Abstract
To study the effects of changes in the rainfall intensity on sediment concentrations in the Loess Plateau, the observed rainfall intensities and sediment concentrations from three typical small watersheds were used to analyze the relationship between these parameters. The results showed that the sediment concentration generally increased with the increasing rainfall intensity on slope scale. However, at watershed scale, a significant threshold phenomenon was observed for the effects of the rainfall intensity on the sediment concentration. When the rainfall intensity exceeds the threshold, the flood sediment concentration will no longer increase with the increase in the rainfall intensity. The rainfall intensity threshold increased with increasing vegetation coverage. The rainfall intensity threshold was 10–315 mm/h during 1956–31969, reached 20 mm/h from 1990 to 1997 and is approximately 40 mm/h at present. Due to a rainfall intensity of 10–315 mm/h almost happened every year, the vegetation did not change much from the 1950s to 1980s. Sediment yield mainly depends on soil erosion caused by surface flow, but the surface flow speed does not increase indefinitely with the increase in the flow discharge. Thus, the annual maximum sediment concentration of the tributaries in the loess area has been basically stable before the 1990s.
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