Simultaneous effects of wind and rain on hydraulic parameters of sheet flow and interrill erosion rate

Many natural rainstorms are accompanied by wind blowing. However, so far no comprehensive research has been reported on the influence of wind on rain-induced erosion under laboratory conditions in Iran. The present study was conducted to investigate the interactive effects of different wind velocities and rain intensities on flow hydraulic parameters and on interrill erosion rate of several agricultural soils. For this purpose, a simultaneous wind, rain and runoff simulator was used, which has been designed and constructed for the first time in the country.
Various combinations of four wind speeds including 0, 6, 9 and 12 m s-1, three rain intensities of 30, 50 and 75 mm h-1 were introduced on three cropland soils with different aggregate size distributions with the largest particle size of 2, 4.75, and 8 mm, each at three replications. Different flow hydraulic parameters including mean flow velocity, flow depth, shear stress, stream power and unit stream power and also the rate of interrill erosion were measured. Afterwards, the effects of wind velocity on the flow hydraulic parameters and also the influence of these parameters on interrill erosion rate were assessed.
The results showed that depending on the wind velocity, the rate of interrill erosion varied from 0.021 and 0.22 g m-2 s-1. In this research, the wind velocity of 6 m s-1 was introduced as a threshold value. With increasing wind speed particularly those speeds higher than the threshold, the flow parameters of velocity and unit stream power increased, whereas, flow depth and shear stress decreased. In addition, stream power increased as the wind speed increased up to the threshold wind velocity, and at the higher wind speeds, the reverse trend was observed. The result indicated that wind speed can control interrill erosion rate by affecting on flow hydraulic parameters. Interrill erosion increased with increasing velocity and unit stream power of the flow, while it was reduced when flow depth, shear stress and stream power increased, this was attributed to the expenditure of raindrops energy for passing through water depth. In fact, the opposite relationship between flow velocity and water depth affects the other hydraulic parameters. Moreover, the presence of coarser aggregates at the soil surface increased water depth and decreased the velocity and unit stream power of flow, resulting in reduced interrill erosion.
The findings of this study showed that in wind-driven rains particularly at those wind speeds higher than 6 m s-1, interrill erosion rate is intensified because of increasing in rain erosivity and flow velocity and decreasing in flow depth. It was found that with appropriate soil management in agricultural lands for increasing the size and strength of surface aggregates, flow velocity and consequently soil erosion can be reduced. From another point of view, the presence of stable and coarser aggregates at soil surface decreases interril erosion. Overall, the findings of this research revealed the importance and necessity of more studies on soil erosion processes due to wind-driven rain.