Rill investigation on landslide scar highlights the shortcomings of current knowledge on erosion of shallow landslides

Widespread shallow landslides typically result in the loss of productive soils and grasslands. Deposits of landslide tails have received much attention, while the severity of rill erosion on the landslide scar is generally neglected or underestimated. Thirty-five rainfall-induced shallow landslides were selected on Lishi (finer texture) and Malan loess (coarser texture) on the hilly and gully Loess Plateau, China. The rill erosion intensity and rill morphological parameters of these landslide scars were measured by on-site investigation 3 years after the landslides. Landslide characteristics were investigated by unmanned aerial vehicle photogrammetry, and soil properties were also evaluated. We found that landslide scars suffered rill erosion for more than 8 years after landslides. Rill erosion intensities of the Lishi (22.91 x 103 t km-2) and Malan landslides (16.90 x 103 t km-2) in 3 years occupied 11.43% of the landslide deposit amounts. The structural equation model demonstrated that catchment area, soil bulk density, soil organic matter, water stable aggregate content (WSA), and sand content explained 87% of the variance in rill intensity through paths of rill depth, width, and distribution. Additionally, the negative effect of rill distribution on rill depth led to the unusual result that coarse soils with low WSA had low rill erosion intensities. In summary, the rill erosion of shallow landslide scars reached a non-negligible value. The effect of initial rill distribution masked the effect of soil properties on rill intensity. Therefore, this study casts fresh insight into the sediment source of landslides, increasing theoretical support for landslide control strategies.