本研究在控制的環境下，研究風速、淨輻射量、蒸汽壓差等環境參數對蒸發皿及裸土之蒸發率、短草(百喜草和地毯草)蒸發散率的影響，實驗結果顯示：在相同的環境狀況下，百喜草的蒸發散率最大，地毯草次之，蒸發皿的蒸發率最小。四者皆會隨著風速、淨輻射量及蒸汽壓差的增加而增加。且短草在夜晚和白天皆有蒸散的現象，在相同風速下，百喜草的夜晚蒸散率為白天蒸散率的54~67%，地毯草夜晚蒸散率為白天蒸散率的60~67%。本研究亦發現蒸發皿的初始水深會影響蒸發皿的蒸發率，當風速U = 6 m/s，初始水深從15公分變為11公分時，皿蒸發率並未有明顯地減少，但初始水深變為7公分時，則皿蒸發率較水深15公分減少55%。此外，本研究依據量測得之皿蒸發量，求得一個新的風函數，結合此風函數與Thom et al. (1981)的模式可用來預測蒸發皿的蒸發率，模式預測值和本研究的觀測值十分接近。 本研究並利用FAO-56 PM公式及Penman-Monteith公式預測裸土蒸發率、短草的蒸發散率。結果顯示FAO-56 PM公式及Penman-Monteith公式皆可用於預測裸土蒸發率及短草的蒸發散率，裸土蒸發率的預測優於短草的蒸發散率之預測。 The influences of wind speed, net radiation and vapor pressure deficit (VPD) on the evaporation rates of Class A pan, bare soil and the evapotranspiration rates of grasses were experimentally investigated in this study. The results demonstrated that the evaporation (and evapotranspiration) rates increase as wind speed, net radiation and VPD increase. Under the same wind speed, the ratio between the night-time and daytime evapotranspiration rate was 54~67% for Paspalum notatum Flügge, the ratio was 60~67% for Axonopus compressus (Sw.) P. Beauv. It was also found that the initial water depth in the pan will affect the pan evaporation rates. Under the same wind speed U = 6 m/s, the evaporation rate of water depth change from 15 cm to 11 cm, the evaporation rate decreases about 0.8%; but when the water depth changes to 7 cm, the evaporation rate decreases 55%. Based on the measured pan evaporation rates, a new wind function is proposed. By comparing with the predictions of Thom et al. (1981), Pereira et al. (1995) and Rayner (2007), the model of Thom et al. (1981) integrate with the new wind function gives the best prediction. The evaporation rates of bare soil and the evapotranspiration rates of grasses were compared with the prediction of FAO-56 PM equation and Penman-Monteith equation. The comparison shows that the predictions of FAO-56 PM equation and Penman-Monteith equation for the evaporation rate of bare soil are better than for that for the evapotranspiration rates of grasses.
