LCBP Technical Report: Tile Drainage Literature Review
Prepared by Stone Environmental, Inc., November 2016
Tile drainage works by providing an open pathway for soil water to drain away, lowering the water table and allowing the upper soil layers to dry out. For farmers, tile drainage has multiple benefits: better growing conditions, improved soil structure, enhanced trafficability, more timely planting and harvest, and improved yields. Tile drainage pipes are typically installed at depths of 0.6 – 1.2 m and spaced 10–100 m apart, depending on soils, crop type, and cost. Historically, tile drainage was often installed strategically, targeting low spots and other frequently saturated areas. Today, drainage tends to be installed in a regular grid pattern, with pipes located 5 to 30 m apart under an entire crop field. Most drainage networks discharge directly to an open ditch or stream.
In agricultural watersheds, phosphorus (P) can enter surface waters through both surface runoff and subsurface flow. In agricultural fields with subsurface (tile) drainage, much of the subsurface flow is conveyed by tile drains directly to surface waters. Once dismissed as negligible, P levels in subsurface tile drainflow are now recognized as potentially significant, and tile drainflow has been clearly shown to influence both hydrology and phosphorus loading at the field and watershed scales in some areas of the United States.
Tile drainage is an essential water management practice on many agricultural fields in the Lake Champlain Basin (LCB). Reliable data on the location and extent of tile drainage in the LCB do not exist, but the Vermont Agency of Agriculture, Food, and Markets (VT AAFM) and the Vermont Agency of Natural Resources (VT ANR) have estimated that about 5% of Vermont’s cropland (9,500 ha on 525 farms) has tile drainage, with cropland drainage in some agriculturally-intensive subwatersheds within the LCB as high as 70%. Of the reported tile drained acres in Vermont, 80% are associated with dairy production.