Assessment of gully erosion in the Upper Awash, Central Ethiopian highlands based on a comparison of archived aerial photographs and very high resolution satellite images

Gully erosion is a burning problem in the Ethiopian Highlands leading to massive soil losses and sedimentation in reservoirs. In this study gully erosion in the Upper Awash River basin in the central part of the Ethiopian Highlands was studied using archived aerial photographs from 1965, 1971 and 1972 in combination with satellite images from Ikonos-2 and Pleiades acquired in 2006 and 2013 respectively. Gully length, areal extent and the accurate position of gully heads were mapped by meansof visual interpretation. Elevation changes due to the gully incision were analysed by calculating the difference between two digital elevation models for the years 1972 and 2006 generated by means of terrain reconstruction using the Structure from Motion approach (SfM) and satellite photogrammetry. In the study area the total gully length increased three times in the period 1965-2013. The initial rapid gully headward extension rate slowed down after 1972. Especially after 2006 effects of remediation activities led even to a decrease in total gully length. However, dendritic gully complexes continue to propagate at an alarming rate. The identified trend of gully evolution in Upper Awash area over the studied period fits to the overall trend identified in northern Ethiopia. In future more attention should be paid to quickly growing gully complexes in order to prevent increasing soil loss and sedimentation in the Koka Reservoir. The presented study proved that the approach based on a combination of archived aerial photos and data from the new satellite systems is effective to monitor gully erosion in semi-arid regions.