台灣地區位於亞熱帶氣候，年雨量雖多達2500公釐，但由於河流短促，降水在集水區停留時間有限，造成可利用的水量相對偏少。為了提供生活與經濟所需，需依賴興建水庫來儲存水量與防洪、發電等功用。翡翠水庫位於北勢溪下游，自民國76年建壩至今蓄水運轉已二十餘年，主要供應大臺北地區近四百萬民生用水，為臺北地區重要水源。
本研究收集基礎水文氣象資料，利用GWLF模式河川模組進行翡翠水庫入流量之模擬。分析結果發現，雖然模式在年與月流量模擬結果相當吻合，但在日的時間尺度上有相當大的誤差。主要原因為蒸發散呈現高估，且地表逕流並無傳輸時間概念，因此本研究修改模式架構，在蒸發散計算中加入土壤阻抗與蒸發散門檻等概念，在地表逕流計算上利用馬斯金更法，使修改模式後能應用於日入流量模擬。
結果顯示，日流量相關係數平方檢定結果從原本的0.744增加到0.871，效率係數從原本的0.724增加到0.834，方均根差從0.902cm進步到0.718cm。而在事件的模擬中修正了洪峰發生時間過早且洪峰流量高估的問題，也修正了降雨停止後退水過快的問題，因此改良後的模式能符合翡翠水庫集水區的日逕流特性。

Although Taiwan has subtropical climate with annual rainfall of more than 2500 mm, limited water resource is still an issue due to short and steep rivers causing relative short residence time of surface runoffs. Reservoirs were often built to fulfill requirements of water resources and provide flood mitigation and power generation. The Feitsui Reservoir has been operated since 1987 and is the most important water resources for the 4 million people lived in the Taipei metropolitan.
The original GWLF model was first applied to simulate the inflow of the Feitsui Reservoir with observed hydrological and meteorological data. Although monthly and annual inflows can be perfectly simulated, errors in daily inflows are significant. The main reasons are overestimate of evapotranspiration and lacking of temporal transmission in surface runoff. Computations of evapotranspiration are modified with effects of landuse, soil moisture resistance, and a threshold to hinder evapotranspiration during small rainfall days. Temporal transmission of surface runoff is modified with the ability of temporal routing by incorporating the concept of the Maskingum method.
The new model has a better capability to simulate daily inflows of the Feitsui reservoir than the original GWLF model. By examining the observed and predicted daily inflows with both models, the correlation coefficient is 0.871 (original is 0.744), the coefficient of efficiency is 0.843 (original is 0.724), the root mean square error is 0.718 cm (original is 0.902 cm). For extreme rainfall events, the new model overcomes the problems of early peak flows, overestimated peak, and steep recession as commonly appeared in the original model. The new GWLF model now can be more appropriate to simulate daily, monthly, and annual inflows of the Feitsui Reservoir.

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