Study On Purple Soil Erosion Process At Sloping Cultivated Field Under Different Surface Hydrological Condition

Purple soil is an important sediment source for Yangtze river. Studying on surface hydrological condition purple soil erosion process has a great significance to agriculture development and water conservancy construction in Yangtze watershed. Based on artificial rainfall simulated experiment, purple soil erosion process and surface hydraulics properties under different slope gradients, different rainfall intensities and different surface hydrological conditions were studied. The main conclusions as follows:1.The time purple soil surface started runoff would deceased with increasing of rainfall intensity and slope gradient. Runoff rate would increased until reached a stable value from soil surface stared runoff, and runoff process divided into no-stable runoff stage and stable runoff stage. In no-stable runoff stage, rainfall intensity and soil surface condition had great impact on runoff, however, in stable runoff stage; runoff had a close relationship with rainfall. No-stable runoff stage was not clear when soil water reached saturation or under seepage condition. When rainfall and slope gradient were same, the time soil surface stated runoff was least under seepage condition, the time soil surface stated runoff was most under free drainage, and the time soil surface stated runoff was in middle of them under saturation condition. When slope gradient and soil surface hydrological condition had not change, runoff increased with increasing of rainfall intensity. When rainfall intensity and soil surface hydrological condition had not change, runoff would increase in the beginning, and decreased finally. When slope gradient and rainfall intensity had not change, runoff was most under seepage condition, runoff was least under free drainage, and runoff was in the middle of them under saturation condition.2.Purple soil surface erosion rate increased with increase of runoff rate, soil erosion rate would decrease slowly under stable stage. When slope gradient and surface hydrological condition had not change, average soil erosion rate increased with increase of rainfall intensity. When rainfall intensity and surface hydrological condition had not change, average soil erosion rate increased with increase of slope gradient, and there were a critical slope gradient, when slope gradient was more than the critical slope gradient, soil erosion increased, when slope gradient was less than the critical slope gradient, soil erosion decreased. When rainfall intensity and slope gradient had not change, runoff was most under seepage condition, runoff was least under free drainage, and runoff was in the middle of them under saturation condition. Increased range of runoff was more than that of soil erosion.3.The impact of soil surface hydrological condition on soil erosion was more than runoff, and the impact was most under free drainage condition, the impact was least under seepage condition, and impact was in the middle of them. Soil surface hydrological condition can affect on soil erosion resistance, soil erosion resistance would decreased with increase of soil water.4. Interaction of slope gradient, rainfall intensity, and surface hydrological condition has an impact on soil erosion. Big slope gradient decreased soil erosion resistance, heavy rainfall and high soil water content aggravated this trend, so the interaction made serious soil erosion.5. The Reynold's number of slope laminar flow is 22.31~71.63, the flow is laminar. The flow's Froude number is larger than 1, it is tranquil flow. The Reynold's number raises with increase of rainfall, runoff, sediment concentration, flow velocity, and the runoff has the most significant effect on the Reynold's number The Froude number raises with increase of slope gradient, runoff and flow velocity, the slope gradient has the most significant effect on the Froude number. The effect of slope gradient, rainfall rate, runoff yield on friction coefficient is significant, the friction coefficient raises with increase of slope gradient,decreases with increase of rainfall rate,runoff. The positive relationship between flow velocity and slope gradient, rainfall, runoff , sediment concentration is extremely significant. With the same rainfall, the flow velocity increases with increase of slope gradient,the hydrological conditions of near-surface has significant effect on hydraulic parameters. Under the same slope gradient and rainfall, the flow velocity, Reynold's number and Froude number increase, friction coefficient decrease, during the change of hydrological conditions of near-surface, from free seepage to soil with saturated water then to interflow...