Managing Water Quantity and Quality with Subsurface Drainage in Eastern South Dakota

Moisture extremes (excess and deficit) impact crop loss and water quality. Due to excessively wet springs and dry summers, crop damage can occur within the same county or field within the same year. To determine the magnitude of this problem in eastern South Dakota, indemnified crop insurance data for drought and excessive moisture claims were assessed for the years 1991-2020 for the occurrence of both excess moisture and drought in four counties in eastern South Dakota. Results show that there is greater than a 70% chance of the moisture extremes happening in the same year, making subsurface drainage, controlled drainage, and irrigation viable options for mitigating the damages. To determine the number of times controlled drainage could have had an impact on soil moisture, a DRAINMOD simulation was analyzed for the years 1950-2012. The results showed on dry and average years, when controlled drainage has potential for the greatest impact on soil moisture, 20 of 47 years had potential to retain soil moisture in the soil profile. In addition to challenges related to water quantity, water quality can be impacted by tile drainage systems. To assess the amount of nitrate-nitrogen entering surface water, 23 tile outlets were monitored weekly for nitrate concentration and flow depth in the tile outlet pipe. The results showed of 352 samples taken (mean 12.4 mg L-1 nitrate-N), 195 samples were above and 157 were below the drinking water standard of 10 mg L-1, with the majority of samples taken at a depth less than 0.15 of the tile diameter, indicating a low flow year