From cultivated land expansion to ecological priority: Coupled socio-hydrological mechanisms of sediment evolution between Hekou-Longmen District in the middle reaches of the Yellow River

Study region: Hekou-Longmen District (the middle reaches of the Yellow River, China) Study focus: A clear understanding of the extent to which various factors influence sediment transport across different time scales constitutes a pivotal cornerstone for unraveling the intrinsic mechanisms underlying runoff-sediment evolution. While conventional analytical methodologies are capable of delineating the processes of sediment evolution and their contributing factors, they fall short in truly capturing the operational mechanisms of human behaviors that underpin sediment variations. Thus, to unravel the coupled evolutionary mechanisms between human activities and sediment transport in the Hekou-Longmen District (HLD) over the past century, this study employs an improved socio-hydrological dynamic modeling framework. By quantifying catchment sensitivity, human demands, and behavioral response, we characterize the interactive processes between anthropogenic decision-making and environmental systems. This approach enables an analysis of the co-evolutionary mechanisms within the sediment-socioeconomic-environmental system during sediment evolution. New hydrological insights for the region: The sediment evolution of the HLD over the last hundred years was influenced by a combination of human demands and catchment sensitivity (V). The V was close to the centennial maximum during 1961–1980, and during the same period, catchment sensitivity function f(V*) was also the centennial maximum (f(V*)max= 1.40). The average sediment transport from 1981 to 1999 decreased by approximately 420 million tons compared with that from 1961 to 1980. This sharp decline coincided with the period when large-scale and extensive soil and water conservation projects were implemented. From 2000–2020, after experiencing a series of fluctuations, behavioral response (X) gradually approached zero, which means that the expansion tendency and the protection tendency roughly reached a relative balance at the basin scale. Overall, the HLD has gone through a relatively long expansion phase and is currently entering an early contraction phase. The sensitivity analysis and validation results of the model indicate that X has a significant positive correlation with the terraces (r = 0.72), check-dam (r = 0.86), and composite structure governance index (r = 0.81) during the observation period. The changes in parameters have little impact on the values of V and X, which showed that the results of model have good robustness.