Modelling interrill erosion in small cultivated catchments

On the cultivated plateau of the loess belt area, the redistribution of precipitation is controlled by topography, cultural techniques and sealing processes. Recent multi-scale studies carried out in northern France have provided quantitative references of interrill erosion rates. These experimental references have highlighted significant differences and evolution trends of mean sediment concentration in interrill flow between different defined categories of soil surface conditions, vegetation and rainfall characteristics. Thus, a classification of sediment concentration in interrill flow has been elaborated (Cerdan O, et al. 2002. Earth Surface Processes and Landforms 27(2): 193-205). The objective of this paper is to incorporate these results in the elaboration of the interrill erosion module of the STREAM model. At the field scale, this classification is used to assign a potential sediment concentration value to each agricultural field. For situations where there is no, or insufficient, experimental references, the classification is completed using a fuzzy logic approach. At the catchment scale, sediment is transported in proportion of the runoff volumes computed with the STREAM runoff module, and is deposited as a function of topography (vertical curvature, slope gradient), or vegetation cover. Sediment concentration thresholds were defined after field experiments; these thresholds define the limits above which sediment mass is deposited. The erosion component of the model has been tested with a data set from a small (ca 90 ha) intensively cultivated catchment. The quality of prediction given by the root-mean-square error (RMSE) is ca 8 t of precision on sediment delivery at the catchment outlet for observed sediment delivery that goes from 0.075 to 21 t (with an RMSE of ca 0.9 t for observed sediment delivery below 1 t and of ca 10.6 t for observed sediment delivery between 1 and ca. 21 1). The evaluation also showed that the results at the outlet of the catchment are more sensitive to the concentration thresholds of the deposition rules than to the potential sediment concentration values assigned to each field. Copyright (C) 2002 John Wiley Sons, Ltd.