摘要 翡翠水庫自民國76年建壩至今蓄水運轉已十九餘年，主要供應大台北地區近四百萬民生用水穩定不虞匱乏。而近年來（2002、2003年）水庫入流量減少，水庫在蓄水量不足的狀況下水位急劇下降，影響民生生活用水供輸。另外雪山隧道於進行開挖工程時遭遇大量湧水問題，除了帶來工程施工上的嚴峻挑戰，更由於近一、二年來的乾旱問題，使一般大眾將此湧水問題與翡翠水庫進帳減少作進一步的聯想。本研究利用基礎的水文氣象資料收集，分析翡翠水庫集水區歷年水文上之變異，探討近年來水庫缺水之主要因素，並嘗試分析雪山隧道開鑿對水庫入流量的影響。 水文分析結果顯示，水庫建壩後逕流比值呈現下降趨勢，造成此現象主要原因為2002、2003年持續性的乾旱所影響，另外隨著溫度的增加，集水區皿蒸發或蒸發散量皆有增加之現象，顯示可利用的水資源逐漸減少。而造成2002、2003年乾旱之主要成因是由於枯水期降水強度明顯偏低，偏低的降雨量導致大部分的降水入滲至土壤或蒸發至大氣中，使得地表逕流不易產生，導致水庫入流量明顯減小。從無降雨記憶河川流量顯示，雪山隧道開鑿對翡翠水庫集水區低流量並沒有顯著的影響，但對其子流域-?魚堀溪流域則有產生低流量減少之可能。 Abstract The Feitsui reservoir has been operated for 19 years since 1987 and supplies municipal water for 4 million people in Taipei areas. The reservoir’s inflow was found to be decreased in recent years especially in 2002 and 2003, and it caused the serious drop of water level affecting the water supply. A speculation was developed to relate the recent water shortage to the construction of the Snow Mountain tunnel. In this study, we collect hydrological and meteorological data for statistical analyses and try to find the main reason causing the inflow reduce. We also attempt to evaluate the effect of the excavation of the Snow Mountain tunnel on the Feitsui reservoir inflow. The results show that the persistent decrease of the runoff ratio was directly linked to the dry condition of 2002 and 2003. With the continuously raising temperature, the pan evaporation and estimated evapotranspiration were found with an increasing trend. This indicates the difficulties on water resource manage management will be increased. The lower winter rainfall intensity was the reason of the dry condition in 2002 and 2003. Lower rainfall intensity will hinder the generation of surface runoff due to higher infiltration and evapotranspiration rates and reduced the reservoir inflow. The analyses of base flow without rainfall memory show that the construction of the Snow Mountain tunnel has no significant impact on the base flow in the Feitsui reservoir basin. But it might affect the base flow in one of the sub-basins, the Tai-Yu-Ku basin.
