There are a couple sites with this claim but I can't find any reliable sources...
http://www.snapple.com/real-facts/cap-view/715 http://www.chacha.com/question/how-much-water-does-a-full-grown-oak-tree-expel-through-its-leaves-each-day
I was hoping someone could give a better explanation or confirmation of this claim... How many leaves does an oak tree have? What is the rate at which water is expelled? What is the definition of a "full grown oak tree"?
Summary: 7 tons seems extreme, but it is probably not more than one order of magnitude too large.
Trees lose water by transpiration through the pore-like stomata of their leaves. This water is primarily drawn up in the tree-sap from the roots.
I expect the transpiration figures vary a lot depending on local climate type, current weather (humidity etc), availability of water, exact tree size, tree variety, etc
a large oak tree can transpire 40,000 gallons of water per year
There are about 240 US Gallons of water in a ton. SO the above figure equates to an average of about 0.45 tons a day. However, trees in temperate climates are much more active in summer than in winter, so the peak daily value might be several times higher.
the full-grown oak (Quercus robur L.) tree ... [has] sap flow rate values ... of up to 400 Kg per day ... 100 years of age, 33 m height.
Sap Flow Rates and Transpiration Dynamics in the Full-Grown Oak ...
400 Kg is about 0.44 tons or exactly 0.4 tonnes.
