Hydrologic impacts of tile drainage in Iowa

Agricultural tile drainage is an integral part of Iowa's landscape, with nearly 30% of Iowa's cropland being drained (Schilling & Helmers, 2008). Tile drainage allows for efficient crop production in Iowa's nutrient rich soils by removing excess water from frequently inundated fields through subsurface pipe networks. These tile systems are suspected of altering the hydrologic regime of Iowa, but the extent of the problem remains unknown. A literature review is performed to assess the current understanding of tile drainage and to help create a framework of analysis to address this problem. The deterministic field-scale model DRAINMOD is used in both a field and catchment scale analysis of the hydrologic impacts of tile drainage in conditions typical to Iowa. The field-scale study explores the influence of soil type, surface storage, rainfall characteristics, and drainage spacing on how tiling impacts the hydrologic response . A range of metrics, including the mean annual peak flow, flow duration curves, and the Richard Baker Flashiness Index are used for the analysis. Subsurface drainage was observed to have no impact on the mean annual peak flow. This is because the largest storms of the year are almost always dominated by surface runoff, rendering the additional storage created by the tiles inconsequential. Metrics that captured the entire flow regime were, however, affected significantly by tiles. The flashiness index of less permeable soils, typical of Iowa, reduced with tile drainage, due to a change from surface dominated to subsurface dominated flow. The reduction varied spatially as a function of rainfall percentiles, and at 25th percentile rainfall, increases were observed in certain areas. A saddle shaped behavior was observed between tile spacing and flashiness index, demonstrating the existence of an optimal spacing minimizes the effect of drainage. The field scale DRAINMOD results are then used in conjunction with a simplified routing equation to analyze the impact of tile drains on the Clear Creek Watershed (CCW) in Iowa. It was found that adding drained fields to the densest portion of the CCW width function leads to the greatest reduction of peak flow at the outlet, as long as the percentage of the watershed drained is maintained constant.