GIS Based Annual Average Soil Loss Rate Estimation: The case of Sor River Watershed

Abstract

Soil erosion is one among the most critical worldwide environmental challenges facing the world nowadays. Erosion by water can be dramatic during storm events resulting in washouts and gullies. Different studies show the occurrence and existence of soil erosion in different parts of the country. Almost all south west parts of Ethiopia have a rainfall throughout the year. Sor river watershed, which is one of the sub-basin of Baro Akobo river basin, is found in this rainy region. Hence, this study was undertaken to estimate the annual average soil loss rate from Sor watershed which shares the severity of the soil erosion problems using Revised Universal Soil Loss Equation (RUSLE) implemented for Ethiopian conditions with the integration of Geographic 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. Digital elevation model(DEM) of 30m spatial resolution, 27 years rainfall records of six stations, land use/land cover map of 2013 soil map of study area and field information were used for the analysis of RUSLES’s soil loss parameters. The parameters were analyzed and integrated using raster calculator in Arc GIS 10.4.1 and the required spatially distributed annual average soil loss rate was obtained. Therefore, the result indicated that the annual soil loss of the watershed extends from none in the lower to 330 t ha-1 y -1 in the steeper slope part of the watershed with a mean annual soil loss 14.86 t ha-1 y -1 . The total annual soil loss from the entire watershed area of 2273km2 was 3.38 M tons. 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 14.86 to 8.57 t ha-1 yr -1 . Hence, applying the specified watershed management reduces the vulnerability of the watershed by 42.3 %. Thus, sustainable soil and water conservation mechanisms should be implemented in order to reduce soil erosion in the watershed.