第一次调查结果展示了全球地表岩层透水性的分布，从图中可以看出，总体上来说透水性能较高的区域占了较大比例，制作这张图的是加拿大范库弗的不列颠哥伦比亚大学水文地质学博士后托姆格里森。

这张图的意义在于：过去的气候预测模型中并没有考虑地下水的运动影响，而这张图展示了占全球99%的不冻淡水的循环区域，而这些地下淡水的存在对当地的气候影响是不可忽视的。它将帮助人们更好地处理气候预测和理解自然淡水的循环过程。

不过，我和一位读者有着同样的问题：

“If "we don't know how much water is hiding underground", how do we know it "makes up 99% of all fresh unfrozen water"? Just asking...”

First water map of Earth's leaky surface

16:25 11 February 2011

Michael Marshall, environment reporter

(Image: Tom Gleeson/Geophysical Research Letters)

The map above (click to enlarge) is the first-ever global survey of Earth's permeability: essentially, how leaky it is. It shows how easily water passes through surface rocks, which will help us understand the planet's water cycle and predict the sustainability of underground water sources.

Crucially, it could help reveal the hidden underground movements of 99 per cent of unfrozen fresh water - water which is not taken into account in computer models used to predict the climate.

The map was put together by Tom Gleeson of the University of British Columbia in Vancouver, Canada, and his colleagues. They assembled data on the kinds of rocks found in different regions, and, using information on how permeable each type of rock is, calculated how leaky different regions are.

Permeability varies over 13 orders of magnitude, so the figures are not very precise, but they offer a rough picture. Gleeson says the map should help hydrologists to work out how much groundwater moves from one basin or aquifer to another, which is important if the water is to be managed sustainably.

At the moment we don't know how much water is hiding underground, or where it is. As a result, groundwater gets left out of climate models. Gleeson says that is a significant omission, as the movements of groundwater could well affect regional climate:

“Groundwater makes up 99 per cent of the fresh unfrozen water on Earth. That huge store could somehow modulate the climate. There may be really complex interactions that we don't appreciate.”

The map is a "very good first step", says Richard Taylor of University College London. However he says the figures for North America are the most reliable, because that area has been heavily studied, whereas those for the rest of the world are more scanty.

Journal reference: Geophysical Research Letters, DOI: 10.1029/2010GL045565

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