Literature Review: Tile Drainage and Phosphorus Losses from Agricultural Land
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.