Soil Erosion And Solute Transport On Red Soil Slope Under The Interaction Of Rainfall And Scouring Flow

Hilly red soil region is an important grain production area, playing a key role in the development of agricultural economy. It is characterized by gentle climate and plenty rainfall. Yet excessive human activities had destroyed large areas of forest and vegetation with the development of economy, which in turn lead to serious problems such as thinning of soil layer, the naking of bed rock, the reduction of farmland area and the lowering of farmland quality. Consequently, the transport of pesticides and nutrients to hydrosphere from slopes can increase water pollution in receiving streams, lakes and reservoirs. Thus, it is imperative to study mechanism of soil erosion and the nutrient losses. As a result, it can provide evidence for the soil erosion control, ecological environment improvement and agricultural production development.Based on the laboratory flume experiment combining the rainfall and scouring flow, we chose the non-adsorptive bromide as a tracer for soil-dissolved solute to study the process of soil erosion and nutrient losses. The objectives were:to evaluate the effect of rainfall intensity and scouring flow rate on the erosion principle and to reveal the hydrodynamic mechanism of erosion and solute transport. The main conclusions are as follows:(1) Effects of rainfall intensity on slope erosion process and solute loss.The rainfall intensity had a significant effect on the runoff and sediment rate under the simulated rainfall. The stronger rainfall intensity, the faster the rate of runoff and sediment. Runoff, sediment rate and sediment concentration of runoff showed a step change trend with the rainfall time. There was a good linear function between cumulative runoff volume or cumulative sediment mass and runoff time. There was a good power function between cumulative runoff volume and cumulative sediment mass. The initial runoff time from beginning to 6 min was the main stage of solute in runoff loss. The sequence of bromide losses has been 120 mm/h> 90 mm/h> 60 mm/h at the any time of the rainfall duration.(2) Effects of flow on slope erosion process and solute loss.The sediment rate displayed different variation under different scouring flow. Sediment rate changed gently at low scouring flow(10 L/min), while it decreased sharply at early 5 min. The solute concentration decreased sharply at early 5 min during which the total loss mass accounted for more than 40% of loss in the whole stage. There was a good linear function between cumulative runoff volume and runoff time. There was a significant power function between cumulative sediment mass and runoff time or cumulative runoff volume. The concentration of bromide decreased as a power function with runoff time.(3) Effects of the interaction of rainfall and scouring flow on slope erosion process and solute loss.Rainfall intensity and scouring flow rate have small influence on. the variation of runoff and sediment rate as temporal variation. The runoff and sediment rate showed an overall decreasing trend. The decay rate of sediment rate was faster than that of runoff rate under the interaction of rainfall and scouring flow. The relationship between cumulative runoff volume and runoff time was linear. The relationships between cumulative sediment mass and runoff time or cumulative runoff volume could be fitted in power function significantly. The power function between bromide concentration in runoff and time could well describe the process of bromide concentration with runoff time. It revealed that the power function model was more suitable for describing the process of bromide concentration on red soil slope under unsaturated conditions.The flow regime was mainly turbulent and torrent under the interaction of rainfall and scouring flow. There was a good power function between the Darcy-Weisbach friction coefficients (f) and Reynolds number (Re). The correlation analysis and path analysis indicated that the Reynolds number was the key hydraulic parameters of soil erosion. Hydraulic shear stress, stream power, unit stream power were all good hydraulic indicators for the average sediment transport rate and solute transport rate.