Some rainfall-related thresholds for erosion and sediment yield in the upper Yangtze River basin

This study examines rainfall thresholds for erosion and sediment yield in the upper Yangtze River basin. Sediment reduction effects of soil conservation measures depend on the magnitudes of rainstorm. When the latter is less than a critical threshold, sediment reduction effects of soil conservation measures are positive; when this magnitude is exceeded, the effect is negative. An analysis based on data from the Jialingjiang River shows that the sediment reduction by soil conservation measures increased with annual precipitation to a peak, and then decreased to a negative value. The annual precipitation at the peak and zero values of sediment reduction are 970 and 1,180 mm, respectively, which can be regarded as two thresholds. Annual precipitation at the zero-value of sediment reduction has a return period of 25 years. In general, the design standard of soil conservation works in China is related with rainstorms with return periods of 10–20 years. When the magnitude of rainstorm exceeds this, the soil conservation works may be partly or totally destroyed by rainstorms, and the previously trapped sediment may be released, resulting in a sharp increase in sediment yield. It was also found in the lower Jinshajiang River that when annual precipitation exceeds 1,050 mm or high-flow season precipitation exceeds 850 mm, the annual sediment yield increased sharply. These can also be regarded as key rainfall thresholds for erosion and sediment yields. When precipitation is less than the two thresholds, dominant erosion types are sheet, rill and gully erosions. When precipitation crosses the two thresholds, debris flows may occur more frequently. As a result, the previously stored loose sediment is released and sediment yield increases sharply.