蒸發散量(Evapotranspiration，簡稱ET)是水文循環基本要素之一，在水資源評估與管理上是相當重要的因子。在國際上，常使用Penman-Monteith法來推估蒸發散量，然而此法會需要用到溫度、輻射量、相對濕度及風速等多種氣象參數，而氣象氣象站的建置與維護需要經費的維持，本研究利用有限或單一的氣象參數推估蒸發散量。本研究主要目的有:(1)利用Penman-Monteith法推估蒸發散量，比較在不同氣象參數缺漏的情況下，使用FAO公布的經驗式替代；(2)選用以輻射量與溫度為主的方法，與Penman-Monteith法做比較，進行推估蒸發散量；(3)修正各估算式之經驗係數，討論在研究地區之適用性。

本研究以台南氣象站與台北氣象站為研究區域，選用Penman-Monteith法與6種以輻射量為主之估算式，有Makkink(1957)、Turc(1961)、Jensen-Haise(1963)、Priestley-Taylor(1972)、Doorenbos-Pruit(1977)及Abtew(1996)等，及4種以溫度為主之估算式，分別為Thomthwaite(1948)、Blaney-Criddle(1959)、Hamon(1961)及 Linacre(1977)等方法來推估蒸發散量並比較其差異性。

本研究結果顯示，當風速資料缺漏或是不足時，對於使用Penman-Monteith法推估蒸發散量影響不大。在經驗係數修正前，輻射量估算法以Turc(1961)表現最好，RMSE在台南氣象站與台北氣象站分別為0.23與0.27，溫度估算法以Linacre(1977)法表現最好，RMSE在台南氣象站與台北氣象站分別為0.63與1.12。整體而言，輻射量估算法皆比溫度估算法評估更準確。最後，本研究進行經驗係數修正後，輻射量法以Makkink(1957)、Turc(1961)與Doorenbos-Pruit(1977)法表現較佳。溫度法則以Thomthwaite(1948)法表現較好。因此本研究建議日後推估蒸發散量，可使用Makkink(1957)、Turc(1961)或Thomthwaite(1948)法進行評估。

Evapotranspiration (ET) is one of the basic elements of the hydrological cycle. The Penman-Monteith method is the globally widespread to estimate evapotranspiration method. However, this method needs to use a variety of meteorological parameters, such as temperature, radiation, relative humidity and wind speed, etc. Besides, to build meteorological weather station and maintenance need funding. Therefore, the study is to use limited or single meteorological parameters to estimate evapotranspiration. The main purpose of this study are : (1) using FAO Penman-Monteith replacing experience function to estimate evapotranspiration when the meteorological parameters miss; (2) Using 6 radiation-based and 4 temperature-based approaches to estimate evapotranspiration and then compare the results with the Penman-Monteith method; (3) to modify the empirical factor for each approach and discuss the applicability of the study region.

The results of this study showed that when the wind speed data missed or lack, using the Penman-Monteith method to estimate evapotranspiration the results just have a little effect. Before the experience factor correction, the best performance of the radiation-based method is Turc(1961). the best performance of the temperature-based method is Linacre (1977). On the whole, the radiation-based methods are more accurate assessment than the temperature-based methods. Finally, this study was corrected the experience factor, and the better performance of the radiation-based methods are Makkink(1957), Turc(1961) and Doorenbos-Pruit(1977) respesticely. The better performance of the Temperature-based methods Thomthwaite(1948) method performed better. Therefore, this study suggests Makkink (1957), Turc (1961) and Thomthwaite (1948) method should be the better methods for estimating evapotranspiration.

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