Subsurface (tile) drainage is an important contributor to crop production in Iowa, but it also provides a pathway for loss of nitrogen from cropland. Conservation drainage is an emerging set of practices designed to maintain the benefits of agricultural drainage while addressing water quality and flow impacts.

Conservation drainage is a critical part of the Iowa Nutrient Reduction Strategy (INRS) for reducing nitrogen losses into Iowa waterbodies. Practices promoted as part of conservation drainage include in-field nitrogen management, cover crops, edge-of-field and drainage system practices as well as land use changes. Practices vary in their nitrogen-reduction effectiveness, site suitability and cost. No single practice can meet the goals of the INRS on its own, so it will take a combination of practices across multiple scales to reach target nutrient reductions.

Edge-of-field and drainage systems will play particularly important roles for cropland with subsurface drainage. Edge-of-field options include bioreactors, saturated buffers, wetlands, shallow drainage and controlled drainage (or drainage water management).

Edge-of-field and conservation drainage system practices have some of the best performance for nitrogen reduction short of land use conversion. With the exception of drainage water management, which is highly dependent on weather, edge-of-field practices generally have less variability (uncertainty) associated with their performance than other INRS options such as nitrogen management and cover crops. Finally, edge-of-field practices are some of the most cost-effective strategies for nitrogen reduction with equalized annual costs of just over $1 per pound of nitrogen removed.

In 2016, the pace of new edge-of-field installations picked up in Iowa. The Iowa Soybean Association (ISA) focused efforts in the Elk Run and Rock Creek watersheds, as well as contracting with the Iowa Department of Agriculture and Land Stewardship (IDALS) to facilitate site determination and design. Three bioreactors and two saturated buffers were installed in ISA’s watersheds of focus with an additional 13 practices planned.

ISA conducted more than 12 site investigations as part of the IDALS edge-of-field project which began in June. An innovative saturated buffer system connecting seven outlets into three separate distribution lines was installed at the farm of ISA District Advisory Council member Tom Vincent as part of the IDALS project.

ISA remains in the forefront on edge-of-field practices in Iowa, and it is encouraging to see the pace of installations increase. In order to meet the ambitious goals of the INRS, the pace and scale of conservation drainage adoption and innovations to the practices themselves will need to continue to increase and intensify.

Chris Hay is an expert on conservation drainage.

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