Sediment connectivity and sediment yield on semi-arid slopes: Integration of unmanned aerial vehicles imagery and field measurements

Semi-arid (semi-)natural terrestrial ecosystems are often vulnerable to soil erosion, but long-term comparable observations are relatively scarce. We measured erosion for thirteen years on a hillslope dominated by tussock grass and pine trees in the Mediterranean region and conducted multiple aerial surveys of landscapes. At plot scales, we found sediment yields of 3.1 g m⁻² yr⁻¹ for open forest, and around 35 g m⁻² yr⁻¹ for tussock grasslands, probably attributed to differences in vegetation type and cover related to exposition. Furthermore, erosion rates in the forest were lower than reported so far. The values obtained could act as a baseline for achieving land degradation neutrality. The tussock grass plots were divided into ten natural subplots. Subplot sediment yields were not sensitive to vegetation cover but significantly negatively correlated with the maximum value of the index of connectivity (IC). This metric, calculated for bare areas not directly connected to measurement gutters for subplots, measures structural connectivity between bare areas and vegetation. Furthermore, spatial clusters of high and low connectivity - hot and cold spots of IC were identified. Sediment yields decreased significantly with a higher percentage of IC hot spots in bare areas, due to vegetation obstruction. However, sediment yields increased significantly with the relative area of IC cold spots plus bare areas adjacent to gutters, which could be useful hints for functional connectivity. At broader catchment scales, the IC cold spot showed promise in identifying potential sediment sources, demonstrating its stability and potential in the upscaling analysis. Our findings support the role of vegetation and topography and the importance of structural and functional connectivity in erosional processes.