Spatial evaluation of a physically-based distributed erosion model (LISEM)

Generally, spatially distributed erosion models are 'validated' using only data on runoff and sediment load collected at the catchment outlet. This im plies that one of the most important aspects of such models, i.e. the predi ction of the spatial variation of erosion and deposition within the catchme nt is not at all tested. After an extreme event. in a small agricultural ca tchment in the Belgium loam belt, erosion and deposition patterns were mapp ed by measuring rill and gully volumes and the thickness of sediment deposi ts. Volumes of interrill erosion were estimated. From these data, the erosi on and deposition budget was calculated. The physically based soil erosion model LISEM was then used to simulate the event. Model parameters which cou ld not be derived from the field data were estimated using guidelines for L ISEM proposed for South Limburg (Netherlands), an area very near and simila r to the Belgian catchment. The model was calibrated on total erosion withi n the catchment by varying the infiltration parameters. The results of the simulations show that the model predicts reasonably well the overall sedime nt delivery ratio of the event (ca. 60%). However, the observed variation i n erosion rates for different crop types is not well-predicted. In general, LISEM strongly overpredicts erosion rates on fields with an important vege tation cover. Furthermore, the correlation between predicted and measured e rosion rates per crop type at locations of the measured rill transects is w eak. The predicted pattern of deposition is generally similar to the mapped deposition pattern. Discrepancies are mainly due to the fact that, in some cases, important (unpredicted) deposition was observed upstream of vegetat ion barriers or alongside roads. The study clearly shows that it is not pos sible to evaluate the performance of spatially distributed erosion models b y using catchment outlet data alone. Such a 'validation' can mask important spatial variations within the catchment which are not accurately predicted . It is therefore possible to obtain a good agreement between observed and simulated outlet data, while the spatial pattern erosion and deposition wit hin the catchment is not well-described. (C) 1999 Elsevier Science B.V. All rights reserved.