蒸發皿為觀測蒸發量的儀器，然而蒸發皿的邊壁在吸收輻射能後會影響蒸發皿的觀測值，本文稱此現象為「邊壁效應」。台灣早期以20 公分蒸發皿觀測蒸發量，於1973 年起陸續加入A 型蒸發皿進行觀測，本研究採兩蒸發皿同時觀測的年份(1974 年至1997 年)，對台灣十個氣象測站進行分析。本研究針對月蒸發資料，應用PenPan V2 Model 及PenPan V3Model，以空氣動力學與能量平衡理論估計A 型與20 公分蒸發皿蒸發量。
由結果顯示，空氣動力學對皿觀測值的影響比例分別約為21%和25%，輻射影響的比例為79%與75%，差異不大，然而邊壁效應卻差距明顯，A 型皿為27%，20 公分皿為58%。本研究將兩蒸發皿的觀測值與邊壁效應進行回歸分析，估計「當邊壁效應為零時所發生的蒸發量」，也是就自由水面蒸發量。分析結果顯示，自由水面蒸發量約為A 型皿蒸發觀測值的0.71 倍，20 公分皿蒸發觀測值的0.52 倍。蒸發為水文循環中的重要過程，對於水文循環及水資源利用，本研究的自由水面蒸發量推估結果能供作參考。

In the early days in Taiwan, the 20 cm evaporation pan was used to observe the evaporation. From 1973, the Class A evaporation pan was additionally used for observation. Based on monthly evaporation data, this study applied PenPan V2 Model and PenPan V3 Model, which are based on aerodynamics and energy balance theories, to estimate the evaporation of Class A and 20 cm evaporation pans, respectively. The results showed that the proportions of the aerodynamic effect on observations of Class A and 20 cm evaporation pans are about 21% and 25%, respectively, and the proportions of radiation effect are 79% and 75%, respectively. Moreover, the side wall effect is 27% for Class A pan, and 58% for 20 cm pan. In addition, this study used the observations of the two evaporating pans and the estimates of the side wall effect to make a regression function to estimate the "pan evaporation without side wall effect" which is known as the free-water evaporation. The analysis results showed that the free-water evaporations are about 0.71 times the observations of the Class A pan evaporation, and 0.52 times the observations of the 20 cm pan evaporation.

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