Affecting Factors And Mechanisms Of Hillslope Soil Erosion Processes In The Mollisol Region

The study of affecting factors of hillslope soil erosion processes in the Mollisol region is an urgent demand to prevent soil erosion and protect black soil resources. This study took the erosivity dynamics(rainfall and confluence) in the water erosion process as the key point, selected eroded croplands of Yushu City in Jilin Province as research site, and a method of laboratory rainfall simulation and inflow experiments was conducted to investigate soil erosion process and mechanism. The study evaluated the effects of erosive agents(raindrop impact, rainfall intensity and inflow water) and soil surface conditions(slope gradient, slope length, soil crust and till depth) on soil erosion processes and flow hydraulic and hydrodynamic mechanism. Main conclusions of this study were as follows:(1) Effects of rainfall energy, rainfall intensity and inflow rate on runoff and soil loss of the Mollisol hillslope were analyzed. Runoff and soil loss with rainfall energy were increased 12.8%~39.7% and 1.7~1.9 times compared with those eliminating rainfall energy treatments, respectively. When rainfall intensity increased from 50 to 100 mm h-1, runoff and soil loss increased 1.4~12.4 and 4.2~6.1 times, respectively. Inflow water from upslope had a significant influence on runoff and soil loss in the inflow experiment, and the effects were affected by the dominant erosion pattern. When sheet erosion dominant, soil loss did not have significant differences among the 50, 100, and 150 L min-1 inflow rates, but soil loss at 200 and 300 L min-1 inflow rates was significantly different; while an increase of inflow rate from 50 to 300 L min-1 casued soil loss to increase 6.2~61.7 times at rill dominant erosion pattern.(2) Soil surface conditions under both rainfall and inflow experiments had significant effects on soil erosion process in the Mollisol hillslope. The result showed that runoff and soil loss increased 0.1~3.1 and 2.1~2.9 times when slope length was increased from 5 to 10 m under the same rainfall intensity and slope gradient, respectively. When slope gradient increased from 5 to 10°,soil loss increased 0.4~0.9 times under the same rainfall intensity and slope length; while soil loss increased 2.6~201.0 times for slope gradient increased from 5 to 10° when inflow rate increased from 50 to 300 L min–1. Effect of soil crust on soil erosion processes was affected by rainfall intensity, slope gradient and slope length conditions. With the increase of till depth, the change trends of soil loss were complex and affected by rainfall intensity and slope gradient under rainfall experiment. Dominant erosion pattern had obviously impact on effects of till depths on soil loss under inflow experiment.(3) The main impact factors of soil loss under both rainfall and inflow experiments at the Mollisol hillslope were quantified. For rainfall experiment, soil loss had the strongest correlation with slope length, and established empirical equations of soil loss with slope length and coupling effects of rainfall intensity—slope length. For inflow experiment, soil loss had the strongest correlation with coupling effects of inflow rate—slope gradient. The empirical equations for predicting soil loss by coupling effects of inflow rate—slope gradient were also established.(4) The effects of different impact factors on flow hydraulic characteristics and dynamic mechanisms of soil erosion under rainfall experiment were explored. Flow velocity, Reynolds number and Froude number were increased with the increase of rainfall intensity, slope gradient and slope length, while Darcy-Weisbach resistance coefficient showed an opposite trend. For treatments with soil crust, Darcy-Weisbach resistance coefficient decreased 25.6%~40.3% compared with those without soil crust treatments. Increasing of till depth from 5 to 20 cm induced flow velocity decreasing by 36.8%~40.2%, while Darcy-Weisbach resistance coefficient increasing by 1.5~1.8 times, respectively. However, no significant linear relationships between soil erosion with flow shear stress, stream power and unit stream power were found. Furthermore, soil erosion at the Mollisol hillslope was sensitive to flow velocity, Froude number and Darcy-Weisbach resistance coefficient.(5) Relationships between soil loss and hydrodynamic parameters for different impact factors and dominant erosion patterns under inflow experiment were discussed. Flow velocity, Reynolds number and Froude number were increased with the increase of inflow rate and slope gradient, while Darcy-Weisbach resistance coefficient decreased. Effects of till depth on flow hydrodynamic characteristic were complex, and which also affected by the impacts of inflow rate and slope gradient. The relational expressions between soil erosion and shear stress, stream power, and unit stream power were respectively established. The key hydraulic and hydrodynamic parameters which influenced soil loss were flow velocity and unit stream power.