Effect of Soil Water Content before Rainfall on Surface Runoff Yield Process in Farmland in Purple Slope Land

[Objective] The study was to provide the reference for the related research of purple soil erosion.[Method] The test was done in 7 runoff plots with different soil slopes and thicknesses in Yanting station.In each spot the rainfall experiments with 2 kinds of soil water content(10% and 20%)was made,and the effect of soil water content before rainfall on the surface runoff yield process in farmland in purple slope land was studied.[Result] When the soil water content before rainfall was higher,the surface runoff yield was faster and the runoff rose more quickly.The runoff yield began after rainfall for 6 min and the surface runoff was stable after runoff for 10 min.When the soil water content was lower,the runoff began after rainfall for 10 min,and 25 min later runoff got stable.When the runoff yield was stable,the stable infiltration rate of 2 kinds of soil water content hadn no obvious difference and the related deviation was smaller than 10%.When the soil water content before rainfall was higher,the surface runoff was bigger.[Conclusion] The effect of soil water content before rainfall on the surface rainfall runoff process was embodied mainly in the early period of rain.