Assessment of erosion hazard with the USLE and GIS:A case study of the Upper Ewaso Ng'iro North basin of Kenya

The Universal Soil Loss Equation (USLE) was used to predict soil erosion hazard in the Upper Ewaso Ng'iro North basin of Kenya using data from erosion plots and reconnaissance surveys. The R-factor (rainfall erosivity) was determined by extrapolation from the rainfall amount-erosivity relationship, using data from autographic records. The K-factor (soil erodibility) was determined using data obtained by laboratory analysis of soil samples collected from 83 test sites in the basin. The LS-factors (slope length and steepness) were determined from a digital elevation model (DEM) of the basin, while the C-factor (crop and management) was determined from vegetation cover data, obtained from SPOT imagery and field surveys. The P-factor (conservation practice) was estimated from maps of soil conservation, obtained from the Ministry of Agriculture. A raster-based Geographic Information System (GIS) was used to interactively calculate soil loss and map erosion hazard. The results obtained were comparable to measured soil loss values from erosion plots. About 36 percent of the Upper Ewaso Ng'iro basin was predicted to have high risk of erosion, most of this land being in the overgrazed rangelands. Land use and management were the major factors associated with soil erosion, and - to a lesser extent - the topography. Due to the reconnaissance scale of this study there were limitations in determining reliable LS, C and P factors for the USLE. In addition, there is a need for research to determine appropriate P-factors for local soil conservation practices, such as trash lines, fanya-juu terraces and stone lines