黄土丘陵区雨强对水流含沙量的影响

以黄土丘陵区的3个典型小流域为重点对象,基于不同时期实测的1300多对雨强和含沙量数据,分析了不同空间尺度情况下的雨强—沙峰含沙量关系。结果表明：在峁坡尺度上,含沙量总体上随雨强的增大而增大;但在既有峁坡、也有沟坡的各级流域尺度上,无论流域层级高低或面积大小,雨强对含沙量的影响均存在明显的阈值现象,当雨强超过该阈值后,洪水含沙量不再随雨强增大而增大。雨强阈值随林草覆盖程度增大而增大,如研究区在1956-1969年下垫面情况下的雨强阈值为10~15 mm/h、1990-1997年为20 mm/h,目前估计达40 mm/h。由于流域尺度上雨强大于10~15 mm/h的降雨几乎每年都有发生、20世纪50-80年代的林草植被变化不大,加上黄土丘陵区产沙主要靠坡面水流对地表土壤的冲刷,而贴地表水流的流速不会随流量的增大而无限增加等原因,形成了20世纪90年代以前黄土区支流的年最大含沙量基本稳定的现象。

Effects of rainfall intensity on sediment concentration in loess hilly region of China

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.