| Purple soil, 60% of which distributes in central Sichuan, is a unique type of soil in China. Since purple soil is unconsolidated and prone to be weathered, 48.8% of it has been damaged by soil erosion. Purple soil area in central Sichuan has become the main source of sediment in Yangtze River. It is quite significant to find a set of accurate and quantitative models which can be applied to the soil erosion of purple soil in central Sichuan.This thesis, with the purple soil in central Sichuan as the subject, soil erosion as the research content and USLE as the research model, applies statistic and GIS methods to collect and analyze the data about the soil erosion of purple soil and rainfall in central Sichuan. By revising and testifying USLE model, a set of forecast equations which are fit for purple soil area are built in this thesis. The amount of soil erosion is calculated and the soil erosion dynamic is tested. The technical parameters and the model are proved to be scientific and reasonable. The results of this thesis are as follows:(1) Rainfall Erosion Factor①Among all the single factors of rainfall features, rainfall is best correlated to erosion, with rainfall kinetic energy and rainfall intensity followed. Thus, rainfall erosion can be estimated by rainfall factor.②PI combination is most positively correlated to the degree of soil erosion, so is EI combination. EI30 is the best in EI combination, so is PI15 in PI combination. In addition, PI15 is better than EI30, but the difference is not quite significant. Thus, EI30 and PI15 can be chosen to be the index of rainfall erosion in purple soil area. But in order to make comparison with other researches, it is suggested here to choose EI30.③Regression relationship between individual rainfall erosion and individual rainfall is set up as follows: R次= 10 .58P-223.91 In the equation, R refers to individual rainfall erosion, with MJ?mm?hm-2?h-1 as its unit. P refers to the rainfall which is above 12mm, with mm as its unit.Based on the equation above and the rainfall data of Suining, the functional relationship between annual rainfall erosion and annual rainfall is set up as follows: R年= 5 .5455P年-3217.2 In the equation, R refers to annual rainfall erosion, with MJ?mm?hm-2?h-1?a-1 as the unit; P refers to the annual rainfall, with mm as the unit.By building the equation, a distribution map of annual rainfall erosion in central Sichuan is drawn.(2) Soil Erodibility Factor①By observing plot data and literature data, Normal Equation and K value formula in EPIC are revised. The revised equation of Normal Equation is as follows: In this equation, M refers to the product of advantage particle size: (0.002～0.1mm particle size content)×(silt content+sand content); OM is the organic matter content in the soil; S refers to the level of soil structure; P is soil infiltration. K refers the soil erosion caused by unit rainfall erosion, with t·h·MJ-1·mm-1 as the unit.The revised equation of EPIC is as follows: In this equation, SAN refers to the 0.1～2mm particle size content; CLA is the clay (<0.002mm) content; SIL is the 0.002～0.1mm particle size content; SN1=1-SAN/100;C is the organic matter content in the soil. ②The K value of 48 types of purple soil in this area is calculated combined with second soil data. And the distribution map of soil erodibility in central Sichuan is drawn according to the revised K value formula of EPIC.(3) Terrain FactorThe calculation results of three LS factors formula are compared by GIS techniques. The results show that the factor value got by LS factor formula of Huang Yanhe ranges from 0 to 5.52 and the average value is 2.18. The modulus of soil erosion is 3.73 t·hm-2·a-1. The result is close to the observed value of this area (3.0～5.8t·hm-2·a-1); therefore, the LS factor formula of Huang is best fit for this area.(4) Soil and Water Conservation Measure FactorBy using the literature data and observation data of runoff plots in Suining Station (1991- 2001) and Yanting Station (1997-2007), the factor values of 5 types of biological measures (B), 11 sorts of tillage methods (T) and 2 kinds of engineering measures are calculated.(5) Application of ModelBy applying the revised model to predict the purple soil in central Sichuan, the average erosion modulus is 3.40t?hm-2?a-1, which is quite close to the previous researches done by different scholars. In terms of the erosion intensity and the spatial distribution of erosion, the result of this thesis is extremely close to the data of remote sensing interpretation in 2000. However, the prediction result is relatively small for two reasons: 1,) the resolution of image data got by remote sensing interpretation is low, which is the main cause; 2,) since 2000, the soil erosion has changed significantly in central Sichuan. Thus, the revised USLE Model in this thesis is accurate and reliable, and it can be applied to study the soil erosion in purple soil area.
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