Combined effects of simulated rainfall and overland flow on sediment and solute transport in hillslope erosion

Abstract

Soil water erosion and nutrient transport, which are strongly affected by rainfall intensity and inflow rate, often lead to severe land degradation and unexpected water eutrophication. However, little research has been conducted on the combined effects of rainfall and runoff on soil erosion. This study investigated soil erosion and solute transport in runoff under the simultaneous influence of rainfall and runoff and evaluated the relationship between soil erosion and hydraulic parameters. The rainfall simulation and inflow experiments were conducted on a slope of 10° under inflow (10, 15, and 20 L min−1) alone and combination of inflow and rainfall (60, 90, and 120 mm h−1). Flow velocity (V), Reynolds number (Re), Froude number (Fr), stream power (ω), shear stress (τ), Darcy–Weisbach friction (f), and the ratio of the Manning roughness coefficient to average flow depth (n/h) were also measured. Both rainfall intensity and inflow rate played important roles in runoff generation, soil loss, and solute transport in surface runoff. Increasing inflow rate substantially increased runoff rate and sediment concentration in runoff either with or without rainfall impact. The combination of rainfall splash and water scouring increased runoff transport, whereas did not causally increase sediment transport. Notably, the sediment concentration or cumulative sediment load under 20 L min−1 inflow rate alone was higher than those of combinations of rainfall and inflow. Most flow belonged to turbulent and supercritical flow regimes. Solute transport in runoff was also affected by both rainfall intensity and inflow rate, and bromide concentration in runoff decreased as a power function of duration on the red soil slope. Both the Reynolds number and the average flow velocity were good indicators of unit sediment load and solute transport in runoff. In a multiple regression model, the Reynolds number was found to be the best predictor of unit sediment load with an exponential relationship. Of the studied hydraulic parameters, average flow velocity was found a better parameter than the Reynolds number when estimating solute transport in runoff under combination of rainfall and inflow.