The net impacts of tile drainage on water quality are not well understood, and the science is sometimes conflicting and often inconclusive. This was the conclusion of a report, submitted by the Agencies of Natural Resources and Agriculture, Food and Markets to the Vermont Legislature on January 31st of this year. The report calls for increased education about management around tile drains, changes to nutrient management plans, and more research and resources to improve our understanding of the function and potential impacts of tile drainage.
Tile drainage—a network of perforated pipes, or tiles, installed at various soil depths to drain excess water from fields—is estimated to be below the surface of 5% of Vermont’s cropland (as reported in the 2012 census by the U.S. Department of Agriculture). However, the percent of tiled farmland is known to be far higher in many parts of the Lake Champlain Basin. Tile drainage increases the economic viability of land that is too wet to crop, and can substantially increase the tonnage of low producing fields, potentially with fewer fertilizer or chemical inputs. Drier fields also allow for increased conservation practices such as cover crops. But some tiles may substantially increase phosphorus draining into the state’s rivers and lakes, decreasing water quality and causing algae blooms.
Before the 1970’s, prior to regulations that now protect wetlands’ water quality and flood resiliency functions, as much as 35% of the wetlands in Vermont were converted into agricultural fields or developed land uses by installing tile drainage. This was often supported by federal funding, and even encouraged by educators and scientists as one method of boosting the agricultural economy.
Tile drainage increases total annual water output from a field an average of 10-25%, and the flow can be a significant contributor to the overall phosphorus load in heavily agricultural watersheds, sometimes contributing phosphorous directly to waterbodies. However, the outcomes of research on water quality impacts of tile drainage vary widely. Factors such as farming methods and the amount of phosphorus in the soil can drastically influence study results. The legislative report highlights the need for more local research before any definitive conclusions can be drawn.
Laura DiPietro of the Agency of Agriculture, Food and Markets and Marli Rupe of the Department of Environmental Conservation discussed the report, the complex questions regarding tile drainage, and next steps for addressing this important issue as part of the Clean Water Initiative Program’s Brown Bag Lecture Series earlier this month. The lecture recording is available on online the Clean Water Initiative’s YouTube channel.