Identifying hotspots of erosion from local knowledge and sediment fingerprinting (Lake Tana Basin, Ethiopia)

To mitigate erosion, soil and water conservation measures have been widely implemented to reduce on-site erosion rates and catchment sediment yield. However, the effectiveness of these measures has often been questioned, particularly in Ethiopia, where gullying has intensified due to conservation programs that primarily target sheet and rill erosion on cropland. This study aimed to identify erosion hotspots and sediment source areas using an integrated approach. The study was conducted in the 211 km² Fota-Gumara catchment, a highly erosion-prone area in Northwest Ethiopia. First, erosion hotspot areas were identified through field observation, farmer’s interview and modelling. Then, sediment sources areas were identified using sediment fingerprinting by employing fallout radionuclide tracers (210-Pbex and 37-Cs). Local communities identified communal grazing lands and croplands as particularly vulnerable to erosion. Field observations corroborated these findings, highlighting steep slope cropland and grazing land as erosion hotspots. The Analytical Hierarchical Process (AHP) model indicated that 23.5% of the area experiences severe erosion, predominantly on steep slopes under cropland, shrubland, and valley bottoms where saturation excess runoff drives gullying. Sediment fingerprinting further revealed that subsoil is the dominant source of sediment. Bayesian models (MixSIAR and BMM) consistently showed that subsoil contributes approximately three-quarters of the sediment, with median contributions of 73% and 81%, respectively. We conclude that prioritizing gully rehabilitation and managing steep slope croplands should be central to land management strategies in Ethiopia.

Keywords: Sediment fingerprinting, Lake Tana Basin, Land Degradation Surveillance Framework, Analytical Hierarchical Process Model, Gully erosion