Assessing hydrological impacts of major engineering projects under intensive ecological restoration through multi-component runoff validation: A case study in a middle-reach Yellow River basin, China

Study region Qin River Basin, China. Study focus Understanding the evolution and attribution of ecohydrological processes under changing environments is critical for sustainable water management. This study establishes and applies a multi-component runoff validation framework for the Soil and Water Assessment Tool (SWAT) model to quantitatively dissect the impacts of a conceptual Returning Agricultural Land to River (RAR) initiative. New hydrological insights for the region Results revealed a distinct runoff component response hierarchy to the RAR initiative: surface runoff (SUR, 0.31–21.88%) > total runoff (TR, 0.42–12.90%) > baseflow (BF, 0.50–6.83%). This hierarchy highlights that ‘blue’ measures preferentially enhance fast flow pathways, contrasting with ‘green’ measures which often reduce overall yield. Key hydrological insights include a pronounced slope threshold effect (higher efficacy on slopes <15°), a strong spatial sensitivity gradient (lower > middle > upper basin), and marginal benefits of TR in the lower basin nearly tenfold those upstream. Spatially differentiated optimal implementation widths were identified (50 m, 50–100 m, and 100–200 m for upper, middle, and lower basins, respectively). These findings elucidate the unique runoff redistribution mechanisms and spatial heterogeneity induced by such "blue" initiatives, offering actionable insights for optimizing nature-based solutions in water-limited basins.