Investigating the downstream sediment load change by an index coupling effective rainfall information with reservoir sediment trapping capacity

•RSTI was developed to investigate the coupled effect of rainfall and reservoir.•Linear or nonlinear regression models were set up to simulate the sediment load.•Sediment load had a striking decreasing trend and a significant change-point.•Nonlinear regression had the better performance than linear regression model.•Coupled effect of rainfall and reservoir can better explain sediment load change. Sediment load is a critical issue in hydrologic process analysis and river basin management. Many studies have analyzed the impacts of rainfall or reservoir separately on the downstream sediment load; however, few researches investigated the coupled effect of rainfall and reservoir on sediment load change. In this study, a rainfall-augmented sediment trapping index (RSTI) that combines the impacts of effective rainfall and reservoir sediment trapping capacity was developed to attribute the sediment load change in the Wujiang River Basin (WRB) during the period of 1952–2017. Eight linear or nonlinear regression models were set up to investigate how to best utilize the proposed RSTI to reveal the coupled effect of rainfall and reservoir on the downstream sediment load of WRB. It is found that observed sediment load has a large decrease after 1984 when the Wujiangdu Reservoir was put into full operation while rainfall had only a slight change in the same period. The nonlinear regression model with RSTI as an explanatory variable (NSE = 0.837) has the best performance in simulating the observed sediment load series. These results might be helpful for the downstream sediment management under a changing environment.