翡翠水庫為大臺北地區主要的用水來源，其水質的優劣備受各界重視，有關單位設有水質採樣站以監測水質變化。趨勢檢定方法可用來檢測水質的長期變化趨勢，本研究利用無母數檢定法Mann-Kendall trend test中針對季節性資料的延伸法—The seasonal Kendall test，分析翡翠水庫上游集水區五個水質測站近十年(1993至2002年) 的水質變化趨勢，包含懸浮固體及氨氮兩個項目。另外也使用時間序列變化圖及盒型圖分析，以評估水質變化趨勢及空間差異，並探討可能影響水質變化的原因。 水質時間序列圖分析顯示，多數測站在近幾年的懸浮固體及氨氮含量有升高的趨勢，且出現較多的極端值。不同測站盒型圖分析的結果指出，黃櫸皮寮站及坪林站的水質較差，這和人為干擾的程度有關；季節變化部分，氨氮含量呈現與施肥季節相同的高峰值。The seasonal Kendall test的趨勢分析檢定顯示，在這十年中除了黃櫸皮寮的懸浮固體外，其餘水質測站的懸浮固體及氨氮含量均有顯著增加的趨勢 (α=0.05)。比較北宜高速公路興建前後兩段期間的差異顯示，各測站水質的年平均值均成長2至3倍，顯示道路興建對河川水質有一定的影響。雖然懸浮固體及氨氮的含量仍在水質安全標準範圍內，但趨勢分析顯示在過去十年來的含量已有顯著增加的趨勢，這值得有關單位注意。

The Feitsui Reservoir is the main source of domestic water for the Taipei region; therefore, its water quality is of great concern for the public and government authorities. Several water sample stations have been established in the Feitsui Reservoir watershed to monitor the long-term changes of water quality. The seasonal Kendall test, an extension method of the Mann-Kendall trend test, is used in this study to assess the water quality trends during the years 1993-2002 by analyzing seasonal data from five water sample stations located in the upper watershed of the Feitsui Reservoir. Water quality indices used in this research are suspended solids (SS) and ammonia nitrogen (NH3-N). In addition, the time-series diagram and box-and-whisker plot are also conducted to assess the spatial differences of the water quality trend among the five stations and the possible causes for the changes of their water qualities. The results of the time-series diagrams indicate that, in recent years, the water contents of SS and NH3-N in most stations have been increasing, and extreme values have been appearing more frequently. The box-and-whisker plot indicates that the water quality is getting worse in the Huang-Ju-Pi-Liao Station and the Ping-Lin Station, which is closely related to human activities in the watershed. Also, the high peak values of the NH3-N concentration coincided with the seasons of fertilizer application. The results of the seasonal Kendall test show that, except for the SS at the Huang-Ju-Pi-Liao Station, the concentration of SS and NH3-N have increased significantly at all five stations (α=0.05). Compared with the period before the construction of Taipei-Ilan Freeway, the annual average values of SS and NH3-N at the five stations have increased by 2-3 times after the construction. This supports the fact that road construction has significant influence on water quality. Although the concentration of SS and NH3-N in the upper watershed of the Feitsui Reservoir is still within safety level, it is worth noting that trend analysis points to increasing decline of water quality in the past decade.