Key Takeaways & Executive Findings
- •• • 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.
Abstract
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.
1. Introduction
Inter-basin water transfer projects are increasingly deployed to alleviate water scarcity, yet they can inadvertently degrade water quality in receiving basins. For instance, the Hanjiang Xiangfan section of the South-to-North Water Diversion Project experienced a 42% reduction in water resource environmental carrying capacity due to increased pollution load intensity. Such projects alter hydrological regimes, affecting pollutant transport and assimilation, thereby complicating water quality management. Existing assessments often rely on single-factor evaluations or isolated models, failing to capture the coupled dynamics of reservoir hydrodynamics and downstream riverine processes.
This study addresses these limitations by integrating EFDC and SWAT models to simulate the reservoir and its downstream reaches as a unified system. The framework enables scenario analysis of combined point-source treatment, land-use management, and diversion volume adjustments. By quantifying the nonlinear interactions among these measures, the research provides a robust tool for optimizing water quality improvement strategies in inter-basin transfer contexts, offering a methodological advance over conventional approaches.
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LI Haixiang, XU Yueping, JIANG Yanming, WANG Xiuling, YANG Le, KONG Linlin, QIU Jiong, JIA Xiaofang (2026). Water Quality Assessment of Inter-basin Water Transfer in Water Supply Areas Based on Coupled EFDC-SWAT Model. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202608015
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Frequently Asked Questions
What are the key limitations of the EFDC-SWAT coupled model when applied to other reservoirs with different morphological and hydrological characteristics?
The model's performance depends on accurate bathymetric and hydrological input data. While EFDC achieved NSE of 0.989 for hydrodynamics in this case, transferability requires recalibration. SWAT's R² and NSE above 0.78 and 0.65 indicate good performance, but these metrics may vary with watershed complexity. Users must ensure adequate data for boundary conditions and land use to maintain predictive reliability.
How does the 20% reduction in diversion volume translate to actual operational constraints, and what are the trade-offs in water supply reliability?
A 20% reduction in diversion volume improved downstream TN by over 50%, but such a cut may compromise water supply objectives. Operational decisions must balance water quantity needs against water quality benefits. The study suggests that even modest reductions can yield significant quality improvements, but a full cost-benefit analysis is necessary to assess impacts on water availability.
What are the specific mechanisms by which land consolidation reduces TN and TP loads by 77% and 45%, respectively, in the reservoir?
Land consolidation, involving the full transfer of agricultural and forest land in protection zones, eliminates fertilizer and manure inputs from these areas, thereby reducing non-point source pollution. The high contribution to TN reduction (77%) indicates that agricultural land is a major nitrogen source, while TP reduction (45%) suggests phosphorus inputs are also significant but may be partially buffered by soil retention.
How does the comprehensive water quality identification index (CWQII) differ from single-factor evaluation in assessing compliance with standards?
CWQII integrates multiple water quality parameters into a single index, providing a holistic assessment. In this study, CWQII indicated that Scenario 3 achieved Class II for the reservoir and Class III for downstream, whereas single-factor evaluation might flag individual parameters as non-compliant. This discrepancy arises because CWQII accounts for the severity and number of exceedances, offering a more realistic representation of overall water quality status.
What are the implications of the nonlinear enhancement effect of combined point and non-point source controls for policy design?
The nonlinear enhancement implies that implementing both point-source treatment and land-use management yields greater than additive benefits. For instance, Scenario 3, which combines highest treatment standards with land consolidation, achieved 80% TN reduction in the reservoir, whereas individual measures would likely produce less. This suggests that integrated policies are more effective and cost-efficient than isolated measures, guiding resource allocation.
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