為了有效利用水資源，台灣地區流量較充沛的河川上皆興建水庫儲蓄水源，然而水庫的營運操作使得下游河川流制變化單調，且發電的放水量造成短延時內的流量大小變動劇烈，原本的河川天然流制受到改變，進而破壞河川生態系統，更甚者會影響人類社會。
本文以可同時呈現訊號在時間域和頻率域特徵的小波分析作為工具，利用其較其他時頻分析工具更高的局域分析彈性，以及更適用於非平穩訊號處理的優點，探討水庫下游流制的改變，並以台灣北部的翡翠水庫作為研究區域，以水庫入流量為天然流量，同時間的水庫放流量為受影響流量，分別進行小波轉換並比較其間的頻譜差異。
本文以小波轉換分析日流量和時流量資料在水庫下游及北勢溪和南勢溪匯流後的二不同河段受水庫營運的影響，由頻譜比較結果可知，時流量可彌補日流量資料因時間尺度不夠細緻無法檢測出水庫因尖峰發電放水操作的影響，此二河段的分析結果可知水庫的營運會造成流制周期在1年以下有較顯著的偏移，但匯流處因有未受影響的流量匯入可略為改善原受水庫影響的流制。本文建議一流制頻譜指數以量化河川天然流制在時間域與頻率域受水庫影響的程度，其為全域小波能量平均值，日流量分析結果顯示水庫下游處受水庫營運影響之流制頻譜指數為7.4%，匯流處約為3.5%，改善幅度為45.5%；時流量資料在不同河段分別為3.8%及2.0%，改善幅度為41.2%。此指數未來可供水庫調整放水策略改善下游河段受影響的參考依據。

In order to have efficient water-resource uses, most rivers in Taiwan have been dammed to regulate streamflow for stable water supplies. However, construction and operation of reservoirs results in severe alterations of natural flow regime such as non-natural variation and hydropeaking induced by hydropower generation. The altered nature flow regime would deteriorate river ecosystem as well as the human society.
In this study, wavelet transform is employed to analyze the hydrologic alterations of flow regime in downstream of reservoirs. Wavelet transform is a flexible approach since it can illustrate the time and frequency characteristics simultaneously. The merits of wavelet transform include that it can decompose variability in temporal and spatial domains, and it can be applied to non-stationary time series data. The Feitsui reservoir located in northern Taiwan is selected to demonstrate the proposed methodology in this study. The reservoir inflow is considered as the unregulated flow, while the outflow from the reservoir in the same period is recognized as the altered flow which is influenced by reservoir operation. Differences between wavelet power spectrums for the natural and altered flow series denote the anthropogenic effects on natural flow regime.
Daily and hourly flow series at two sites, a site located immediately downstream of Feitsui reservoir and another site located at the reach of Hsintien Creek (confluence of the Peishi Creek and Nanshi Creek), are analyzed in this study. The results show that spectrum of both temporal-scale data have obvious shift when the period is less than 1 year. An index called the flow regime spectrum is proposed in this study to quantify effects of reservoir operation in time-frequency domains. This index is defined as the mean value of spectrum in time and frequency domains. The results demonstrate that effects of reservoir operation at both sites are 7.4% and 3.5%, respectively, for daily flow series. Improvement of altered spectrum is around 45.5%, which is attributed to unregulated flow from the Nanshi Creek. Hourly flow series have similar results. Improvement of 41.2% is observed. This index can be used to guide reservoir operation and improve altered flow regime.

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