氣象變化及入流水溫對新山水庫優養化之影響

Abstract

位於基隆市安樂區的新山水庫為一離槽水庫，提供基隆市以及鄰近新北市部分區域之公共給水。近年來新山水庫於夏季分層時期常處於優養化狀態。本研究利用CE-QUAL-W2模式搭配浮昇射流機制來模擬新山水庫之水質，分析進流水水量、溫度與水庫優養化程度之關係，探討不同進水條件對水庫優養化及水理、水質之變化情形。 本研究以2014年之實測數據做為模式參數校正之依據，而以2010年之實測資料做為驗證。分析結果顯示，以CE-QUAL-W2模式搭配浮昇射流模擬新山水庫之水位、水溫等水理變化情形，其結果較無浮昇射流之模式好。而本研究模擬的水質項目有總磷、硝酸鹽、氨氮及葉綠素a等，其結果顯示總磷模擬結果有偏低之情形，葉綠素a模擬結果，大致與實測狀況相吻合。 由模擬結果可看出，經浮昇射流計算後，入流水溫的變化，將影響入流水上浮最終停留之位置。當停留位置接近表面時，便會使入流營養鹽濃度累積於表層或斜溫層中，導致水庫優養情形更為嚴重。 整體而言，以CE-QUAL-W2模式搭配浮昇射流來模擬新山水庫之水質，對夏季分層水體之水質模擬有較良好的結果，顯示浮昇射流運用於分層水體之必要性。在水庫分層時期，由底層進水可有效減緩表層之藻華現象。另外入流水溫度會影響入流最後停留位置，若進流溫度較低時，如夜間進水，營養鹽較不易進入表層，可有效抑制藻類的繁殖；而進流水溫高時，將造成藻類大量生長。因此水庫管理單位在連續高溫，河水溫度較高時，應注意水庫水質之變化。可依不同之操作，舒緩水庫優養化現象，維持水資源的安全性與利用性。

Shin-Shan Reservoir located at Anle Dist., Keelung City in Taiwan is an off-channel reservoir, providing water for people living in Keelung City and surrounding area of New Taipei City. In recent years, Shin-Shan Reservoir had heavy algal blooms in summer when the water body was stratified. In this study, we adopted CE-QUAL-W2 model and integrated the buoyant jet mechanism into it to simulate the water quality of Shin-Shan Reservoir. The relationships between inflow rate, temperature and the degree of eutrophication were investigated, as well as those between the variation of eutrophication and the characteristics of inflow. The model was calibrated and verified with the field data in 2014 and 2010, respectively. The results show that the predicted concentrations of phosphorus, nitrate, ammonium and chlorophyll a are close to real data. However, the total phosphorus concentration was underestimated by the model. Model simulations reveal that temperature of inflow water affects the final position of inflow which becomes a buoyant jet in a stratified water body. When the inflow jet is dispersed near the water surface, the nutrients will be loaded in the epilimnion or metalimnion, leading to severe eutrophication. CE-QUAL-W2 model with the buoyant jet mechanism is able to simulate the water quality of Shin-Shan Reservoir better than the model without consideration of the behavior of the buoyant plume. Furthermore, river temperature affects the final position of inflow. If the river water is cooler than the water of the thermocline, the nutrients will not be loaded into surface water, and there will be less growth of algae. On the contrary, if it is warmer, the algae will bloom heavily. Therefore, the authority of Shin-Shan Reservoir should closely watch the temperature of river water when it is climbing up quicker than that of the water in the reservoir, and choose appropriate operational approach to preserve the water quality in the reservoir.