利用風洞實驗研究蒸發皿蒸發率與風速之關係

本研究利用風洞實驗來研究風速對A型蒸發皿蒸發速率的影響。實驗結果發現在風速U > 5 m s-1的狀況下，蒸發皿內的水會產生蕩漾波，波高會高於皿高，最後導致水濺出蒸發皿，濺出水量遠大於蒸發量。這表示在無人觀測的狀況下，濺出水量會誤認為蒸發量，造成高估蒸發速率的現象。且在相同的風速下，風所驅動之盪漾波及濺出水量會因為不同水位計而有所不同，水位計在蒸發皿外的濺出水量比水位計在蒸發皿內的濺出水量大。比對現地觀測的四年(2004-2008)資料發現，濺出水量會造成年蒸發率4.31 %的誤差。除此之外，研究結果亦發現蒸發皿的初始水深會影響蒸發速率，當風速U = 4 m s-1時，初始水深在8公分以上時，蒸發速率並未有明顯減少，但當初始水深為6公分時，則蒸發速率較標準水深20公分的蒸發速率減少26%。但當風速愈小，皿壁的遮蔽效應就愈不明顯。風速U = 2 m s-1，初始水深從20公分變為6公分時，蒸發速率幾乎相同。本研究也發現在相同風速U = 4 m s-1、淨輻射量Rn = 200 W m-2的狀況下，蒸發皿底下有無一木製棧板，蒸發皿的蒸發速率皆相同，亦即棧板不會影響蒸發皿的蒸發速率。

關鍵字

風速；蒸發皿；盪漾波；風洞實驗

並列摘要

This study used wind tunnel experiments to investigate the wind effect on the evaporation rates of Class A evaporimeter pan. The experimental results revealed that high wind speed (U > 5 m s-1) could generate seiche wave inside the pan. The wave height was higher than the rim of the pan, and water was spill out of the pan. Namely, the water loss from the pan is not entirely due to evaporation and the evaporation rates could be over-estimated at high wind speeds. By checking the wind speed and evaporation rates of a four-year (2004-2008) field observation collected in Taiwan, it was found the ratio of yearly water loss due to the spill to the average evaporation rate is 4.31 %. In addition, this study examined the influence of initial water depth in the pan on the evaporation rate. The results demonstrated that the evaporation rate decreased as the water depth decreased when the water depth is less than a critical depth. The evaporation rate of water depth 6 cm is about 26% that of standard depth (20 cm) when wind speed U = 4 m s-1. It was also found that with or without a wooden platform underneath the pan, the evaporation rates are the same.

並列關鍵字

Evaporation pan ； seiche wave ； wind tunnel experiment

參考文獻

Allen, R. G., Pereira, L. S., Raes, D., and Smith, M. (1998). Crop evapotranspiration: Guidelines for computing crop water requirements, FAO Irrigation and drainage paper, Paper 56. Food and Agriculture Organization of the United Nations, Rome, Italy.

Brutsaert, W., and Parlange, M. B. (1998). “Hydrologic cycle explains the evaporation paradox.” Nature, 396, 30.

Chattopadhyay, N., and Hulme, M. (1997). “Evaporation and potential evapotranspira- tion in India under conditions of recent and future climate change.” Agricultural and Forest Meteorology, 87, 55-73.

Chu, C. R., Li, M. H., Chen, Y. Y., and Kuo, Y. H. (2010). “A wind tunnel experiment on the evaporation rate of Class A evaporation pan.” Journal of Hydrology, 381(3-4), 221-224.

Dawson, T. E., Burgess, S. S. O., Tu, K. P., Oliveira, R. S., Santiago, L. S., Fisher, J. B., Simonin, K. A., Ambrose, A. R. (2007). “Night-time transpiration in woody plants from contrasting ecosystems”, Tree Physiology, 27, 561-575.

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利用風洞實驗研究蒸發皿蒸發率與風速之關係

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