Evidence from field-based studies of rates of soil erosion on degraded land in the central Karoo, South Africa

Abstract

Degradation in Sneeuberg Region of the central Karoo, South Africa, is characterized by the development of badlands on the footslopes of upland areas and by gully systems in valley bottoms. Evidence from aerial photographs suggests that there has been a slight decrease in degraded areas between 1945 and 2002, although gully systems have increased in density, if not in length. Field measurements were carried out using a grid pattern of erosion pins at ten sites over a three-year period. On interfluves within existing degraded areas erosion continues at an average rate of 5.6 mm yr − 1 . On channels, footslopes, and sidewalls erosion rates are 2.6 mm, 4.7 mm and 16.7 mm respectively. The overall figure masks the high degree of cut and fill experienced within degraded areas. In channels, approximately eight times as much sediment is being moved compared to the overall loss at the end of 3 years. These figures indicate that the relief of most badland areas is decreasing. Erosion pin results are compared with losses from stones on pedestals and from the area around shrubs. There is evidence that rainfall has increased in amount and intensity over the last century which would promote erosion on existing bare ground. At the same time stock numbers have continued to decrease since a high point in the 1930s, reducing grazing pressure. This, coupled with the fact that most available colluvial footslopes are already degraded, may explain the overall decrease in the extent of degraded areas. However, monitoring results, observational evidence and sedimentation in dams indicate that badlands remain active and an important source of sediment.