The use of multi-temporal SfM-photogrammetry to monitor gully degradation : an example from the Ethiopian drylands

Recent advances in Structure from Motion (SfM) photogrammetry offers the possibility to collect multi-temporal DEMs at very high spatio-temporal scale to study gully erosion dynamics and to enhance our knowledge on gully erosion processes and controls. In this case, we studied the degradation and aggradation of a gully at the gully headcut and four re-incisions along a gully reach in Northern Ethiopia. The recorded environmental controls are rainfall, land use and cover, land management, topography and soil characteristics. With an average soil loss of 11.83 t ha-1 y-1, as calculated by the RUSLE, the overall vulnerability to soil erosion in the catchment is low. This reflects the success of efforts made in land management over the past years. On average, the runoff coefficient was 7.3% (maximum 18.2%). Most geomorphic change was caused by one major rainfall event of 54.8mm d-1. Other runoff events caused both degradation and aggradation, without a link between the studied sites. Slumping of the gully bank also occurred on dry days. Although most studies focus on gully heads alone, re-incisions at lower locations can cause important gully degradation