A coupled upland-erosion, instream hydrodynamic-sediment transport model for assessing primary impacts of forest management practices on sediment yield and delivery

The purpose of this research was to develop a modeling system/framework for assessing forest management-related erosion at its sources and tracking sediment as it is transported from hillslopes to stream channels, and transported through a channel network to a watershed outlet. The ultimate goal was to develop a land management assessment tool capable of accurately assessing the primary impacts of spatiotemporally varied forest management practices on sediment yield and delivery at hillslope- and watershed-scales.; The modeling framework developed consists of four components: (1) the TOpographic ParameteriZation model for discretizing hillslope and channel elements, (2) the Water Erosion Prediction Project model for evaluating hillslope-scale surface erosion processes, (3) the National Center for Computational Hydrodynamics and Engineering One-Dimensional hydrodynamic-sediment transport model, and (4) an interface program to manage relational databases and data transfer between modules.; The coupled model was calibrated and validated with observed flow and sediment load data from Caspar Creek Experimental Watershed in coastal, northern California. The coupled model predicts peak flow rates, total flow volume, and sediment loads significantly better than the empirical methods used by the WEPP Watershed model. The coupled model predicted flow rates that were not significantly different from observed values, and sediment loads that were within typical ranges for sediment transport equations.; The most significant finding of this research project was the limits of applicability of the WEPP Hillslope model. It was found that the results of WEPP Hillslope erosion simulations became more divergent from actual values as the critical source area for delineation of first order channels increases. Critical source areas (CSAs) between 5 and 10 ha yield runoff rates that are not significantly different from observed values. However, as the CSA is increased, runoff rates and sediment loads become exponentially divergent from observed values. It is concluded that the governing equations used to represent hillslope-scale erosion processes in the WEPP Hillslope model begin to break down for assessment areas greater than 10 ha; and that this area delineates a point where hillslope-scale runoff and erosion processes give way to more dominant watershed-scale open channel flow and sediment transport processes.