Event-scale analysis of runoff-sediment inconsistencies and their attribution in the Yanhe River Basin

This study aimed to investigate the evolution characteristics and driving mechanisms of runoff-sediment events under changing environmental conditions in the Yanhe River Basin, a representative watershed on the Chinese Loess Plateau. Particular attention was given to the non-synchronous responses of runoff and sediment transport at the event scale and to the relative contributions of climatic and anthropogenic factors to hydrological and sediment processes. Long-term daily hydrological and sediment records from the Yanhe River Basin were collected to identify individual runoff-sediment events. Trend analysis and change-point detection were conducted to characterize temporal variations in annual runoff and sediment yield as well as event-scale hydrological responses. The identified runoff-sediment events were classified into different response types using the K-means clustering algorithm. Partial Least Squares Structural Equation Modeling (PLS-SEM) was then employed to quantify the direct and indirect effects of multiple environmental factors on runoff and sediment transport under different event types and to identify the dominant driving pathways. Annual runoff and sediment yield both showed significant decreasing trends during the study period, and an abrupt change occurred around 1996. Following the change point, annual runoff and sediment yield decreased by approximately 35% and 88%, respectively, indicating that sediment reduction greatly exceeded runoff reduction. Intensive human activities, especially large-scale ecological restoration and soil and water conservation practices, were identified as the dominant causes of the substantial decline in sediment transport. At the event scale, precipitation associated with runoff-sediment events increased significantly, whereas runoff volume exhibited no significant long-term change. In contrast, peak discharge and sediment yield both declined significantly, indicating an evident inconsistency between runoff and sediment responses. Although rainfall events became more intense, enhanced vegetation cover and conservation measures effectively reduced hillslope erosion and sediment connectivity, thereby weakening the linkage between runoff generation and sediment transport.Cluster analysis classified the runoff-sediment events into three distinct types with significantly different hydrological response patterns and sediment transport characteristics. Type I events were characterized by the strongest influence of precipitation on runoff generation, with a precipitation–runoff path coefficient of 0.553. These events responded rapidly to rainfall but produced relatively weak runoff and the lowest sediment transport intensity. Type II events exhibited a stronger dependence of sediment transport on runoff, with a runoff-sediment path coefficient of 0.598, while vegetation exerted a significant inhibitory effect on sediment transport, with a path coefficient of −0.358. Type III events showed the strongest control of runoff on sediment transport, with a runoff-sediment path coefficient of 0.840, whereas the influence of vegetation became much weaker under high-intensity hydrological conditions. Structural equation modeling further revealed that the response pathways and driving strengths differed considerably among event types. Runoff consistently acted as the dominant direct driver of sediment transport, whereas vegetation maintained an important sediment-retention effect under most conditions. Climatic factors mainly affected sediment transport indirectly through their influences on runoff generation and vegetation development. Runoff-sediment processes in the Yanhe River Basin changed substantially under the combined influences of climate variability and ecological restoration. The relationship between runoff and sediment transport became increasingly decoupled at the event scale, resulting in pronounced non-synchronous hydrological and sediment responses. The dominant driving mechanisms varied among different runoff-sediment event types, highlighting the importance of considering event-scale heterogeneity when evaluating watershed responses to environmental change. These findings provided a scientific basis for improving soil and water conservation practices and optimizing watershed management strategies in the middle reaches of the Yellow River.