A Comparison of Watershed Models in the Clear Creek, IA, Watershed

The Clear Creek, IA watershed (CCW), which drains to the Iowa River, experiences severe surface erosion due to a combination of high slopes, erodible soils, and extensive agriculture. Several computer models are currently available that can evaluate the magnitude of erosion occurring in the CCW at the watershed scale. Each model is inherently different resulting from unique combinations of possible assumptions and algorithms; moreover, each model is bounded by its own limitations regarding spatial scaling. Two public domain models were used in this study to evaluate the water and sediment fluxes from a sub-watershed in the headwaters of the CCW. The Water Erosion Prediction Project (WEPP) is a dynamic, process-based, continuous simulation, erosion prediction model that is most applicable to hillslope erosion processes, namely sheet and rill erosion. Simulation of the hydrologic and erosion processes in WEPP is best performed in small watersheds up to 2.6 km². The ANNualized AGricultural Non-Point Source Pollution Modeling System (AnnAGNPS) is designed to evaluate hydrology, soil erosion, and nutrient pollution at the watershed scale. AnnAGNPS will process data on a single cell scale (0.16 – 1 km²) but will integrate the information from these cells into sub-watersheds ranging from 200 to 1650 km². The runoff, gross upland erosion, and the fluxes of water and sediment from the CCW sub-watershed were compared for 2-, 5-, and 10-year simulations assuming a single soil type and land-use throughout the catchment. These simulations were also compared to more detailed simulations from AnnAGNPS that implemented the various heterogeneities of soil type and land-use in the CCW. Predicted runoff was similar for both models; however, for the homogeneous simulations, WEPP produced higher upland erosion rates and sediment fluxes. The heterogeneous simulations for AnnAGNPS produced sediment fluxes similar to estimated fluxes from the watershed.