Quantitative assessment of drivers of sediment load reduction in the Yangtze River basin, China

Abstract

River sediment flux, the most critical element of land-ocean interaction, is undergoing significant changes due to climate variability and anthropogenic activities. Thus, identifying the driving forces and their contributions to the variation in sediment load is of great significance for sustainable management of soil and water resources. However, the dynamics behind these changes are not well understood, and the quantification of drivers of sediment load change remains scarce. In this study, we investigated the changes in hydrometeorological variables and quantitatively separated drivers of sediment load change in the Yangtze River (YZR) basin, China, during 1956–2015. The Mann-Kendall test and sequential cluster method were employed to examine the trends and abrupt changes in water discharge and sediment load. Results showed that the mean annual precipitation for most sub-basins and water discharge at most stations exhibited no apparent downward trend, while annual temperature showed a significant upward trend for most sub-basins. Sediment load decreased significantly at most stations, with breakpoints occurring around 1968 and in the early 2000s. The whole study period was split into three sub-periods (1956–1968, 1969–2002 and 2003–2015) to quantify the change in sediment load. As indicated by water-sediment relationships at 12 stations, the slope of the double mass curve showed a considerable decreasing trend during the period 2003–2015. A decomposition method based on sediment identity factor analysis was applied to quantify the impact of driving forces on the variation in sediment load. It was found that a decrease in suspended sediment concentration accounted for approximately 100–125% of the basin-wide reduction in sediment load in the YZR basin. Construction of large reservoirs played a dominant role in sharp decline in sediment load across the basin, which was responsible for about 157 Mt yr−1 and 384 Mt yr−1 of sedimentation for the period 1969–2002 and 2003–2015, respectively. Soil conservation projects played the secondary role in reducing sediment load, which led to an average decrease of 234 Mt yr−1 in the entire basin during 1989–2015. Hence, more attention should be paid to changes in sediment load resulting from increasing human activities in the future.