Soil erosion and sediment yield in Africa: Processes and factors

Abstract

Africa experiences diverse and often intense soil erosion processes, yet the interplay between these processes and observed sediment yield (SY) at catchment scales is insufficiently understood. Building on previous continental-scale assessments, this study integrates a newly verified database of measured SY at 425 river outlets with, model-based estimates of Rill and Sheet Erosion (RSE), Landslide Mobilization Rates (LMR), and Gully Head Susceptibility (GHS). Our approach addresses a key gap in earlier work, which primarily relied on environmental predictors without explicitly quantifying the contribution of multiple geomorphic processes.

Our results indicate that the mean observed SY is 6.07 t⋅ha⁻¹⋅yr⁻¹, whereas modelled RSE and LMR average 9.42 t⋅ha⁻¹⋅yr⁻¹ and 1.51 t⋅ha⁻¹⋅yr⁻¹, respectively. Although direct estimates of gully erosion were not available, the moderate to strong correlations between SY and both LMR (r = 0.47) and GHS (r = 0.57) underscore the importance of gullies and landslides in delivering sediment to catchment outlets. In contrast, RSE shows a weaker relationship with measured SY, suggesting that traditional RUSLE-type models may substantially over- or underestimate total sediment yield if they neglect gully erosion, landslides, and in-channel deposition processes.

This study contributes novel insights into continental-scale sediment dynamics by explicitly linking observed SY to distinct geomorphic processes. Our findings highlight the need for integrated modeling frameworks that account for rill and sheet erosion, landslides, and gully erosion in order to enhance the accuracy of sediment yield predictions. Such approaches are vital for guiding land management and soil conservation strategies across Africa's diverse biophysical and climatic regions.