Long-term, process-based, continuous simulations for a small, nested rangeland watershed near Tombstone, AZ (USA): Extending model validity to include soil redistribution.

Soil or sediment redistribution prediction along hillslopes and within small watersheds is considered to be a great challenge for the application of watershed erosion models in predicting the impact of soil and water conservation measures as well as for the redistribution of pollution such as radioactive fallout. In this study, long-term soil loss and deposition were estimated for two nested semi-arid watersheds within the Walnut Gulch Experimental Watershed in Southeastern Arizona using the process-based Geo-spatial interface of WEPP (GeoWEPP). While soil parameters were previously parametrized and validated through watershed outlet runoff and sediment yields, the channel parameters were adjusted and validated based on reference values of soil redistribution generated from fallout radionuclide Cs samples within the watersheds. Two methods were applied for the soil redistribution analysis by comparing observed and simulated soil loss/deposition rates (a) at single pixels and reference values at the specific location of each Cs sample site; and (b) for average values of a 5 m radius around each Cs sample site to compensate for measurement and model uncertainties. Surprisingly, soil redistribution predictions improved as topographic data resolution increased from 5 m to 3 m and were best at 1 m without changing key model parameters that were originally derived at the watershed scale.