| The quantitative analysis of the effect of soil properties on erosion characteristics is mainly focused on soil erodibility factor K. Owing to more than 70% percentage of coarse sand content of purple soil, it is difficult to figure out accurately its value of erodibility factor K. Therefore, the efficient approach to achieve quantitative analysis is to find out the quantitative index through the qualitative analysis of relationship of different purple soils' internal properties and the erosion characteristics. Vegetation bestows are considered to be the most important erosion environment. It is generally believed that as long as there is vegetation bestow, the soil erosion can be reduced. But the research presently emphases on the grassland, crop field and woodland separately and comparisons of such three types of vegetation bestows are rare. The analysis of erosion characteristic differences of three vegetation bestow varieties and bare land can demonstrate the mutual and respective discrepancy among them, which concern about the effective way of purple soil erosion reduction and how to maintain the erosion in the permissible scope. The specific characteristics determine the particularity of permissible erosion and give a value for the scientific research. Permissible soil erosion rate not only can accurately provide a basis for the soil erosion intensity classification criterion but also supply an essential basis for assessing the reliability of soil and water conservation. On the basis of 20 fields observation to runoff and soil erosion and their physical and chemical analysis, this article estimated the rate of permissible purple soil erosion by chemical method and analyzed the variation of the permissible purple soil erosion rate under different circumstances. The main results go follow:1 The effect of vegetation bestows on purple soil erosion characteristicsOn the unit of one year, as for the different vegetation bestows of purple soil hillslope, both the erosion modules and runoff coefficient manifest: maize > grassland + woodland >grassland. Whileerosion module and runoff coefficient of each rainfall in a year varied greatly, the capability of soil and water conservation behaves unstable. On the unit of one month, runoff coefficient of each rainfall has no obvious co-relationship with the vegetation bestow ratio. Soil structure and soil moisture before runoff have the greatest relationship with the runoff coefficient as well as the rainfall intensity. Whatever kinds of vegetation bestow, the runoff coefficient and erosion module vary proportionally, so the first step to control the silt production is to control the amount of runoff. However, the ability of runoff reduction obviously exceeds that of silt reduction, and the ability of soil and water conservation of vegetation varies greatly with the change of rainfall characteristics. Especially when successive erosion rainfall happens, the ability of soil and water conservation of vegetation will be weakened dramatically with the increase of rainfall intensity, therefore the timely prevention should be adopted.2 The effect of mother rock on purple soil erosion characteristicsThe average runoff coefficient order in rainy season of three different mother rock purple soils is: Shaximiao>Penglaizhen>Suining, when the soil particle diameters between 0.25~2.0mm have the maximum positive correlation with runoff coefficient and the soil particle diameters between 0.002~0.02mm have the maximum negative correlation with runoff coefficient. The effects of rainfall characteristics on varieties of purple soil runoff coefficient include the following three instances: (1) when excessive infiltration is the dominant surface runoff, runoff coefficient of Shaximiao soil and Penglaizhen soil are similar and both of them are much greater than Suining soil. (2) when saturation runoff is dominant, the surface runoff of three various soils is the same Shaximiao> Suining >Penglaizhen. (3) When saturation and excessive infiltration happen simultaneously with...
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