Abstract: The characteristics of runoff-sediment variation of Dongting Lake in recent six decades and its relationship with human activities were analyzed using Mann-Kendall trend test and double cumulative curve method based on the runoff-sediment data of Dongting Lake from 1955 to 2018. Under the influence of the waterway changes in Jingjiang reach of Yangtze River and the construction of water conservancy projects (WCP), declining trends were observed for the time series of runoff and sediment discharge at the three outlets of Yangtze River into Dongting Lake and the four rivers into Dongting Lake, of which the former was most significant. Affected by the water-sediment change at the three outlets, both the annual runoff and sediment discharge at Chenglingji presented dropping trends. An obvious downward trend was also discovered for the sediment deposition inside the Dongting Lake from 1955 to 2018. Such downward trend can be mainly attributed to the decrease of three outlets' sediment discharge caused by human activities such as the construction of WCP, which reduced the sediment concentration and deflected the dual cumulative runoff-sediment curves. The impact of human activities on runoff-sediment variation of Dongting Lake can be quantified by building the relations between cumulative runoff and sediment discharge before and after the deflection point. Calculations in this paper showed that human activities cut the annual sediment discharge at the three outlets, four rivers, and Chenglingji by 0.69×10⁸ t/a (2003-2018), 0.22×10⁸ t/a (1986-2018), and 0.21× 10⁸ t/a (1981-2018).

Key words: runoff-sediment variation, runoff, sediment, Mann-Kendall trend test, double cumulative curve, human activities, Dongting Lake