Impact of the gravel content on rill erosion and soil losses on colluvial deposit slopes

This study investigated rill morphology and soil loss characteristics during rill erosion on slopes with different gravel contents, aiming to clarify the regulatory mechanism of gravel content on rill erosion and soil loss. A runoff experiment with four flow discharges (2, 4, 8, and 12 L min^-1) and four gravel contents (0%, 10%, 30%, and 50%) was used to assess the effects of rill morphology on soil losses and concentration. With the increase of gravel content from 0% to 10%, both rill width and depth showed a decreasing trend. However, when the gravel content exceeded 10%, this inhibitory effect on rill width and depth weakened, and rill development tended to be more extensive (i.e., more rills formed). Notably, gravel contents higher than 10% led to higher initial soil loss rates and sediment concentrations, and cumulative soil loss increased with the increase of gravel content. In contrast, the sediment concentration and soil loss rate on slopes with 10% gravel content were significantly lower than those on slopes with other gravel contents. Correlation analysis indicated that sediment concentration was significantly negatively correlated with average rill depth, number of depressions, depression depth, and rill longitudinal profile index (p<0.01), while significantly positively correlated with rill width-depth ratio (p<0.01). The soil loss rate was significantly negatively correlated with rill cross-sectional shape index (p<0.01) and significantly positively correlated with average rill width (p<0.05). Path analysis revealed that the rill width-depth ratio had the highest path coefficient (0.656) with respect to sediment concentration, and the rill cross-sectional shape index had the highest path coefficient (0.859) with respect to soil loss rate. Conclusions The intensity of rill erosion on slopes first decreased and then increased with the increase of gravel content, with the lowest erosion intensity observed at a gravel content of 10%. Path analysis shows that rill width-depth ratio (0.656) is the key morphological indicator for characterizing runoff sediment concentration, while the rill cross-sectional shape index (0.859) is the best indicator for soil loss rate.