Soil Erosion across Scales: Assessing Its Sources of Variation in Sahelian Landscapes under Semi-Arid Climate

Soil erosion varies in space and time. As the contributing surface area increases, heterogeneity effects are amplified, inducing scale effects. In the present study, soil erosion processes as affected by the observation scale and the soil surface conditions are assessed. An experimental field scale setup of 18 plots (1–150 m2) with different soil surface conditions (bare and degraded, cultivated) and slopes (0.75–4.2%) are used to monitor soil losses between 2010 to 2018 under natural rainfall. The results showed that soil loss rates range between 2.5 and 19.5 t.ha−1 under cultivated plots and increase to 12–45 t.ha−1 on bare and degraded soils, which outlines the control of soil surface conditions on soil erosion. At a larger scale (38 km2), soil losses are estimated at 2.2–4.5 t.ha−1, highlighting the major contribution of scale. The scale effect is likely caused by the redistribution of sediments in the drainage network. These findings outline the nature and contribution of the emerging and dominant soil erosion processes at larger scales. At the plot scale, however, diffuse erosion remains dominant, since surface runoff is laminar and sediment transport capacity is limited, resulting in lower soil erosion rates.