Hydrologic and Crop Production Impacts of Drainage District Outlet Improvements

Highlights Improvement of drainage district outlets to the modern standard reduced cropland ponding by 86.7%. Average payback periods for improvements and improvements + wetlands are between 18 and 33 years. Improved districts had less ET than control districts in the early growing season. District improvement + wetland projects can benefit both crop production and the environment. ABSTRACT. In the Des Moines Lobe region of Iowa, district tile drainage infrastructure is essential for highly productive row crop agriculture. However, in many cases, drainage district infrastructure is old, undersized by modern standards, and contributes to negative water quality outcomes. In this study, we assessed the hydrologic and crop production impacts of five Iowa drainage districts that coupled district outlet improvements with water quality wetlands. Our objectives were to (1) quantify the impact of district improvements on cropland ponding, (2) evaluate the economic viability of drainage improvement projects, and (3) assess the effects of improvements on evapotranspiration. We mapped cropland ponding using satellite images and the Modified Normalized Difference Water Index (MNDWI) and found that its extent was reduced by 86.7% following district improvement. A simple economic analysis based on the ponding data showed that the payback period for an improvement-only project is 18 years and the payback period for an improvement + wetland project is 33 years in an “average-case” scenario. Using the OpenET dataset, we measured 11.9 mm less annual ET in improved districts than control districts, which we attribute to soil moisture and ponded water differences in the early growing season. Thus, drainage district improvements could potentially increase annual drainage flow volume slightly (4%) if we assume ET reductions are transferred to additional flow. This study showed that district improvement projects can significantly reduce the extent of cropland ponding, likely pay for themselves before their expected 100-year design life ends, and slightly reduce ET.