水文極端事件發生頻率及強度增強會加劇自然環境和社會經濟的負面影響，乾旱亦屬極端事件之一，由於其具有重複發生與因地而異之特性，故需探討台灣地區乾旱變化情況，以增加水資源管理之參考依據。本文應用Mann-Kendall檢定分析乾旱趨勢，其為廣泛利用的無參數檢定方法，並利用小波分析(wavelet analysis) 探討乾旱時頻變化，小波函數經由壓縮或擴張的特性得以分析非定常性(non-stationary)資料，因此常用於水文資料分析。本文研究資料採用台灣北、中、南及東部地區各一測站之月雨量資料，測站分別為台北、日月潭、高雄及大武站，並將月雨量轉換為1個月時間尺度的標準化降雨指數(standardized precipitation index, SPI)以定義乾旱，雨量及SPI資料亦以個別月份時序列應用於趨勢與時頻分析。乾旱趨勢研究結果顯示台北、日月潭及高雄站的雨量與SPI資料為增加趨勢，但不具有統計顯著性，大武站則為顯著減少趨勢；乾旱時頻分析結果顯示4個測站連續月雨量小波頻譜皆有明顯一年週期，在大於一年週期的部份，台北站的主要週期沒有明顯趨勢變化，日月潭和大武站的主要週期為增加趨勢，高雄站則為減少趨勢；連續SPI的分析結果顯示台北和日月潭站的主要週期為增加趨勢，高雄和大武站則沒有明顯趨勢變化。綜合趨勢檢定和時頻分析結果，台北、日月潭和高雄站趨於濕潤，大武站的乾旱情況則越趨嚴重。

Drought is a recurring climatic phenomenon and has various characteristics in regions. Investigating changes of droughts in Taiwan is necessary in water resources management. This study aims to detecting time-frequency changes of droughts using Mann-Kendall test and the wavelet analysis. Mann-Kendall test is a widely used non-parametric test for detecting trends. The wavelet analysis has capability for demonstrating the correlation between time and frequency domain of non-stationary signal through the extension and compression of the wavelet. Monthly rainfall series of four stations from north, central, south, and east regions in Taiwan are selected to explore time-frequency changes of droughts. The standardized precipitation index (SPI) is used to defined droughts in this study. The results show that north, central and south stations have an increase in rainfall but the eastern station has the opposite result. The wavelet power spectrum of monthly rainfall at the north station shows no evident variation in the longer-than-1-year period. The longer-than-1-year period has an upward trend in rainfall at the central and eastern stations, but it reveals decreasing trend at the south station. The main periods of SPI at the north and central stations reveal increasing trend, but show no evident variation at the south and eastern stations. The results suggest that the north, central and south stations lead to lessened droughts, while the eastern station leads to more severe drought.

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