植生過濾帶於營養鹽負荷削減效益評估：以德基水庫為例

Abstract

本研究以德基水庫流域為研究區域，建立耦合式SWAT-Vollenweider水質模型，透過彙整歷史30年水質觀測數據，分析集水區內特徵營養鹽與貢獻較高之熱區，評估不同設置情境之植生過濾帶的削減效益，進而最佳化其設置策略。本研究進行庫區水質參數之相關性與主成分分析，訂定總磷為德基水庫的限制營養鹽，並建立SWAT土壤水文模式，評估集水區之輸砂量與總磷負荷等貢獻，接續匯入Vollenweider模式模擬庫區水體的總磷濃度。結果顯示單位面積輸砂量與總磷貢獻較高之區域大多為農業利用，且SWAT模式可合理模擬集水區逕流、輸砂量與總磷負荷。根據污染熱區分析，本研究選擇前10高之水力反應單元模擬四種操作情境之結構性最佳管理作為，分別為沿著區域邊界或沿著河岸設置植生過濾帶，寬度設定為5或10公尺。模擬結果顯示單位面積貢獻較高之操作情境為沿著河岸設置5公尺的植生過濾帶，可減少各熱點輸砂量約每公頃2.7至38.8噸，總磷輸出則減少約每公頃4.0至34.4公斤。本研究率定庫區總磷之沉降速度為25 m/yr，並透過Vollenweider模式模擬緩衝帶設置情境下的總磷濃度趨勢；結果顯示植生帶寬度為10公尺之削減量較5公尺高，整體營養鹽削減率約介於0.5至9.1%。本研究透過最佳化分析找出可達到目標削減量之植生過濾帶區位與最小設置面積，並推得設置面積為9、13與23公頃時出現削減量臨界值。綜合上述，可知植生過濾帶可確實減少德基水庫集水區的輸砂與營養鹽輸出，並可進一步探討不同限制條件下的植生過濾帶最佳配置策略，提升以自然為本之集水區保護措施效益。

The study examines the effects of establishing vegetative filter strips (VFSs) by the water quality model systems of SWAT and Vollenweider in Techi Reservoir watershed. Historical 30-year observed data is analyzed for correlation and principal component analysis to identify total phosphorus (TP) as the specific nutrient in Techi Reservoir. This study applies Soil and Water Assessment Tool (SWAT) model to assess sediment and TP load from Techi Reservoir watershed, and analyzes the areas within the watershed that contribute higher sediment and nutrient loads. Additionally, Vollenweider model is chosen to simulate the trends of TP in the reservoir. The analysis reveals that areas with higher sediment and TP load per unit area are primarily for agricultural uses. On the other hand, SWAT model demonstrates satisfactory performance in simulating streamflow, sediment, and TP load within the watershed. Based on the analysis of pollution hotspots, this study selects the top 10 Hydrologic Response Units (HRUs) to establish structural BMP with VFS scenarios of along the field edge or along the riverbank, and widths of 5 or 10 meters. The results show that under scenario of 5-meter VFS along the riverbank contributes the most reduction per unit area, which effectively reduce sediment yield by approximately 2.7 to 38.8 tons/ha and TP load by about 4.0 to 34.4 kg/ha. Vollenweider model is also calibrated and validated, with the TP settling velocity in the reservoir set at 25 m/yr. The model simulates the trends in TP concentration under different width of VFS, and the results indicate that the 10-meter VFS achieves a higher reduction rates than the 5-meter VFS, with the reduction rates around 0.5 to 9.1%. This study employed optimization analysis to ascertain the optimal placement of VFSs and calculate the minimum VFS areas necessary to achieve the target reduction amounts. Additionally, this study identified the critical reduction values for VFS implementation at areas of 9, 13, and 23 hectares. The findings indicate that VFSs are effective in reducing sediment and nutrient discharge in Techi Reservoir watershed. Further investigation into the optimal configuration strategies for VFSs, considering different constraint factors, has the potential to enhance the efficacy of nature-based measures for watershed protection.