Validation of WEPP sediment feedback relationships using spatially distributed rill erosion data

Process-based soil erosion models have not been thoroughly evaluated due to the lack of spatially distributed, instantaneous rill erosion data. The objective of this study was to quantitatively evaluate the sediment feedback relationships of the Water Erosion Prediction Project (WEPP) model using distributed instantaneous rill erosion data derived with rare earth element (REE) tracers. Four REE oxide powders (Ce2O3, Nd2O3, SM2O3, and DY2O3) were separately mixed with a loessial silt loam soil, and each mix was packed in a 1-m segment of a 4-m flume. Each packed flume was subjected to flow scouring at a selected slope and inflow rate for 13 to 17 min, depending on slope and discharge. Three slopes (10.5, 15.8, and 20.2%) and five inflow rates (2.5, 3.5, 4.5, 5.5, and 6.5 L min(-1)) were used, and two replicates were made for each combination. Runoff and sediment samples were collected at 1- or 2-min intervals. Flow velocity and width were monitored. Sediment samples were analyzed for the REE composition by Instrumental Neutron Activation Analysis (INAA). The REE concentrations in each sample were used to estimate sediment deliveries from each I-m tagged segment. Net rill detachment rates tended to decrease linearly as sediment loads increased in the downslope direction. The negative slope of linear regression (or rate of the decrease), which became more negative with inflow rates at the 10.5% slope but less negative at the 20.2% slope, substantiated the sediment feedback relationships assumed in the WEPP model. The WEPP-calculated and REE-measured rill detachment rates agreed reasonably well, with the model efficiency being 0.511. Overall, the results show that the assumed sediment feedback relationships used in the WEPP model are reasonable for simulating rill detachment.