Effect of rainfall gradient and vegetation restoration on gully initiation under a large-scale extreme rainfall event on the hilly Loess Plateau: A case study from the Wuding River basin, China

As the most serious form of soil erosion, gully erosion can be triggered by individual high-intensity rainfall events. In this study, a total of 369 small catchments in 24 sites were sampled to investigate the relationship between rainfall and gully erosion on hillslopes and to study the impacts of vegetation restoration following heavy rainstorms in the central Loess Plateau, China. A total of 280 newly formed gullies on hillslopes were identified by comparing pre-storm Google Earth images and post-storm unmanned aerial vehicle (UAV) images. The results showed that the dimensions and density of gullies increased significantly with rainfall gradient increasing from the periphery to the storm center. When the rainfall amount exceeded 200 mm, gully volumetric density reached up to 928.39 m3/km2 and the mean gully volume was 15.74 m3, 12.8 times and 2.3 times the mean gully volume for rainfall amounts of 106 and 150 mm, respectively. In the sampled small catchments, where cropland was dominant, the relationships between the gully densities and rainfall amount could be fitted with exponential functions. Vegetation restoration was found to reduce the densities and dimensions of gullies on hillslopes. Compared to those in cropland-dominated catchments, the density of gullies in grassland-dominated catchments was found to be lower by > 60%, while the individual gully volume was found to be 1.6 times higher. In small catchments, no new hillslope gullies were observed when the rainfall amount fell below 106.7 mm. Therefore, the rainfall thresholds for (1) ephemeral-gully initiation on grassland hillslopes, (2) permanent-gully initiation on grassland hillslopes, and (3) permanent-gully initiation on cropland hillslopes are concluded to be not >106.7 mm, not >136.1 mm, and not >110.2 mm, respectively. This suggests that the restoration of cropland to grassland would reduce the rainfall threshold for gully initiation.