蒸發散為水文循環中重要因子之一，現今量測蒸發量最普遍的方法就是採用A型蒸發皿進行現地觀測。在台灣，中央氣象局所屬的氣象觀測站、水庫管理單位及農業試驗所皆是利用A型蒸發皿來監測各地的蒸發量。但中央氣象局的氣象觀測站將蒸發皿漆上白色，與世界氣象組織的建議不同，且台灣各單位對於蒸發皿表面是否塗上白漆並不一致，因而影響蒸發皿所接受的淨輻射量及蒸發量。本研究比較同時放在同一氣象觀測坪的兩個A型蒸發皿，一個未上漆之金屬蒸發皿與另一個白色蒸發皿的蒸發量。觀測的結果顯示:因為白色蒸發皿所吸收的輻射量低於金屬皿的輻射量，白色皿年蒸發量為1041 mm year-1低於金屬皿為 1392 mm year-1，亦即白色皿年蒸發量約為金屬皿年蒸發量的75%。
本研究並利用兩個蒸發皿的蒸發量差異研究淨輻射對皿蒸發量的影響，亦建立蒸發皿能量收支式來探討其能量平衡以及預測皿蒸發量。因為皿邊壁的淨輻射量難以量測，故本研究由水面上的淨輻射之量測值，乘上一個修正係數來計算蒸發皿邊壁所吸收的輻射量，使得蒸發皿能量收支得以閉合，金屬皿的修正係數為1.50，白色皿的修正係數為1.26，且可用能量收支式來預測蒸發量。本研究亦修正現有的蒸發量預測模式，修正後可更正確的預測蒸發量。

關鍵字：A型蒸發皿、蒸發量、淨輻射量、反照率

This study experimentally investigated the effect of the net radiation on the evaporation rate of the Class A evaporation pan. The experiment was conducted by comparing evaporation rates of two Class A evaporation pans, one in original galvanized metal color, another one in white color, placed in the same meteorological station. The experimental results revealed that, because the white pan absorbed less net radiation than that of the metal pan, the annual evaporation rate of white pan (1041 mm year-1) is smaller than the annual evaporation rate (1392 mm year-1) of the metal pan. The evaporation rate of the white pan is about 75% of the evaporation rate of the metal pan.
This study also used the difference of pan evaporation rates to examine the influence of net radiation on the evaporation rate. Furthermore, the energy budget of the evaporation pan and suggested a correction coefficient to include the net radiation Rn absorbed by the pan wall to ensure the energy closure. The correction coefficients were 1.50 for the metal pan and 1.26 for the white pan. In addition, the measured daily evaporation rates Epan were compared with several prediction models. A modified Penman-Monteith prediction model was proposed to incorporate the net radiation on the water surface and pan wall, and to give better accuracy for the pan evaporation rate.

Keywords: Class A evaporation pan, Evaporation rate, Net radiation, Penman-Monteith model, Albedo.

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