應用SWAT與CE-QUAL-W2耦合模型於翡翠水庫水質模擬

Abstract

翡翠水庫為北臺灣重要水源，其深水型槽式水庫特性使水質高度受上游集水區營養鹽輸入與庫區垂向結構所控制。然而，集水區地形陡峭且非點源污染具高度時空變異性，單一模式難以完整模擬由集水區至庫區內部之傳輸與反應過程。本研究透過耦合SWAT與CE-QUAL-W2模式，連結上游負荷輸出與水庫垂向水質模擬，並針對2017–2021年進行系統性再現能力評估。 SWAT模式用於模擬北勢溪、𩻸魚堀溪與金瓜寮溪之日尺度入庫流量、水溫及營養鹽負荷，並作為CE-QUAL-W2之邊界條件。經率定與驗證結果顯示，SWAT模式於流量模擬具良好再現能力，營養鹽負荷 (輸砂、硝酸鹽氮、總磷) 亦能反映集水區非點源污染之季節變化趨勢，顯示其作為水庫模式入流條件具合理性。上述模擬成果進一步輸入CE-QUAL-W2，以模擬翡翠水庫之垂向溫度結構與水質變化。 結果顯示，耦合模式可合理再現翡翠水庫之季節性熱分層結構，並能有效掌握硝酸鹽氮於不同水層間之季節變動趨勢。此結果亦揭示，硝酸鹽氮濃度之變化主要受集水區營養鹽輸入所驅動，且模式對氮循環相關過程之描述具備整體穩定性與一致性。相較之下，總磷之模擬表現相對較弱，推測除與上游正磷酸鹽監測資料不足有關外，亦可能受到庫區內部營養鹽轉化與藻類過程簡化假設之影響。 整體而言，本研究所建立之流域至水庫耦合架構，能有效連結上游負荷與庫區垂向水質反應，相較傳統以簡化方式估算入流水量與水溫，提供更具過程基礎之CE-QUAL-W2入流條件設定方式，並可作為後續水庫水質情境分析與管理應用之基礎。

The Feitsui Reservoir is a critical water supply source for northern Taiwan. Owing to its deep, narrow valley–type morphology, reservoir water quality is strongly controlled by nutrient inputs from upstream catchments and by vertical stratification processes within the reservoir. However, the steep terrain of the catchment and the high spatiotemporal variability of non-point source pollution make it difficult for a single model to fully represent transport and transformation processes from the watershed to the reservoir interior. In this study, a coupled SWAT–CE-QUAL-W2 modeling framework was developed to link upstream load outputs with vertical water quality simulations in the reservoir, and its overall reproduction performance was systematically evaluated for the period 2017–2021. The SWAT model was applied to simulate daily inflow discharge, water temperature, and nutrient loads from the Beishi Creek, Yushikeng Creek, and Jingualiao Creek, which were subsequently used as boundary conditions for the CE-QUAL-W2 model. Calibration and validation results indicate that SWAT exhibited good performance in streamflow simulation, while simulated nutrient loads—including sediment, nitrate nitrogen, and total phosphorus—were able to capture the seasonal variability of non-point source pollution in the catchment, supporting the suitability of SWAT outputs as inflow inputs for reservoir modeling. These simulated inflow conditions were then incorporated into CE-QUAL-W2 to simulate the vertical thermal structure and water quality dynamics of the Feitsui Reservoir. The coupled modeling results demonstrate that the seasonal thermal stratification of the reservoir was reasonably reproduced, and that seasonal variations of nitrate nitrogen across different water layers were effectively captured. These results indicate that nitrate nitrogen dynamics in the reservoir are primarily driven by nutrient inputs from the upstream catchment, and that the model provides a stable and consistent representation of nitrogen-related processes. In contrast, the simulation performance for total phosphorus was relatively weaker, which is likely attributable to limited availability of upstream orthophosphate monitoring data as well as simplifications in the representation of internal nutrient transformation and algal processes within the reservoir. Overall, the watershed-to-reservoir coupled modeling framework established in this study effectively links upstream nutrient loads with vertical water quality responses in the reservoir. Compared to conventional approaches that rely on simplified estimates of inflow discharge and water temperature, this framework provides a more process-based method for defining CE-QUAL-W2 inflow boundary conditions, and can serve as a foundation for future reservoir water quality scenario analysis and management applications.