整合觀測及模擬分析土地利用改變對七家灣溪水質、水溫影響之研究

Study on Integration of Monitoring and Modeling for Analyzing the Effects of Land Use Changes on Water Quality and Stream Temperature in Chichiawan Creek

李宗祐(Tsung-Yu Lee)

指導教授 ： 童慶斌

國立臺灣大學/生物資源暨農學院/生物環境系統工程學研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.01546

摘要

櫻花鈎吻鮭（學名Oncorhynchus masou formosanus）是台灣本島之自然資產，更是世界重要的生態資產，七家灣溪是櫻花鈎吻鮭碩果僅存的棲地，但其棲地範圍受到人為活動的影響而逐漸縮減，引發我們對於水溫及非點污染在集水區內空間及時間的分佈的高度興趣，因唯有透過深入瞭解水溫及非點源污染在集水區內的傳輸機制，才能有效管理土地利用及維護國寶魚生存之棲地環境。
透過高時間頻率及高空間解析度之水質監測，研究中發現集水區內NO3與PO4濃度變化仍與子集水區內過去的農業活動密切相關，儘管部分耕地已經回收進行復育，但廢耕蔬菜園之NO3-N及PO4-P產出量，仍分別高達356.57 kg-N/ha/yr及4.21 kg-P/ha/yr，雖然顯著低於現有蔬菜園之產出量（分別為2698.1 kg-N/ha/yr及5.38 kg-P /ha/yr），但均高於現有果園產出量（124.16 kg-N/ha/yr及1.19 kg-P/ha/yr），可見過去農業行為對土地殘存之影響，進一步將現存蔬菜園廢耕將有效降低各子集水區年NO3-N及PO4-P輸出量分別達58.7%∼80.9%及2.6%∼15.1%；以集水區內元素輸出的時間而言，NO3-N主要隨著一般降雨逕流事件流出集水區，不若PO4-P主要附著在懸浮顆粒上，並集中在颱風事件輸出集水區；而污染傳輸路徑之研究結果顯示，暴雨期間地表逕流提供超過總流量之50%為主要河川物質來源，其次為地表下逕流及地下水，但平時仍以地下水為主，地表下逕流為輔，但需要更多的現地觀測資料來輔助目前的研究成果。
在水溫研究方面，透過連續監測之溫度紀錄並配合模式之計算，發現河川走向與兩岸植栽影響河川水溫之物理特性，進一步提供了七家灣溪河岸植栽可考量的優先順序，其中以上游植栽能獲得最好之日最高溫平均降溫效率，在上游達到完全庇蔭的情況下，將為櫻花鈎吻鮭在夏季多創造出至少1公里之適合棲地；研究中並精進過去所發展之水溫模式，加入雲遮蔽與逕流匯入之影響，突破以往水溫模式使用上受到天氣條件之限制，結果顯示此模式可以合理模擬連續水溫，將可有效做為評估短期氣候變異對於櫻花鈎吻鮭棲地影響之評估工具，而雲遮蔽對於水溫影響遠大於側流匯入之分析結果，亦呼應了以河岸植栽策略控制水溫之可行性。
建議櫻花鈎吻鮭所在之七家灣溪流域應持續進行水質監測，將可釐清農耕地回收所產生之一連串效應，並做為台灣其他山坡地經營管理的借鏡與典範；而水溫為影響國寶魚之關鍵性因子，持續監測與尋求有效降低水溫的方法可及早因應全球暖化對國寶魚的衝擊。本研究之成果，將可提供有關單位做為集水區管理方式之參考依據以減緩氣候變遷及土地利用改變所造成之棲地衝擊。

關鍵字

非點源污染 ； 污染傳輸 ； 水溫 ； 植栽策略 ； 櫻花鈎吻鮭

並列摘要

Formosan Landlocked Salmon (Oncorhynchus masou formosanus), an endangered species that can be only found in Chichiwan Creek in Taiwan, is not only the natural asset for Taiwan but also for the world. The decreasing habitat due to human activities motivates us to understand the temporal and spatial distribution of water quality and stream temperature in this watershed. The investigation and understanding on the mechanism of water quality and stream temperature changes within the watershed will help the managers to efficiently resotre the habitat.
Through the intensive investigation on water samples in temporal and spatial aspects, it was found that the expropriated vegetable farms (inactive vegetation) still contributed lots of NO3-N and PO4-P to the stream. Even if the expropriated farms has stopped growing vegetable for 2~3 years, the annual NO3-N and PO4-P yield from the inactive vegetation were still as high as 356.57 kg-N/ha/yr and 4.21 kg-P/ha/yr, respectively. Although the yields were much lower than those from the current vegetable farms (2698.1 kg-N/ha/yr and 5.38 kg-P /ha/yr), they were still higher than the yields from the current orchard (124.16 kg-N/ha/yr and 1.19 kg-P/ha/yr) indicating the consequences of fertilization in the past. If all the rest vegetable farms were expropriated, the reduction of annual NO3-N and PO4-P export among all the subwatersheds would reach 58.7%∼80.9% and 2.6%∼15.1%, respectively. Most of the NO3-N was leached out via the light and moderate rainfall events. It is not like PO4-P that often attached to the surface of eroded sediment was flushed out in the very short time span, i.e. typhoon events. According to the results of flow pathway, it was found that when typhoon invaded, surface runoff contributed more than 50% streamflow and then subsurface runoff and groundwater. Except storm events, groundwater discharge was the primary contributor then subsurface runoff. However, we still need more field observations to validate our conclusion on flow pathway.
For study on stream temperature, it was found that the orientation of river and riparian canopy would affect stream temperature. Hence, several planting strategies along Chichiawan Creek have been evaluated. Upstream planting having the best performance on reducing daily maximum stream temperature was strongly recommended. If the riparian canopy in the upstream could create shades on the entire stream surface at any time, more than 1 km suitable habitat would be created for Formosan Landlocked Salmon. To improve the stream temperature model, cloud effect and surface/subsurface runoff input has been incorporated into the model making it capable of continuously simulating stream temperature when rain or shine. The well-validated model can be applied to evaluate the impacts of short-term weather variabilities on stream temperature. The finding, the effect of cloud is more siginificant than surface/subsurface runoff input, support the idea of riparian planting which can diminish solar radiation.
To keep monitoring water quality in Chichiawan Creek is essential to understand the consequent effects of farm land expropriation. The Chichiawan watershed can be an ideal example demonstrating the effects of land use changes on water quality. Stream temperature is one of the most critical environmental factors for Formosan Landlocked Salmon. To keep monitoring and looking for alternatives to reduce stream temperature is the most critical issue, particularly important because of stress from global warming. The results of this research can be a reference for the associated authorities to manage the watersheds to mitigate the impacts on aquatic habitat resulting from climate changes and land use alterations.

並列關鍵字

Non-point Source Pollution ； Pollutant Transport ； Stream Temperature ； Riparian Planting Strategy ； Formosan Landlocked Salmon

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