Multi-temporal gully expansion and volumetric soil loss estimation in Kakia and Esamburmbur sub-catchments, Enkare-Narok Basin, Kenya

Gully erosion is a major driver of land degradation and sediment flux in transitional highland environments, yet its magnitude and time dynamics are poorly understood in East Africa. This study quantified multi-temporal gully expansion and volumetric soil loss in the Kakia and Esamburmbur sub-catchments (Enkare-Narok Basin, Kenya) using an integrated Earth Observation framework adopting very-high-resolution imagery (2009–2024), UAV photogrammetry (0.10 m DEM), and DEM-of-Difference (DoD) analysis. The mapped gully area increased from 5.93 ha in 2009 to 14.94 ha in 2024, with the largest mapped extent occurring in 2019 (16.60 ha). UAV-derived morphometry revealed substantial geometric variability, with a mean depth of 1.02 m and a maximum width of 40 m. DoD analysis estimated erosion volumes of 116,633 m 3 (FABDEM) and 130,497 m 3 (WorldDEM) within the UAV survey footprint. High-volume gully identification flagged 39 gullies exceeding a threshold of 1665.20 m 3 and accounting for a combined predicted erosion volume of 203,345 m 3 . A near-linear area–volume scaling relationship (b = 1.02, R 2 = 0.41) enabled extrapolation of volumetric estimates beyond the UAV footprint. Scaling-derived estimates indicated increasing erosion volumes between 2009 and 2019, followed by a reduction by 2024. Comparison of DEM sources showed an uncertainty range of approximately 11.9–13.3% in volumetric estimates, while RTK-GNSS validation confirmed high vertical accuracy of the UAV-derived DEM (RMSE = 0.062 m). The results show the value of integrating multi-temporal imagery with UAV-derived topography to link long-term gully expansion with volumetric erosion assessment. Although uncertainties remain regarding DEM-source selection and scaling hypotheses, the framework provides a practical approach for evaluating erosion magnitude and spatial concentration in data-limited environments.