Significantly reduced extreme streamflow and sediment load in the Yellow River Basin: Impacts of climate change and human activities

Over the past decades, the increasing frequency and intensity of extreme events due to climate change and anthropogenic climate change have greatly increased the threat to the production and livelihood of people along the riverbank. Hence, it is crucial to analyze the extreme variations of streamflow and sediment load observed in large rivers to better predict future changes in the world's water resources and hydrological extremes. In this study, we show the spatiotemporal variations of streamflow extremes and sediment load extremes in the mainstream of the Yellow River based on the daily streamflow and sediment load data from 1956 to 2019 and multiple mathematical and statistical methods. Then, we identify the main factors related to human activities and climate change by establishing quantitative relationships to figure out how these factors altered extreme streamflow and sediment load. Our results reveal that extreme streamflow and sediment load have decreased significantly since the 1950s (p < 0.05) except for the Yellow River source. However, the extreme streamflow increased significantly (p < 0.05) during 2000-2019, likely due to increased precipitation, and the extreme sediment load at most stations tended to stabilize. The contribution of extreme streamflow to the annual streamflow declines remarkably in the middle-upper reaches and increases significantly in the Lower reaches. While the contribution of extreme sediment load to the annual sediment load decreased significantly in the middle-lower reaches. Besides, the occurrence dates of extreme streamflow and sediment load showed an overall trend to disperse from the flood season to the four seasons of a year. We also have evidence that the fundamental cause of extreme water-sediment yield is extreme precipitation. Yet the extremity and hazard of water-sediment extremes are strongly affected by anthropogenic activities. Among them, mainstream dams can artificially change the water-sediment extremes, relationship, and synchronization, while anthropogenic engineering and vegetation measures can reduce the maximum possible peak of water-sediment extremes. Changes in the water-sediment relationships across the basin also confirm that changes in sediment source availability or erosive power dominate sediment reduction in each sub-basin. This study provides a scientific basis for risk management and water resources development and utilization in complex watersheds.