Rainfall threshold–conditioned land use impacts on sediment connectivity in the Yellow River Helong District

Soil erosion and sediment transport are shaped by complex interactions between land–use management and rainfall forcing, yet the conditional nature of this interaction remains incompletely understood. Taking the Hekou–Longmen (Helong) District in the Yellow River Basin, a typical sediment source area, as the study region, this study investigated how rainfall conditions may influence the effectiveness of land–use change in regulating sediment connectivity. We reconstructed the spatiotemporal dynamics of sediment connectivity from 1990 to 2020 using the Index of Connectivity (IC) framework integrated with NDVI–derived vegetation dynamics, and explored its nonlinear relationship with rainfall thresholds using generalized additive models (GAMs). The results showed that large–scale vegetation restoration and soil and water conservation measures were associated with a long–term decline in sediment connectivity across most sub–basins, suggesting a gradual weakening of sediment transfer pathways in the study area. However, this relationship appeared to vary under different rainfall conditions. When rainfall was below the statistically inferred inflection point of approximately 65 mm in the Wuding River Basin, sediment connectivity showed a stronger association with land–use conditions, and vegetation restoration corresponded to lower sediment transfer potential. When rainfall was above approximately 65 mm, the influence of rainfall appeared to become stronger, and sediment connectivity showed signs of partial reactivation despite ecological restoration. These findings suggested that the inferred inflection point might vary under long–term land–use and vegetation changes. By linking land–use regulation with rainfall conditions, this study offered a potential perspective for understanding the coexistence of long–term sediment decline and episodic high–sediment events, and highlighted the importance of adaptive soil and water conservation strategies under different rainfall regimes.