Water Quality Assessment of Inter-basin Water Transfer in Water Supply Areas Based on Coupled EFDC-SWAT Model
Inter-basin water diversion projects can profoundly alter the water quality dynamics of receiving basins. Taking the Qincun Reservoir and its downstream reaches in the Huangze River Basin as a case study, this research quantitatively evaluates water-quality responses under multiple coordinated management measures. An integrated Environmental Fluid Dynamics Code-Soil and Water Assessment Tool (EFDC-SWAT) modeling framework was established, coupling a two-dimensional hydrodynamic-water-quality model for the reservoir with a hydrology-water-quality model for the downstream reaches. Seven management scenarios were designed to reflect various combinations of point- and non-point-source pollution control strategies. Simulations focused on spatiotemporal variations in key indicators—total nitrogen (TN), total phosphorus (TP), ammonia nitrogen (NH3-N), and permanganate index (CODMn)—and assessed pollution-load reduction effectiveness. Comparative analysis using the comprehensive water quality identification index (CWQII) revealed that under Scenario 3 (highest pollution-control standards with lowest diversion volume), TN and TP concentrations in the reservoir decreased by 80% and 50%, respectively, achieving Class II water-quality standards. Downstream TN and TP levels declined by 36% and 33%, and the CWQII improved from 4.211 to 3.410. Land consolidation contributed 77% and 45% to TN and TP load reductions in the reservoir, respectively, while a 20% reduction in diversion volume was most effective in improving downstream TN (>50%). These results demonstrate that the coupled EFDC-SWAT model effectively elucidates mechanisms through which inter-basin water diversion influences water quality in supply areas. Moreover, synergistic point- and non-point-source controls exhibit a nonlinear enhancement effect on overall water-quality improvement.