Deciphering runoff-sediment-nutrient dynamics in agricultural watersheds supplied by large feeder Rivers: A multi-scale analysis

The runoff-sediment-nutrient continuum served as a pivotal hydrological-ecological indicator system underpinning agricultural sustainability and the integrity of water resources in river watersheds. However, the spatiotemporal dynamics and intrinsic mechanisms governing these coupled processes remained incompletely characterized in intensively managed agricultural catchments drained by major tributaries. This study employed multi-source daily monitoring data (2007–2022) from an agricultural watershed, integrating change-pattern analysis, change-point detection and scale-sensitive correlation analysis to decipher the coupling mechanisms and hysteretic responses of the runoff-sediment-nutrient continuum under anthropogenic perturbations. Our results showed that runoff displayed no significant long-term trend, driven by the offsetting effects of reservoir regulation and precipitation homogenization. In contrast, sediment load at Waizhou Station underwent a marked 74 % reduction, accompanied by a sustained downward trajectory. A 5-year lag was observed between runoff-sediment dynamics and regional precipitation changes, which maybe related to reservoir storage saturation and delayed anthropogenic land-water interaction effects. Along the river course, runoff showed a gradual increase while sediment underwent non-linear changes. Spatial nutrient analysis showed the short-term spatial differences of nitrogen and phosphorus observed from 2021 to 2022, with higher concentrations in the northern and southern regions and lower concentrations in the central region. Based on two years of observational data from this study, downstream total nitrogen (TN) may exhibit significant fluctuations due to the intensive interactions between urban and agricultural activities, with extreme values reaching a ratio of 200. Scale-dependent reversals characterized the relationships between precipitation, runoff, sediment and nutrients. Runoff closely followed precipitation, whereas sediment became decoupled owing to reservoir buffering. TP was strongly correlated with runoff at the daily scale, whereas TN dynamics might driven by dilution-biological interaction processes. This study qualitatively demonstrated how agricultural and hydraulic infrastructure reconfigured source-sink dynamics of material translocation. The findings provided preliminary directional guidance for adaptive water-nutrient management and sediment control in agricultural watersheds sustained by regulated tributaries.