lower the water table and water removal rate are a function of drain depth, spacing, soil permeability. Drain depth typically ranges from 3 to 6 ft and spacing from 30 to 100 ft. Laterals drain to mains and submains where the flow rate is governed by inside pipe roughness, pipe size and slope. Mains and submains must be sized to convey the flow from all upstream laterals. Tile drain systems eventually discharge into a surface water conveyance system or ditch. These ditches can be part of a legal (Wis. Stat. Chap. 88) public drainage system or county drainage system administered by a county drainage board. The drainage board oversees the maintenance on the county ditch system and assesses benefited land owners to cover the costs.
Soil Science Extension
University of Wisconsin Madison