Modelling the spatial variation of processes in the redistribution of soil: digital terrain models and 137Cs in southwest Niger

An interpolated grid-based digital elevation model (DEM) was created from survey data to derive two compound topographic indices, a stream power index to provide a surrogate measure of water erosion and an index of susceptibility to aeolian processes. Maps of both indices were compared with a map of interpolated percentage 137Cs residuals, used as a surrogate for net soil redistribution over the last ca. 30 years. Results suggest that wind erosion is pervasive throughout the study area, whereas water erosion is concentrated on the steeper slopes. Areas of potential aeolian deposition were identified on the lee-side of obstacles, but were validated in only a few cases, by the map of 137Cs residuals. This anomaly probably results from considerable reworking of aeolian material deposited in the lee of obstacles, on slopes highly susceptible to water erosion. Although the digital terrain models (DTMS) could not explain the pattern of 137Cs residuals in some locations, the inclusion of the percentage of vegetation cover further elucidated the patterns of soil redistribution. Vegetation cover was found to explain a considerable amount of variation in the patterns of soil redistribution over the medium-time scale. Further research into the modelling of these processes is necessary to elucidate the pattern of vegetation cover removal and its link with soil redistribution.