TOPOGRAPHY AND WATER EROSION IN NORTHERN SHAANXI PROVINCE, CHINA

The relationship between slope features and water erosion in the north ern region of Shaanxi Province in the Loess Plateau of China is presen ted. Most of the slope angles of loess ridges and hills vary from 10 d egrees to 28 degrees. The upper limit of the characteristic angles cor responds to the angle of internal friction of loess and the lower limi t to one third of the angle. This is as predicted from the literature. The slope forms in the study area can be separated into five types, n amely straight, convex, concave, convex-concave and step slopes. Soil erosion is greatly influenced by slope angles. Statistical analysis of observed data suggested that slopes of 15 degrees, 28 degrees and 45 degrees are important thresholds in relation to soil erosion. Sediment yields rise rapidly and washing is greatly intensified as slopes incr ease from 15 to 28 degrees. At 28 degrees the dominant type of erosion changes from running-water wash to gravitational erosion. Overall ero sion is most intense on slopes of 45 degrees. This result is supported by mathematical deduction. From the results it is suggested that 15 d egrees should be the maximum slope for agricultural land; lands with s lopes between 15 degrees and 28 degrees should be returned to grass, a nd lands with slopes over 28 degrees should be under forest.