| The purple soil widely distributed in Sichuan, Chongqing mountain hilly region is the important agricultural resource of the Three Gorges Reservoir Area, at the same time purple soil is the primary source of soil and water loss. By studying purple soil erosion prediction model, we can understand the process of soil to detachment, sediment transport and deposition caused by raindrop impact and runoff flowing, and quantitative evaluate the efficiency of different soil and water conservation measures, and provide the scientific theory foundation for soil erosion control in the Three Gorges Reservoir Area.The WEPP model is a complex computer program so far and with remarkable superiority compared with the other erosion predication models, which describes the processes that lead to erosion. The research on the application of WEPP model in purple soil indicate that, the forecast accuracy of the sediment yield is worse than the forecast accuracy of runoff yield.This study attempted to determine the key factors of influencing the forecast values predicted by WEPP model and explore the reasons for the forecast error by analyzing the weight and sensitivity of the parameters in the WEPP model database impacted on the forecast values. On the one hand, it can promote us to understand the mechanism of the purple soil erosion in-depth, on the other hand it can provide reference for revising the parameters of the WEPP model when applicated in purple soil region, so as to lay a foundation for WEPP model to predict purple soil erosion accurately.There also is a practical guide to reduce the slope farmland soil erosion, reduce sediment entering the Three Gorges Reservoir, and protect the security of the middle and lower reaches of the Yangtze River.Using 22 years of the meteorological data observated in Soil and Water Conservation Experimental Station of Suining City, Sichuan Province in 1985-2006, the meteorological database of WEPP model are established. Through the observation of the fallowland (15 degrees) in the runoff experiment station in the rainy season from 1999 to 2002, 28 times erosion process caused by rainfall are observed. the runoff yield and sediment yield of the runoff plot are obtained respectively, and the results of calculation are compared with the easured values.The weight of meteorological parameters influencing the simulation results of the WEPP model is analyzed by path analysis method.The sensitivity of the soil parameters on the results of the simulation is analyzed by disturbance analysis method. The main results are as follows:(1) Using the WEPP model to predict the runoff yield and sediment yield in single rainfall erosion incident of the fallow purple soil, the model of effectiveness (E) are 0.735 and 0.692, respectively. The results show that the forecast values are close to the measured values in certain degree.The forecast accuracy of the runoff yield is well than the forecast accuracy of sediment yield.Although there is a poor accuracy relatively, it can reflect the process and development trends of the actual soil erosion when simulated sediment yields. Relatively speaking, the runoff erosion incident simulation accuracy is higher than that of the mini-erosion event simulation accuracy.(2) Of the rainfall parameters, the rainfall, the average rainfall intensity and the rainfall duration had great remarkable positive correlation with the runoff yield, with the correlation coefficient of 0.9823, 0.5790 and 0.4929, respectively; The rainfall and the average rainfall intensity had great remarkable positive correlation with the sediment yield, with the correlation coefficient of 0.9478 and 0.6118; The rainfall duration and the maximum rainfall intensity had remarkable positive correlation with the sediment yield, with the correlation coefficient of 0.4272 and 0.3780.(3) Rainfall is the most important parameter to impact on the runoff and sediment yield, its direct decision coefficient to the runoff and to the sediment yield is 1.3555 and 1.4173, respectively;The rainfall duration and the average rainfall intensity had larger indirect effects on runoff and sediment yield through the rainfall. The the direct decision coefficient of the maximum rainfall intensity (0.0214) is greater than that of the average rainfall intensity (0.0163) to the sediment yield, this can indicate that the direct effect of the maximum rainfall intensity is greater than that of the average rainfall intensity on the sediment yield.(4) The runoff yield in single rain is not sensitive to the change of Soil Albedo, Interrill Erodibility, Rill Erodibility and Critical Shear, but highly sensitive to the change of Initial Saturation Level and appreciably sensitive to the change of Effective Hydraulic Conductivity; and the runoff yield had negative correlation with Effective Hydraulic Conductivity. These indicate that the main parameters in the soil database caused the error are Initial Saturation Level and Effective Hydraulic Conductivity when using the WEPP model to predict the runoff yield in single rainfall.(5) The change of Initial Saturation Level, Interrill Erodibility, Rill Erodibility and Critical Shear had no impact on simulated value of average annual runoff when using WEPP model to predict the runoff yield in years of rainfall. But the runoff yield is sensitive to the change of Effective Hydraulic Conductivity and appreciably sensitive to the change of Soil Albedo; and the runoff yield had negative correlation with Effective Hydraulic Conductivity. These indicate that the main parameter in the soil database caused the error is Effective Hydraulic Conductivity while the minor parameter is Soil Albedo when using the WEPP model to predict the runoff yield in years of rainfall.(6) The change of Soil Albedo had no impact absolutely on simulated value of sediment yield and the change of Interrill Erodibility almost had no impact on simulated value of sediment yield in single rainfall. But the sediment yield is sensitive to the change of Initial Saturation Level, Rill Erodibility, Critical Shear and Effective Hydraulic Conductivity; and the runoff yield had negative correlation with Critical Shear and Effective Hydraulic Conductivity. These indicate that the main parameters in the soil database caused the error are Initial Saturation Level, Rill Erodibility, Critical Shear and Effective Hydraulic Conductivity when using the WEPP model to predict the sediment yield in single rainfall.(7) The average annual sediment yield in years of rainfall is not sensitive to the change of Initial Saturation Level and Interrill Erodibility, but highly sensitive to the change of Rill Erodibility, Critical Shear and Effective Hydraulic Conductivity and appreciably sensitive to the change of Soil Albedo; and the sediment yield had negative correlation with Critical Shear and Effective Hydraulic Conductivity. These indicate that the main parameter in the soil database caused the error is Rill Erodibility, Critical Shear and Effective Hydraulic Conductivity while the minor parameter is Soil Albedo when using the WEPP model to predict the sediment yield in years of rainfall.
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