Supplemental information for "Hydrologic and Crop Production Impacts of Drainage District Outlet Improvements"

In the Des Moines Lobe region of Iowa and other poorly drained U.S. Corn Belt areas, artificial subsurface (tile) drainage infrastructure shared by neighboring landowners within the same watershed/drainage district is essential for highly productive row crop agriculture. However, in many cases, this district infrastructure is over 100 years old, past the end of its original design life, and possesses outlets undersized by modern standards. Additionally, tile drainage has contributed to significant negative impacts on downstream water quality and regional biodiversity. In this study, we analyzed five north-central Iowa drainage districts that had coupled district outlet improvements with water quality wetlands and five similar unimproved districts that were selected as control districts in a paired experimental design. Specifically, we compared cropland ponding and evapotranspiration between these districts to understand the crop production and hydrologic impacts of drainage district improvements. We mapped cropland ponding using publicly available satellite images and the Modified Normalized Difference Water Index (MNDWI) and found that its extent was reduced by 86.7% (p-value = 0.0192; CI = 46.5% < x < 96.7%) following drainage district improvements. A simple economic analysis based on the ponding data showed that the payback period for an improvement-only project is 11.2 years and the payback period for an improvement + wetland project is 25.1 years, in an “average-case” scenario. These results highlight the potential and economic feasibility of district outlet improvements to enhance crop production. Using the OpenET Ensemble dataset, we also measured 11.9 mm less annual ET in improved districts than control districts, which can be attributed to soil moisture and ponded water differences in the early growing season. Thus, drainage district improvements alone could increase annual drainage flow volume by nearly 4% assuming that ET reductions are transferred to additional flow. Pairing nutrient reduction practices like wetlands with district improvements is critical to account for increased flow and associated additional nitrate export.