Pan Evaporation Trends and the Terrestrial Water Balance. II. Energy Balance and Interpretation
Pan Evaporation Trends and the Terrestrial Water Balance. II. Energy Balance and Interpretation
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DOI：
10.1111/j.1749-8198.2008.00214.x
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发表时间：
2009-03
影响因子：
3.1
通讯作者：
M. Roderick;M. Hobbins;G. Farquhar
中科院分区：
文献类型：
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作者：
M. Roderick;M. Hobbins;G. Farquhar
据报道，美国、前苏联、印度、中国、澳大利亚、新西兰和加拿大等地的蒸发皿蒸发量都有所下降。这一趋势很大-大约比基于模式的大气层顶辐射强迫估计值大一个数量级。蒸发皿蒸发量的趋势也有一个不同的迹象（即下降），从通常持有的概念。这是一组非常有趣的观察结果。在这个两部分系列的第一篇文章中，我们讨论了测量本身，然后总结了全球观测结果。在这篇文章中，第二篇文章，我们概述了使用能量平衡的方法来属性所观察到的蒸发皿蒸发量的变化，在基本的物理变量，即辐射，温度，蒸汽压力赤字和风速的变化。我们发现，蒸发皿蒸发量的下降主要归因于辐射（即变暗）和/或风速（即静止）的下降。然后，我们讨论了陆地水平衡的变化的解释。这是一个存在许多误解和混乱的领域，其中大部分可以通过使用熟悉和长期存在的供求框架加以纠正。关键是在使用蒸发皿蒸发数据，使陆地水平衡的变化推断是区分水和能量限制条件下，不同的解释适用。
Declines in pan evaporation have been reported across the USA, former Soviet Union, India, China, Australia, New Zealand and Canada, among other places. The trend is large – approximately an order of magnitude larger than model-based estimates of top of the atmosphere radiative forcing. The pan evaporation trend also has a different sign (i.e. decline) from commonly held conceptions. These are a remarkably interesting set of observations. In the first article of this two-part series, we discussed the measurements themselves and then presented summaries of the worldwide observations. In this, the second article, we outline the use of energy balance methods to attribute the observed changes in pan evaporation to changes in the underlying physical variables, namely, radiation, temperature, vapour pressure deficit and wind speed. We find that much of the decline in pan evaporation can be attributed to declines in radiation (i.e. dimming) and/or wind speed (i.e. stilling). We then discuss the interpretation of changes in the terrestrial water balance. This has been an area of much misunderstanding and confusion, most of which can be rectified through use of the familiar and longstanding supply/demand framework. The key in using the pan evaporation data to make inferences about changes in the terrestrial water balance is to distinguish between water- and energy-limited conditions where different interpretations apply.