Hydrologic impacts of tile drainage in Iowa

ii ACKNOWLEDGMENTS I would like to acknowledge the following for completion of this thesis: God for giving me the patience, guidance and strength to succeed in this research. I am so grateful for everything I have been given and everything I was able to do. Without God I can do nothing. Dr. Nandita Basu for challenging and guiding me throughout graduate school and this research. I have grown and learned so much from her and am so thankful for her efforts. Dr. Ricardo Mantilla for his creativity and assistance in the catchment scale portion of this thesis. All the staff and students at IIHR for their assistance and support during my research. I could have never learned as much as I did without their help. Finally, to my family and friends who have been unwavering in their support of me. I love you all very much. iii ABSTRACT 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, …