結合季長期天氣預報與標準化降雨指標建立有勝溪斷流預警系統

Abstract

近年來研究人員發現域外放流至羅葉尾溪的櫻花鉤吻鮭有向其下游的有勝溪擴張之趨勢，卻遭到有勝溪斷流事件的阻礙，羅葉尾溪成功的域外放流，卻可能因有勝溪頻繁地斷流事件而直接衝擊臺灣國寶魚棲地生態的完整性。因此，本研究欲藉由航遙測影像、雨量與流量資料之比對來探討有勝溪斷流發生機制，並結合季長期天氣預報與標準化降雨指標建立有勝溪斷流預警系統。 結果顯示，有勝溪自2012年之後，河道受颱風影響從直流而成曲流的情況下，單月標準化降雨指標(SPI-1)小於0或日流量低於0.531CMS，即可能會發生斷流。因此，本研究採用中央氣象局提供季長期天氣展望資料，並配合修正後有勝溪每月雨量屬於偏高(Above)、正常(Normal)、偏低(Below)之機率分類區間的界定門檻值，依月分別輸入台北、台中與花蓮測站的季預報結果，以氣象合成模式繁衍有勝溪月雨量資料。本研究採歷史季預報預測下一個月的結果，以SPI-1小於0做為斷流預警指標，應用於推估2012年8月之後斷流事件，其準確率達57%。本研究另外發現，以IHA計算2012年前後兩時期的各項流量指標，結果並無明顯差異，顯示位在斷流處下游流量站的流量變化無法真實反映上游頻繁斷流的現象，顯示斷流河段現地觀測之必要性。

Recently, the success in the reintroduction to the Louyewei Creek inspired researchers who have been working on the salmon conservation. The offspring of the reintrodueced the Formosan landlocked salmons (Oncorhynchus masou formosanus) have a tendency to expand their habitat to the Yousheng Creek, the downstream of the Loyewei Creek. However, the habitat expansion was limited by the river fragmentation in the Yousheng Creek. In order to understand the occasions of the river fragmentation, we have collected and analyzed the historical satellite image, rainfall and streamflow data, also combined seasonal forecast with Standardized Precipitation Index to establish river fragmentation warning system of Yousheng Creek. It is found that after Aug., 2012, the flow direction was changed from the straight to the meandering, and the flow was braided. In this condition, i.e. meandering and braided, river fragmentation occurred while SPI-1<0 or streamflow less than 0.531CMS. The study therefore constructed the river fragmentation warning system according to the forecast of seasonnal long-term weather outlook of Central Weather Bureau (CWB). The thresholds of three outlook classes, i.e. A(Above), N(Normal) and B(Below), of Yousheng Creek’s monthly rainfall were modified firstly, ensuring the class of monthly Yousheng’s rainfall to match the class of the coresponding CWB weather station (one among Taipei, Taichung and Hualien station). Afterwards, the daily rainfall could be generated by Weather Generator (WGEN) according to the CWB’ outlook forecast and the rainfall statisitics for the outlook class of Yousheng’s rainfall. After generating the rainfall, the study utilized SPI-1<0 as the river fragmentation warning indicator to assessment fragmentation events after October, 2012. The accuracy of predicting fragmentation is 57%. Besides, IHA was used to assess the flow changes between 1996-2012 and 2013-2016. It is found that three was no statistically significant changes among the two peridos. It indicats that the frequency occurance of fragmention in the upstream could not be reflected by the streamflow data measured in the downstream watershed outlet. In-situ measurement in the fragmented reaches is therefore strongly suggested.