• • Under Scenario 3 (highest pollution-control standards, lowest diversion volume), reservoir TN and TP concentrations decreased by 80% and 50%, respectively, achieving GB 3838-2002 Class II standards; downstream TN and TP declined by 36% and 33%, with CWQII improving from 4.211 to 3.410, demonstrating that stringent point/non-point controls combined with reduced diversion yield substantial water quality gains.
• • Land consolidation (full transfer of agricultural land and forest in protection zones) contributed 77% of TN and 45% of TP load reductions in the reservoir, highlighting non-point source management as the dominant lever for reservoir quality improvement.
• • A 20% reduction in diversion volume was most effective for downstream TN improvement, achieving >50% reduction, indicating that hydrological manipulation can be a powerful tool for downstream water quality management.
• • The EFDC model achieved NSE of 0.989 for hydrodynamics, while SWAT attained R² and NSE above 0.78 and 0.65, respectively, confirming the coupled framework's reliability for multi-scale simulation of water quantity and quality.