Temporal changes in annual runoff and influential factors in the upper and middle reaches of Yellow River from 1919—2010

Of the total water resources in the Yellow River, the contribution of surface water accounts for about 84%. However, the surface water status has undergone a significant change during the past nine decades mainly due to the combined influence of climate change and human activities. The study analyses the variability of annual runoff in the upper and middle reaches of the Yellow River. The analysis uses methods of the Mann-Kendall rank correlation test, the flow duration curve and the double mass curve. Observed stream flows for 1919—2010 at the Shanxian and Hekouzhen stations on the river are used for the analysis. Results show that there have been abrupt reductions in the observed annual runoff in both upper (at Hekouzhen) and middle reaches (between Hekouzhen and Shanxian) of the river since 1985, with a slight higher reduction in the middle reach. This is despite the fact that there have been no significant changes in precipitation over the drainage areas for the two stations during the same period of time. Apparent periodicities (low—high—low) can be observed in the variation of annual runoff in both upper and middle reaches of the river. At present, the river is experiencing it low-water period. The analysis of the double mass curve method indicates that human activities are respectively responsible for 88.1% and 84.9% reductions of annual runoff in the upper and the middle reaches of the river. Therefore, human activities appear to be the main influential factor for the change in annual runoff in both upper and middle reaches of the Yellow River.