Objective This study aimed to investigate the relationship between hydrodynamic characteristics and microtopographic factors in rill erosion, which was significant for advancing our understanding of hillslope erosion mechanisms.

Method A series of simulation tests were conducted on a 0.10 m wide limited rill of saturated red soil under different slopes (5°, 10° and 15°) and flow rates (4, 8 and 16 L min⁻¹). Microtopographic factors were obtained using the Structure from Motion (SfM) photogrammetry. Flow velocity was measured using the electrolyte tracer method, and other hydrodynamic characteristics were calculated.

Result Under the experimental design conditions, flow velocities ranged from 0.166 to 0.848 m s⁻¹ and exhibited a power function relationship of slope and flow rate. As slope and flow rate increased, flow velocity and Froude number showed an increasing trend, while Dancy resistance coefficient and Manning roughness coefficient decreased, and Reynolds number fluctuated irregularly. The ranges of variations in surface roughness, average topographic relief and average surface cutting depth were 0.79 ~ 3.98 cm, 0.08 ~ 6.20 cm and 0.07 ~ 1.16 cm, respectively. These values increased with slope and showed different variation patterns with flow rate. The microtopographic factors showed a positive correlation with flow velocity, Froude number, and Reynolds number. Additionally, these factors could be effectively characterized by linear functions of flow velocity and Froude number respectively.

Conclusion Slope has a greater influence on hydrodynamic characteristics and microtopographic factors compared with flow rate. There exists a critical erosion slope between 10° and 15°, which causes flow velocity, Froude number and microtopography to increase significantly, and the response of variation amplitude of surface roughness to hydrodynamic characteristic parameters transitions to be from strong to weak.