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Combined seasonal forecast with Standardized Precipitation Index to construct river fragmentation warning system of Yousheng Creek
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.
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