Effect of soil and water conservation measures on the water–sediment relationships at different time scales in the Loess Plateau, China

The increasing prevalence of extreme rainfall events has further complicated the water–sediment relationship. Understanding the multiscale evolution of this relationship and its response to extreme rainfall is critical for effective sediment regulation. In this study, we employed multiple methods, including change point detection, percentile-based thresholds, and water–sediment rating curves, to examine changes in extreme rainfall across the Loess Plateau (LP)from 1961 to 2020. We studied the evolution and driving mechanisms of the water–sediment relationship at multiple temporal scales in the multi spatial scale watersheds of the middle reaches of the Yellow River (YRB). The results showed the following: From 1961 to 2020, the LP experienced increased spatial extent, higher frequency, and greater intensity of extreme rainfall events. Although the period 2001–2020 recorded the highest values of these indices over the 60-year period, our event-scale analysis, based on flood events grouped by comparable areal rainfall depths, reveals that since 2000, mean flood runoff under such comparable conditions decreased by 69% relative to pre-1980 levels, while sediment transport per event declined by 87%. After 2000, at the interannual scale, the rainfall-sediment correlation weakened considerably, exhibiting varying degrees of negative correlation in the Kuye, Huangfuchuan, and Wuding River Basins, as conservation measures altered underlying surface conditions, reduced runoff erosion and caused structural decoupling between sediment production and transport. At the event scale, extreme rainfall intensified the nonlinear water–sediment relationship, increasing the risk of high-sediment flood events. Soil and water conservation measures, however, substantially prolonged flood durations, attenuated peak discharges, and dissipated energy, which resulted in an approximately 70% reduction in sediment yield. In conclusion, frequent extreme rainfall events are progressively reshaping water and sediment dynamics on the LP. Although soil and water conservation measures have markedly reduced sediment loads, their marginal returns are diminishing. These findings provide a scientific foundation to optimizing soil and water conservation strategies in the YRB.