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Rain-induced overland flow involves the detachment of soil particles by raindrop impact and the transportation by the resultant overland flow. The purpose of this study was to investigate the relationship between sediment concentration and different hydraulic parameters including flow depth, flow velocity, shear stress, stream power, and unit stream power. The effects of soil particle size distribution, rain intensity, and slope steepness on measured sediment concentration in rain-induced sheet flow were also examined.

Two arid land soils with different particles size distributions (D-2mm and D-4.75mm) were subjected to simulated rains using a detachment tray under infiltration conditions. Two rain intensities of 57 and 80 mm h(-1) were simulated on slope gradients ranging from 0.5 to 20 %, resulting in rain-induced overland flows. After pre-wetting each soil, the sediment-laden overland flow was sampled at several time intervals (2, 5, 15, 20, 30, and 40 min) and the sediment concentration was determined. Different hydraulic parameters including flow depth, flow velocity, shear stress, stream power, and unit stream power were measured. The hydraulic parameters were used to model the sediment concentration, and the model performance was evaluated.

The result showed that the measured sediment concentration was greater in the higher rainfall intensity and at steeper slopes. With increasing slope steepness, sediment concentration increased from 4.3 to 15.5 kg m(-3) and from 3.8 to 12.5 kg m(-3) for soils D-2mm and D-4.75mm, respectively. There was a direct relationship between sediment concentration and the rain-induced flow velocity, shear stress, stream power, and unit stream power. Nevertheless, the values of sediment concentration increased as flow depth decreased on steeper slopes. Also, sediment concentration was lower in the soil containing larger aggregates than in the finer soil. The hydraulic parameters tended to overestimate low amounts of sediment concentration and underestimate high values.

In general, the accuracy of the hydraulic parameters in predicting sediment concentration was: flow velocity > stream power > shear stress > unit stream power > flow depth. Flow velocity was the best predictor of sediment concentration with a linear relationship, whereas the other parameters showed nonlinear relationships. This study revealed that rain-induced sediment concentration at small scales can be modeled precisely on the basis of the flow velocity parameter.