Sediment Yield Potential in South Africa's Only Large River Network without a Dam: Implications for Water Resource Management

Soil erosion not only involves the loss of fertile topsoil but is also coupled with sedimentation of dams, a double barrel problem in semi‐arid regions where water scarcity is frequent. Due to increasing water requirements in South Africa, the government is planning water resource development in the Mzimvubu River Catchment, which is the only large river network in the country without a dam. However, previous soil erosion studies indicate that large parts of the catchment are severely eroded. These studies, nonetheless, focussed only on rill and interill erosion processes and provided insufficient information about sediment yield. The present study maps and models the sediment yield potential by considering important connectivity factors (flow path length, slope steepness and surface roughness) over a 5‐year timeframe (from 2007 to 2012). Sediment yield contribution from rill–interrill erosion was modelled with ArcSWAT‐2012, whereas gully erosion contributions were estimated using sequential gully mapping with SPOT 5 imagery followed by gully‐derived sediment modelling in a GIS. Integration of the rill–interrill and gully results provided the total sediment yield potential, with an average of 5100 Mg km−2 y−1. The average sediment yield where the irrigation and hydropower dams are planned is around 20 000 Mg km−2 y−1. This study provides relative differences of the potential sediment yield and a framework to predict sediment yield to assist area‐specific rehabilitation in large, ungauged catchments. Without catchment rehabilitation, the life expectancies of the future irrigation and hydropower dams could be 55 and 43 years, respectively. Copyright © 2017 John Wiley & Sons, Ltd.