Effects Of Near-surface Soil Hydrological Conditions On Critical Rill Erodibility Of Yellow Soil

Knowledge of critical rill erodibility is an essential requirement for erosion prediction. Whereas,at present, the influence mechanism of different near-surface soil hydrological conditions on critical erodibility dynamical changes and its key factors were not clear yet. Therefore, deep understanding mechanisms of rill erosion to occur different near-surface soil hydrological conditions has important theory significance. Which is imperative demand to protect yellow soil resources. The typical eroded yellow soil area of the upper and middle reaches of the Yangtze River was taken as the research area Flush tank and V-shaped soil pan were taken for runoff scouring test. In order to discuss response of occurrence of rill erosion on yellow soil area critical erodibility to different subsurface hydraulic gradients, the critical erodibility were analyzed using the method of observed the critical condition for inception of sediment. And the results showed that(1) The regression relationships between rill runoff shear stress under the different subsurface hydraulic gradients and detachment rate were obtained. The average value of rill erodibility values (Kr) of five hydraulic gradients was 2.51×10-2s/m, and3.07×10-2s/m for saturated seepage conditions. On average, the rill erodibility values(Kr) for the soil under saturated seepage conditions was then 1.78 times higher than that of the soil drainage conditions. The effect of critical shear stress of slope on the subsurface hydraulic gradient was analyzed using actual measurement method. The linear regression analysis showed that the fitted value of critical sheer stress fitted well with the measured value. The fitted value of critical sheer stress in the range of -0.71～1.43m/m of hydraulic gradients overestimate range from 14.24% to 55.02% and average overestimate 36.85% compared to the measured value. Moreover, an exponential relationship was observed between the fitted value of WEPP model to predict the critical sheer stress and the measured value.(2) With the increase of the antecedent soil moisture content and slope gradient, the critical flow rate and water depth of rill erosion to occur of yellow soil showed a gradually decreasing trend. The results of multivariate nonlinear regression showed that a decreasing power function relationship were observed between and among critical flow rate of rill erosion to occur, slope gradient and antecedent soil moisture content. Flow depth of rill erosion to occur also shows the same result.(3) With the increase of the antecedent soil moisture content and slope gradient, the Reynolds number of rill erosion to occur decreased significantly. The Reynolds number was less than 500 which belonged to laminar flow. The Froude number of rill erosion to occur, except antecedent soil moisture content in the range of 5%～13% under the slope of l,was less than 1 which belonged to subcritical flow.(4) Critical sheer stress of rill erosion to occur increased with increasing the slope gradient and antecedent soil moisture content, and the relationship between and among them could be established by a increasing power function. With the increase of the antecedent soil moisture content, the critical shear stress showed a gradually decreasing trend. While with the increase of the slope gradient, the critical sheer stress showed power function increasing trend. The effect of the antecedent soil moisture content on soil force properties was analyzed. It is discovered that soil cohesion is not major influential factor of critical inception of yellow soil in the process of rill erosion.