Seasonal nutrient removal from agricultural drainage ditches using low-grade weirs

Jeffrey S. Strock, Southwest Research & Outreach Center, University of Minnesota, Lamberton and Andry Z. Ranaivoson, Southwest Research & Outreach Center, University of Minnesota, Lamberton, MN

Background/Question/Methods
In order to improve productivity, extensive agricultural areas in the Midwest require artificial drainage systems consisting of subsurface drainage (tile) and open ditches. These drainage systems are known to transport particulate and dissolved phosphorus, nitrate-nitrogen, and sediment to streams and rivers. The objective of this research was to measure the seasonal variability of nitrogen and phosphorus removal from experimental drainage ditches with and without low-grade-weirs in a cold climate. This field research took place in the Cottonwood River Watershed at the University of Minnesota, Southwest Research and Outreach Center near Lamberton, MN. The site included a paired drainage ditches equipped with H-flumes and water level sensors for determination of discharge and flow rates along with automated samplers for collecting water to quantify water quality parameters.
Results/Conclusions
Monthly and annual precipitation influenced ditch discharge and nutrients loads. On average, ditch management between 2017 and 2020 resulted in a 55% reduction in discharge from the ditch with the low-grade weir compared to the ditch without the low-grade weir. The ditch containing the low-grade weir resulted in nitrate-nitrogen and dissolved reactive phosphorus load reductions of 62% and 63%, respectively, over the four years. Flow weighted nitrate-nitrogen concentration was reduced by 51% whereas flow weighted dissolved reactive phosphorus concentration was reduced by only 6%. Minimally invasive ditch management has the potential to mitigate nutrient fluxes from agricultural drainage and runoff without impacting drainage system operation.