Abstract: The processes of soil erosion may be determined by two aspects:the erosive agents and resistance to erosion, and it is necessary to study flow hydraulics to understand mechanisms of water erosion and to predict soil loss. A series of experiments were conducted to assess the difference of flow hydraulics under the coexisting conditions of rainfall and inflow and to understand the relationship between flow hydraulics and soil erosion amount. A soil pan (3.0 m long, 0.5 m wide and 0.35 m depth and with an adjustable slope gradient of 0°-30°) was subjected to designed rainfall intensity (60 mm·h^-1, 90 mm·h^-1 and 120 mm·h^-1) and inflow rates (10 L·min^-1, 15 L·min^-1 and 20 L·min^-1). Our results suggested that both rainfall intensity and inflow rate played important roles in runoff formation, soil erosion and hydraulic characteristics. The average flow velocity (V), the mean flow depth (h) and Reynolds number (Re) and the stream power (w) increased with rainfall intensity and inflow rate, and the rate of Manning roughness coefficient to mean flow depth (n/h) showed opposite trends. There were no obvious trend for the other flow hydraulics. The average flow velocity ranged from 0.21 m·s^-1 to 0.45 m·s^-1 and the mean flow depth from 5.6 mm to 9.4 mm. Overland flow regimes existing in the slope were almost considered as turbulent and supercritical. The results indicated that the parameter of Manning roughness coefficient to mean flow depth was probably a better hydraulic parameter to predict sediment load and runoff rate in hillslope erosion processes of red soil under the coexisting conditions of rainfall and runoff.

Key words: red soil, hillslope erosion, runoff scouring, simulated rainfall, flow hydraulics