Integrated numerical modeling of spatial and time-variant hydrologic response in subsurface drained watersheds

Existing steady state and time-variant formulations for simulating flow to tile drains were studied extensively for their applicability to single tile drainage systems. Review of the literature indicated that available models were inadequate to represent the characteristics of the Upper Little Vermilion River Watershed because it has extensive networks of subsurface drains predominantly laid in irregular patterns. The importance of preferential flow in the watershed was identified from measured rapid responses of tile drains to rainfall events. A general-purpose, two-dimensional variably-saturated model was used with appropriate modifications to include preferential flow and water flow in soils with single tile drains. Tile drains were represented as a single node seepage boundary with adjusted hydraulic conductivities in the surrounding elements. Preferential flow was modeled assuming one-dimensional non-Darcian flow in vertically connected cracks and pores. The subsurface module was integrated with physically-based routing components to simulate 1D overland runoff, stream flow, and tile flow in a distributed framework.; Field-scale studies showed that the subsurface module could provide good predictions with accurate descriptions of soil hydraulic properties, climatic data and drainage parameters. Simulated results at the field scale also indicated that the subsurface model was highly sensitive to estimates of evapotranspiration during the summer. Hydraulic parameters obtained from calibration studies failed to simulate post-rainfall event recessions. Drain flow simulations that included macropore flow resulted in higher magnitudes of response and better agreement with observed data, compared to simulations that considered sorptive flow alone. Simulations also showed that drain responses were in better agreement with observed flows during low rainfall, as compared to high rainfall. Results from the application of the integrated model to the Upper Little Vermilion River Watershed showed that the integrated modeling framework underestimated stream flows, indicating the importance of contributions from surface runoff and ditch drainage.