Topography and water erosion in northern Shaanxi Province, China

The relationship between slope features and water erosion in the northern region of Shaanxi Province in the Loess Plateau of China is presented. Most of the slope angles of loess ridges and hills vary from 10° to 28°. The upper limit of the characteristic angles corresponds to the angle of internal friction of loess and the lower limit 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, namely 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°, 28° and 45° are important thresholds in relation to soil erosion. Sediment yields rise rapidly and washing is greatly intensified as slopes increase from 15 to 28°. At 28° the dominant type of erosion changes from running-water wash to gravitational erosion. Overall erosion is most intense on slopes of 45°. This result is supported by mathematical deduction. From the results it is suggested that 15° should be the maximum slope for agricultural land; lands with slopes between 15° and 28° should be returned to grass, and lands with slopes over 28° should be under forest.