Abstract: This paper presents a systematic analysis on spatiotemporal variations in the runoff in the middle Yellow River and its driving forces, using the methods of Spearman rank correlation test, regime shift index, and double mass curve, based on the 1957-2018 hydrological series at four mainstream stations and eleven typical tributaries in Toudaoguai-Huayuankou reach. The results reveal a significant decreasing trend at the significance level of 0.001 in the annual runoff in the middle Yellow River mainstream and two abrupt changes in its hydrological regime in 1969 and 1987. Since the 1970s, the runoff depth in these tributaries has reduced with time significantly; the runoff proportion of Toudaoguai-Longmen reach to the entire middle Yellow River has decreased from 31% to 23.5% around the year 2000, while the proportions of all the reaches below Longmen have increased obviously. The average contribution rate of human activities to runoff variations was 62.68% in Toudaoguai-Longmen reach and 79.85% in Longmen-Tongguan reach. The major driving forces of runoff reduction are the operation of water conservancy projects and the implementation of soil and water conservation measures, such as reservoirs, check dams, irrigation water diversion works, and terraces.

Key words: middle Yellow River, runoff variation, mainstream and tributaries, water conservancy project, soil and water conservation, driving factors