Mapping potential soil erosion using rusle, remote sensing, and GIS : the case study of Weenen Game Reserve, KwaZulu-Natal.

Abstract

Accelerated soil erosion is drawing a growing attention with the recognition that the rate of soil loss is too great to be met by soil formation rate. Weenen Game Reserve (WGR) is an area with an unfortunate history of prolonged soil erosion due to excessive overgrazing that led to severe land degradation with prominent visible scars. This problem triggered the general objective of estimating and mapping potential soil erosion in WGR. Assessing soil loss in the area objectively has important implications for the overall management plans as it is reserved for ecological recovery. The most important variables that affect soil erosion are considered as inputs in soil loss estimation models. In this study the RUSLE model, which uses rainfall, soil, topography, and cover management data, was employed. From the rainfall data, an erosivity factor was generated by using a regression equation developed by relating EI30 index and total monthly rainfall. The soil erodibility factor was calculated using the soil erodibility nomograph equation after generating the relevant data from laboratory analysis of soil samples gathered from the study area. Using exponential ordinary kriging, the point values of this factor were interpolated to fill in the non-sampled areas. The topographic effect, which is expressed as the combined impact of slope length and slope steepness, was extracted from the DEM of the study area using the flow accumulation method. For mapping of the land cover factor, in situ measurements of cover from selected sites were undertaken and assigned values from the USLE table before being related with MSAVI of Landsat 7 ETM+ image. These values were then multiplied to get the final annual soil loss map. The resulting potential soil loss values vary between 0 and 346 ton ha-1 year-l with an average of 5 ton ha-1 year-l. About 58% of the study area experiences less than 1 ton ha-1 year-1 indicating the influence of the highest values on the average value. High soil erosion rates are concentrated in the central part extending as far as the south and the north tips along the eastern escarpments and these areas are the ones with the steepest slopes. The results indicate a high variation of soil loss within the study area. Nevertheless, the majority of the area falling below the average might foresee that the soil erosion problem of the area can be minimized significantly depending largely on soil management. The most important areas for intervention are the medium and low erosion susceptible parts of WGR, which are mainly found in the flatter or gently sloping landscapes. The steepest areas are mostly covered with rocks and/or vegetation and hence less effort must be spent in managing them. Overall, the reported increasing density of the vegetation community in the area that reduces the exposure of soil from the impact of direct raindrops and surface-flowing water must be pursued further.