Estimation of Annual Soil Loss Rate from Hangar River Watershed Using RUSLE through the Application of GIS Technique

Abstract

Ethiopia has been described as one of the most seriously affected nation in the world by soil erosion. Several studies indicate the existence of sever soil erosion problem in different parts of the country. This erosion problem has on-site and off-site effects. The study area, Hangar River watershed, which is a sub catchment of Didesa River Basin, shares this sever erosion problems. Hence, the aim of this study is to estimate the spatially distributed mean annual soil loss rate and mapping of the vulnerable areas in the watershed using the Revised Universal Soil Loss Equation (RUSLE) adopted for Ethiopian conditions with the aid of Geographical Information System (GIS) techniques. The RUSLE parameters; such as rainfall erosivity factor (R-factor), soil erodibility factor (K-factor), slope steepness and slope length factor (LS-factor), vegetative cover factor (C-factor) and conservation practice factor (P-factor), which consists of a set of logically related geographic features and related attribute data were used as data input for the analysis. In order to quantify the soil loss rate in the study area, spatial and non-spatial source of data were used as an input. A digital elevation model (DEM) with 30 x 30 meter resolution was implemented for catchment delineation and analysis of the LS-factor of the study area. The land use/ land cover map of 2013 was used for the analysis of C-factor and the Soil map of the study area was also used for the analysis of the K-factor. The analysis of R-factor was derived from mean annual rainfall data of the nearby rain gauge stations. Eventually, each of the RUSLE factors, with associated attribute data were digitally encoded in a GIS database to create five thematic map layers of each factor. By integrating these five map layers in GIS raster calculator, the required spatially distributed annual average soil loss rate was determined. Accordingly, the result of the analysis for the existed conditions depicted that the amount of soil loss from the study area ranges from 1 to 500 t ha-1 yr-1 with average annual soil loss rate of 32 t ha-1 yr-1 from the whole catchment. About 84.2 % of the total area experienced soil loss above tolerable limit of 11 t ha-1 yr-1. The total annual soil loss from the entire watershed area of 7790 km2, was about 24.93 Mtons. It shows that, it could be difficult to maintain the sustainability of the soil productivity if the specified amount of soil is removed annually. To evaluate the effect of watershed management, particularly contour ploughing with terracing; if it is fully developed, and adjusting P-factor values for such conditions, the average annual soil loss rate would decrease from 32 to 19.2 t ha-1 yr-1. Hence, applying the specified watershed management reduces the vulnerability of the watershed by 40 %. Based on the result, most critical soil erosion areas were situated in the steepest upper part of the watershed due to intensive agricultural activities on the specified part of the watershed. Hence, this area needs immediate and appropriate intervention of soil conservation measures.