Improvement of the Water Erosion Prediction Project (WEPP) model for quantifying field scale subsurface drainage discharge

In the poorly drained regions of the world, subsurface drainage systems are required to remove excess water for crop growth. Plastic drains alter a field’s hydrology by lowering the water table, reducing surface ponding, and reducing surface runoff. One significant concern with the use of subsurface drainage systems is adverse environmental effects because of the modification of the soil water dynamics. Some effects include the reduction of ecological services since wetlands change to croplands, water quality concerns, particularly sediment, nitrogen, and phosphorus losses in agricultural subsurface discharge water, as well as changes in the volume and timing of off-site discharges. Hydrological simulation models predict surface and artificial subsurface flow at different scales. Often in these models, Hooghoudt-based expressions are adapted in their internal algorithms. In this study, the Water Erosion Prediction Project (WEPP) model, developed by the United States Department of Agriculture - Agricultural Research Service (USDA-ARS) for soil and water conservation planning activities, was tested and improved to simulate surface and subsurface discharges. The modified WEPP model was tested and validated on an extensive dataset collected at four experimental sites managed by USDA-ARS within the Lake Erie Watershed. Predicted drainage discharges show Nash-Sutcliffe Efficiency (NSE) values ranging from 0.50 to 0.70, and Percent Bias ranging from −30% to +15% at daily and monthly resolutions. Evidence suggests that the WEPP model can be used to produce reliable estimates of subsurface flow with minimum calibration. Future work includes the extension of the model for quantifying subsurface drainage under controlled water table and watershed-scale simulations.