Abstract:

Based on the data of rainfall intensity and sediment concentration measured in different periods of three typical small watersheds in the loess hilly region, the relationship between rainfall intensity and sediment concentration at different spatial scales was analyzed in this study. The results show that the sediment concentration generally increases with the increase of rainfall intensity on the slope scale. However, on the catchment scale of both slopes and gullies, regardless of the level of the watershed or the area of the valley, there is a significant threshold phenomenon in the effects of rainfall intensity on sediment concentration. When the rainfall intensity exceeds the threshold, the flood sediment concentration will no longer increase with the increase of rainfall intensity. The rainfall intensity threshold increases with the increase of forest and grass coverage. For example, the threshold of rain intensity under the condition of underlying surface in the study area from 1956 to 1969 was 10-15 mm/h, and it reached 20 mm/h from 1990 to 1997, and about 40 mm/h now. Because the rainfall with the intensity of 10-15 mm/h happened almost every year on the basin scale, the vegetation did not change much from the 1950s to the 1980s, and the sediment yield in loess hilly area mainly depended on the soil erosion caused by surface flow while the surface flow speed did not increase indefinitely with the increase of the flow discharge, so the annual maximum sediment concentration of the tributaries in loess area was basically stable before the 1990s.

Key words: the Loess Plateau, loess hilly region, rainfall intensity, sediment concentration, influence