Assessing Watershed Vulnerability Using Weighted Overlay and Analytical Hierarchy Techniques: The Case Study of Gojeb River Watershed–Kaffa, Ethiopia

Abstract

Understanding the susceptibility of Watershed is very crucial issue in identification of the health status of water bodies. The watershed characteristics such as size, shape, slope, drainage density, land use, geology, soil type etc. are some common factors affecting the health of watershed. As this watershed characteristic can influence the vulnerability to pollution, it’s better to assess them. Assessing Watershed vulnerability and identification of risk is very important input for understanding the status of watershed health condition. Gojeb Watershed was investigated to indentify the extent of vulnerability so that parts and zones of vulnerability extent to be well defined. Therefore, the aim of this study was to assess the vulnerability of Gojeb River watershed using weighted overlay and Analytical Hierarchy techniques. The method used to study Gojeb Watershed was done by using the combination of Software. GIS spatial analysis tool was used to produce the maps that show the vulnerable parts of the Watershed. Using these maps AHP analyzed which parts of Gojeb Watershed were affected with respect to the extent of susceptibility in percentage of area coverage. The analysis was done using five main factors that influence water quality. Land use land cover, soil type, annual average rainfall, slope gradient, and bedrock type were the main factors considered based on their contribution to water pollution. The main findings obtained were identifying the vulnerable parts of the watershed and showing it by thematic maps. Accordingly, 0.24 % were very high vulnerable, 5.7 % were high vulnerable, 38.6 % were moderately vulnerable, 55 % were low vulnerable and 0.47 % were very low vulnerable to contamination. This study clearly defined and relates how land use, slope, precipitation, geology and soil factors affect the watershed of Gojeb River. Therefore, It is concluded that Gojeb River Watershed to be exposed to vulnerability to contamination in some parts and hence, need reclamation work. The recommended measures for reclamation were to take the action of reforestation and terrace building.