Soil erosion risk modelling in the Limpopo River Basin, crossing borders and mountains in Southern Africa

Soil erosion represents a significant yet often underrecognised threat to water management in southern Africa, particularly within water source areas located in mountainous regions such as the Limpopo River Basin (LRB). Since previous soil erosion maps of the LRB are global assessments coupled with spatial uncertainties, a need to improve techniques of mapping the soil erosion risk in the LRB was identified by the Limpopo Watercourse Commission (LIMCOM). The aim of this study is to create an improved soil erosion risk map and model the sediment yield potential of the LRB. This was achieved by interfacing the Revised Universal Soil Loss Equation (RUSLE) in a GIS, as well as the application of a sediment delivery ratio (SDR) model and the index of connectivity method to estimate the potential sediment yield. Multiple geo-spatial datasets were formatted at a regional and transboundary scale to improve the most important water erosion risk factors, namely rainfall erosivity, soil erodibility, topography, vegetation cover and support practices. According to the results, the average soil erosion rate of the LRB is 7.0 t/ha·yr−1, and the area of land with a moderate to extremely high erosion risk (>12 t/ha·yr−1) totals nearly 6 million ha (14% of the basin land surface). Results indicate that a combination of erodible soils and poor vegetation cover leads to the occurrence of erosion on flat, low-lying areas (mountain footslopes and valley floors) instead of steep mountainous terrain (midslopes and crests) as frequently determined in other parts of the world.