| Land degradation and its factors are among the most important research themes in Land Change Sciences. Karst areas in southwest China, with a large population and local non-sustainable land use, have the most serious land degradation in China and even the whole world. Therefore research on soil erosion in Karst areas in China, analyzing its factors, using model to simulate the soil erosion are quite necessary for regional management. In this article, we adopt methods like RS, GIS, wavelet analysis, neural network, RUSLE model and multivariate analysis to analyze soil erosion in Wujiang River Basin (WRB) in Guizhou Province. This article is constituted by three parts:1. Scale effect(1) Time scale. Using wavelet analysis method, we calculated the periodicity of annual runoff modulus, precipitation and sediment transport modulus based on their monitoring data from 1980 to 2000. The results show precipitation and runoff modulus data sequences are quite synchronous, which has a primary period of 16 years and a secondary period of 4 years. Sediment transport modulus is a little different, which has a primary period of 11 years and a secondary period of 5 years. We draw the wavelet transform coefficient figures of these three factors using 16-years-peroid and 4-years-period separately. The figures show asynchronization in both two time scales. This is related to different river basin development in different time.(2) Space scale. Through constructing relationships between sediment yield and drainage area of hydrological stations in WRB, it is found that there is a quadratic function relationship. 189 sub-river basins are generated from the whole Wujiang River Basin by setting the threshold value of convergence area (150 km2). We analyzed the regression relationship between 5 factors of soil erosion and the area of sub-river basin. The conclusion is that these 5 factors will change along with the changing of sub-river basin area. To eliminate the influence of scale effect, we further use 1000 km2 as standard sub-river basin area to calculate the sediment transport modulus map using Kriging interpolation method.2. The effect factors of soil erosion(1) Precipitation. The rainfall erosivity for every meteorological station was calculated on the Wujiang River Basin. The results show that the average annual rainfall erosivity changed from 1751 to 4349 MJ?mm/(hm2?h?a), and the mean of it is 2788 MJ?mm/(hm2?h?a) with the distribution of Southern North high-low, East West high-low. The rainfall erosivity is concentrated in summer (May-August), and which account for 90.42% of the annual rainfall erosivity. During 1980-2000, the trend rate of the average rainfall erosivity was positive. It indicates that the rainfall erosivity on the whole Wujiang River Basin is increasing. By using the conception of region gravity center, the change for the gravity center of rainfall erosivity in the different years is estimated as well as relation to the gravity center of slope, land use, geometric and the spatial location. Also, the effect of the distribution of rainfall erosivity on soil erosion is analyzed.(2) Topographic features. The type of erosion is water erosion on the Wujiang River Basin. According to the strength, the erosion can be divided into five degree which are less slight, slight, moderate, strength and pole-strength. The less slight and slight erosion are the main erosion. Based on the overlay analysis of soil erosion and DEM in 2000, the elevation, slope, amplitude and surface roughness are selected to analyze the geomorphology of soil erosion. From elevation, the less slight and slight erosion appear single peak which focus on the rocky hills basin areas in Guizhou with the elevation of 1100-1300 meters. The moderate and above degree erosion concentrate on areas with the elevation of 500-900 meters and 1300-1700 meters. From the slope, the distribution of the moderate and below degree erosion decrease. The area scale of the strength and pole-strength erosion increase with the surface rolling increase firstly and then decrease; 15°is a threshold. The erosion first increases and later decreases along with the increase of amplitude, with a threshold of 7-16 meters. The rougher the land surfaces, the lighter soil erosion. Soil erosion intensity is not sensitive to amplitude and surface roughness. (3) Land use. Analysis on the relationship between soil erosion and land use suggests that different land use types have different degrees of soil erosion, i.e. barren land < other land < construction land < forest land < water area < paddy field < grassland with low coverage < shrub land < grassland with high coverage < open forest land < grassland with medium coverage < dry land. Among these land use types, dry land, grassland with medium coverage and open forest land are mostly frequently accompanied with soil erosion. All types of soil erosion concentrate on areas with vegetation coverage of 50%-60%. When the vegetation coverage falls to 50%-60%, the most serious soil erosion happens. In Karst areas, the degree of soil erosion is strongly related to soil thickness, which is different with the situation in non-Karst areas where soil erosion intensified along with the degradation of vegetation.3. Soil erosion simulation and predictionUsing GIS platform, we calculated the average annual soil erosion modulus in the 1980s and 1990s. Our results coincide with previous soil erosion investigation. However, difference exists because RUSLE model does not have gravitational erosion included. Potential soil erosion modulus in the 1990s is higher than that in the 1980s, which suggests an increase in potential soil erosion in Wujiang River Basin. San-Cha river basin and Ma-Ti river/Yin-Jiang river have the highest value of potential soil erosion modulus. Among three main land cover types, forest land has the most soil conservation, grassland the least, while farmland in the middle. This is different with non-Karst area where grassland is better than farmland in soil conservation.We use BP Neural Network model to calculate soil erosion modulus in the 2000s based on the data in the 1980s and 1990s. The results suggests soil erosion decreases from 2313 t/(km~2·a) in the 1990s to 101 t/(km~2·a) in the 2000s, the amount of soil erosion from 115.18×10~6 t/a to 5.03×10~6 t/a. Soil erosion in San-Cha river is under control. Qianxi, Jinsha, Xifeng, Xiuwen, Guiyang, Pingban, Sinan, Shiqian, Yanhe and Songtao need strict soil management in 12th Five-Year Plan period.
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