Dynamics of sediment transport in the Yangtze River and their key drivers

Understanding the dynamics of sediment transport and their underlying driving mechanisms is critical to developing strategic plans for sustainable river basin management. However, the quantification of various drivers (e.g., dam construction, soil and water conservation measures) of sediment load change remains scarce, and the projections of sediment transport under the 1.5 °C and 2.0 °C global warming scenarios are not well understood. Here we quantify the sediment load dynamics in the Yangtze River and also estimate the relative contributions of climate change and human activities to these changes using the Soil and Water Assessment Tool (SWAT) hydrologic model and statistical methods. Results show that the sediment load exhibited a significant decreasing trend at most stations in the Yangtze River, with breakpoints occurring around 1993 and 2003. The study period of 1972-2016 can be split into three sub-periods: 1972-1992 (P1), 1993-2003 (P2), and 2004-2016 (P3). From P1 to P3, 95.6 % of the decline can be attributed to climate change, the remainder resulting from the construction of dams and reservoirs (3.6 %) and improved soil and water conservation (0.8 %). Specifically, human activities contributed ∼67.96 % to sediment changes during the period P1-P2, whereas climatic change contributed ∼81.76 % during the period P2-P3. The sediment load significantly increased in the RCP 2.6 (slope = 0.87 Mt·yr^-1, P < 0.01) and RCP 4.5 (slope = 0.84 Mt·yr^-1, P < 0.05) under the global warming of 1.5 °C and 2.0 °C towards the entire Yangtze River. Compared with the 2.0 °C (2040-2059) global warming scenario, the mean annual sediment load in the 1.5 °C (2020-2039) global warming scenario is smaller by 17.88 Mt. Our investigation found that climate variations will provide a more remarkable contribution to sediment load dynamics in the Yangtze River in the future than the historical average due to the effectiveness of reservoirs decreases and the saturation of the capacity of the soil and water conservation measures to capture sediment. Although these findings indicate the importance of climate change's impact on sediment load changes, it is necessary to apply them to appropriate management to adapt to climate changes in future river basin management policies.