Sediment Response to Rill Micromorphology Developed Under Rainfall and Inflow Simulations on Steep Sandy Slopes

Rill erosion is an important source of sediment on steep disturbed hillslopes, although the interaction among runoff accumulation, rill micromorphological evolution, and sediment response remains insufficiently understood. This study investigated rill development on sandy slopes of 15° and 20° under controlled rainfall-only and rainfall-inflow conditions using direct measurements and LiDAR-derived three-dimensional topography. Under rainfall-only conditions, rills initiated approximately 3 min after the onset of rainfalland propagated upslope, whereas additional inflow induced rapid incision within approximately 30 s, thereby promoting downslope extension and network connectivity. Additional inflow substantially enhanced rill development across both slopes; however, the dominant mode varied, with the 15° slopes exhibiting a pronounced 2.9-fold increase in network length and density, while lateral enlargement was enhanced on the 20° slopes, resulting in a 67% increase in mean rill width. Manually estimated and LiDAR-derived rill volumes were strongly associated. The ratios of measured rill volume to the volume-equivalent of exported sediment were 0.62 and 0.73 at 15° and 20°, respectively. Sediment yield was strongly correlated with mean rill flow velocity, as well asgeometrically estimated rill volume, and LiDAR-derived rill volume, while runoff discharge and individual rill dimensions showed weaker relationships. These findings indicate that sediment yield was more closely associated with concentrated-flow hydraulics and integrated rill development than with runoff magnitude alone. These 3D morphological changes imply that rill volume can serve as a key structural indicator of sediment response, capturing the cumulative effects of concentrated flow on steep disturbed slopes.