Gully Erosion Susceptibility Assessment and Mapping Using Integrated Geospatial Techniques and Multi-Criteria Decision Analysis for Selected Watersheds of Bale Highland, Oromia, Southeast Ethiopia

Gully erosion is one of the main causes of land degradation, which poses a serious threat to agricultural production, ecosystem stability, and the livelihoods of people living in the Ethiopian highlands. The objective of the study was therefore to analyze the spatial variability and relative importance of gully erosion conditioning factors and generate a map of gully erosion susceptibility to facilitate the selection of priority sub-watersheds for soil and water conservation works in the Bale Highlands. Different sources of spatial data, such as DEM, land use/land cover, soils, rainfall, and gullies inventoried through GPS, were combined in a GIS environment. Several terrain and hydrological parameters, like the Stream Power Index and the Topographic Wetness Index, were calculated to express the erosion processes. The 10 gully erosion conditioning factors were evaluated in terms of their weights based on expert opinion and pairwise comparison using the Analytic Hierarchy Process. Weighted Overlay Analysis (WA) in a GIS environment was used to generate a gully erosion susceptibility map with five levels, namely very low, low, moderate, high and very high. High to very high susceptibility areas are dominantly located in steep cultivated lands and hydrologically convergent zones in drainage channels. It has been found that the validation process has confirmed that 80% of the gully sites are classified into moderate and very high susceptibility categories with an R² value of 0.75, which shows adequate model performance.