Variation characteristics of flow length of grass slope under different grass strip positions

Vegetation cover and its pattern generally determine the hydrodynamics process and mechanisms in the runoff and sediment reduction. In this study, an indoor scouring experiment was carried out to investigate the variation characteristics of flow length under different vegetation coverage and patterns. A relationship was established for the flow length with the hydrodynamic parameters, yields of runoff and sediment in different vegetation patterns and coverage under different flow rates. Furthermore, the flow length of the slope was proposed to serve as an indicator for measuring the effects of vegetation patterns on soil erosion. The results showed that: 1) The vegetation coverage significantly affected the hydrodynamic parameters. There were different trends of hydrodynamic parameters with the increase of vegetation coverage. The velocity and unit stream power presented a linear trend of increase relationship (P<0.01). There was an exponentially increasing trend in the roughness coefficient, resistance coefficient, and runoff shear stress. The increase of flow resistance and the decrease of kinetic energy can cause a gradual reduction in the amount of runoff yield and sediment production, where the greater the discharge was, the greater the reduction was. The vegetation reduced runoff yield and sediment production by changing the hydrodynamic parameters of water flow. 2) The flow length, one of the important indexes of hydrological connectivity, was widely used in water and soil conservation. There were significant correlations between the flow length and vegetation coverage, hydrodynamic parameters, as well as runoff and sediment production. Hydrodynamic parameters, runoff and sediment production varied with the flow length. The velocity and unit stream power increased logarithmically, leading to the increase in the kinetic energy of slope flow. The roughness coefficient and the resistance coefficient presented a logarithmical decrease, indicating the decrease in the retardation capacity of the slope surface. But the runoff shear stress decreased exponentially, leading to the increase in the unit stream power of runoff. Obviously, the ability of slope block was declined, whereas, the erosion capacity of water flow was enhanced. This case finally led to the change of erosion status of the slope. There was also a logarithmic increase in the runoff and sediment yield of the slope. Thus, the flow length with the aid of the flow path change can be used to indicate the hydrodynamic parameters of slope surface water flow and sediment yield in the change. Particularly the flow length can be expected to serve as an indicator for the dynamic process of vegetation runoff and sediment reduction, and thereby predict the runoff and sediment yield on the slope. This finding can provide a scientific basis for the relationship between vegetation pattern and hydrological connectivity, and further establish vegetation patterns and evaluate vegetation impacts on erosion. The simulation experiments can contribute to the effective distribution and utilization of water resources for the regional ecological environment in modern agriculture.