Simulating the Effects of Drainage and Agriculture on Hydrology and Sediment in the Minnesota River Basin

Monday, 15 December 2014
Charles W Downer1, Nawa Raj Pradhan1, Brian E Skahill1, Ann M Banitt2, Greg Eggers3 and Ryan E Pickett1, (1)Engineer Research and Development Center Vicksburg, Coastal and Hydrualic Laboratory, Hydrologic Systems Branch, Vicksburg, MS, United States, (2)USACE St Paul District, Water Management and Hydrology Section, St Paul, MN, United States, (3)Minnesota Department of Natural Resources, EWR, St Paul, MN, United States
Throughout the Midwest region of the United States, slopes are relatively flat, soils tend to have low permeability, and local water tables are high. In order to make the region suitable for agriculture, farmers have installed extensive networks of ditches to drain off excess surface water and subsurface tiles to lower the water table and remove excess soil water in the root zone that can stress common row crops, such as corn and soybeans. The combination of tiles, ditches, and intensive agricultural land practices radically alters the landscape and hydrology. Within the watershed, tiles have outlets to both the ditch/stream network as well as overland locations, where the tile discharge appears to initiate gullies and exacerbate overland erosion. As part of the Minnesota River Basin Integrated Study we are explicitly simulating the tile and drainage systems in the watershed at multiple scales using the physics-based watershed model GSSHA (Gridded Surface Subsurface Hydrologic Analysis). The tile drainage system is simulated as a network of pipes that collect water from the local water table. Within the watershed, testing of the methods on smaller basins shows the ability of the model to simulate tile flow, however, application at the larger scale is hampered by the computational burden of simulating the flow in the complex tile drain networks that drain the agricultural fields. Modeling indicates the subsurface drains account for approximately 40% of the stream flow in the Seven Mile Creek sub-basin account in the late spring and early summer when the tile is flowing. Preliminary results indicate that agricultural tile drains increase overland erosion in the Seven Mile Creek watershed.