Have human activities been accurately evaluated in sediment yield changes in the middle reaches of the Yellow River?

The middle reaches of the Yellow River (MRYR) in China are some of the most severely eroded areas in the world. Knowledge of the changes in sediment yield in the MRYR is of great significance for understanding the impact of human activities on soil erosion and sediment transport. Using data from the MRYR and 13 sub-basins, this study aims to evaluate the actual contributions of human activities to sediment yields and to examine whether the widely used Mann-Kendall test has underestimated this contribution. Sediment data (1960-2020) were analyzed using the Mann-Kendall test and based-on-model fitting (BMF) to identify sediment yield breakpoints, while the double mass curve was used to evaluate the contributions of human activities. The results indicate a significant decline in sediment yields across all basins, with human activities contributing substantially more than climate change. The analysis also showed that in all basins, the contribution from human activities calculated by the Mann-Kendall test was lower than that of the BMF method, with an average underestimation of 42.37%. The constructed regression equations all have R² values greater than 0.9. In the MRYR, the reduction in sediment volume increased by 27.31% between 2002 and 2015, while the marginal benefit decreased by 11.15%, and the ratio of detained runoff to sediment decreased by only 1.73%. Sediment reduction volume per unit area of soil and water conservation measures (SWCMs) gradually diminish. The study results provide a scientific reference for optimizing SWCMs planning in the MRYR.