ARS | Publication request: Managing farmed closed depressional areas using blind inlets to minimize phosphorus and nitrogen losses

Submitted to: Soil Use and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 24, 2012
Publication Date: N/A

Interpretive Summary: Potholes, or closed depressions are an important feature in the glacial till landscapes of the Midwestern United States. Often, these low points in fields offer some of the most fertile soils with the field where they appear. Through watershed scale research in the St. Joseph River watershed in the United States, farmed potholes have been identified as contributing to nutrient loading of streams. Most farmed potholes are drained with tile risers, which are direct conduits for runoff water and associated contaminants directly to nearby streams. The objective of this study was to determine if blind inlets could be used as an alternative to tile risers to improve the quality of water drained from potholes. We instrumented and monitored a pair of potholes, and installed both a tile riser and a blind inlet at the low point of both potholes, which we constructed so that researchers could switch between the two drainage practices. A pair of small watersheds (ca. 700 ac) was also used to compare the effectiveness of this practice at a larger scale, as all the tile risers in one of the watershed were replaced with blind inlets in Spring 2010. Discharge, sediment and nutrient loads were lower when the experimental fields were drained with the blind inlet compared to the tile riser. Late Spring 2010 was much wetter than normal, and increased nutrient loading was observed in both small catchments compared to previous years. The increased loading from the treated watershed was less than the increased loading in the control watershed. Results from this study indicate the replacing tile risers with blind inlets in this landscape may be one method to effectively reduce nutrient and sediment loading to Lake Erie and other sensitive water bodies.

Technical Abstract: Through watershed scale research in the St. Joseph River watershed in the United States, farmed potholes have been identified as contributing to nutrient loading of streams. Most farmed potholes are drained with tile risers, which are direct conduits for runoff water and associated contaminants directly to nearby streams. The objective of this study was to determine if blind inlets could be used as an alternative to tile risers to improve the quality of water drained from potholes. In a pair of monitored potholes, a tile riser and a blind inlet were placed at the low point of both potholes, and were constructed so that researchers could switch between the two drainage practices. A pair of small catchments (ca. 300 ha) were also used to compare the effectiveness of this practice at a larger scale, as all the tile risers in one of the catchments (B catchment) were replaced with blind inlets in Spring 2010. Discharge, sediment and nutrient loads were lower when the experimental fields were drained with the blind inlet compared to the tile riser. Late Spring 2010 was much wetter than normal, and increased nutrient loading was observed in both small catchments compared to previous years. The increased loading from the B catchment was less than the increased loading in the control (A) catchment. Results from this study indicate the replacing tile risers with blind inlets in this landscape may be one method to effectively reduce nutrient and sediment loading to Lake Erie and other sensitive water bodies.