Gully network expansion and spatial and temporal dynamics of catchment geomorphic characteristics and gully topographical thresholds in the semi-arid Ethiopian Rift Valley

To analyse the driving forces of gully erosion using a present dataset of geomorphic parameters and land use/cover involves limitations because past datasets at the time of gully incision may best explain the gully formation and evolution at that time. The recent development of photogrammetric techniques enabled to estimate temporal gully volume changes. This study conducted in semi-arid Ethiopian Rift Valley used field measurements and gully volume–length relation to analyse spatial and temporal dynamics of catchment geomorphology and topographical threshold of gully heads to explain the difference in the gully volumes and area-specific gully volumes between two study sub-areas. The topographic thresholds of the gully heads, expressed by the slope (= s) and drainage area (= a), (i) formed in each catchment and (ii) that had the same land use/cover items (forest, grassland, and farmland) in all the catchments of each sub-area were approximated by power functions (s = ka-b). Analysis of covariance found that these threshold lines had clear spatial and temporal patterns: the threshold lines maintained almost the same exponent b specific to each sub-area while the threshold coefficient k significantly decreased in the order of forest, grassland, and farmland. The spatial variability and its temporal changes in relief aspect of the catchment morphology can responsible for the difference in the area-specific volumes of gullies between the sub-areas, while the continuous reduction in vegetation cover over time can be the main driving force of the similar scale and changing patterns of the gully volumes between the sub-areas.