[ejohnchat (OP)]

Dear Naked Scientist,
In May 2007, Connie Godmanchester asked:
"Where does a tree's water come from?
If a large tree will drink 60 gallons of water per hour, where does all that water come from and where does it go?"
And you answered:
"60 gallons per hour is a lot of water.  But it’s estimated that a big tree, such as a 48 foot maple has on average 177,000 leaves, which adds up to a leaf area of 1/6 of an acre.  Each leaf has thousands of tiny pores called stomata on the underside which open up on a nice day and lose water.  They need to do this to pull water up through the roots into the stem and up to the leaves, in a process called transpiration.  All this water must come from ground water, water stored underground."
I am quite impressed with your show and enjoy listening to the old Podcasts every day on my way to and from work.  But the answer to the above question just doesn't seem to add up.  As far as I know, the ultimate source for fresh water is rain.  Even ground water comes ultimately from rain.  Is there a large underground storage of fresh water that didn't come from rain?  And if so, trees must be using it up at an alarming rate and we should be concerned about it drying up.  Here is my logic.  In a typical summer month in central Illinois, we receive about 4 inches of rain.  At the 60 gallons per hour, that is only enough for 2 trees per acre.  We have massive forests with a much higher concentration of trees than 2 per acre.  And those forests are still there even after this summer's drought with less than 10 inches of rain in three months.  You say that these stomata only open up on a nice day.  What about an average day?  Is this where have I gone wrong with my math?  The 60 gallons per hour must be an extreme case of a very large tree with many more leaves than what you would find on a typical forest tree.   Based on the 4" per month, the average forest tree only gets about 25 gallons of rain per day (assuming 100 trees per acre, 20 ft between each tree).  It is difficult to believe that over 90% of the water comes from ground water.  How was this value 60 gallons per hour value determined?  Please reconsider and let me know what you think.

Thanks for providing such a great show.  You should do a TV show.

John Chatfield
Illinois, USA

[CliffordK]

Whew...

So we should cut down all the trees to preserve the world's water supplies?  Of course, the trees actually help produce rain in the rain forests.

In my backyard, I have a year-around spring coming out of a moderate sized tree stump, with what I would estimate a flow of about 1 gallon per minute.  I have always found it odd that the spring comes directly out from under the stump, but I've wondered if the tree had used all the water when it was alive.

I'm seeing lots of notes about varying consumption of water by trees. 
http://answers.google.com/answers/threadview/id/720450.html
http://forums2.gardenweb.com/forums/load/trees/msg020925009699.html

The water amount for each type of tree varies.  The consumption also varies by the water availability.

A mature, large Doug fir can consume upto 800 gallons a day if available but will subsist on far less as necessary (Peter Rennie, RPF Consulting Forester and Arborist). And that is often the case -- many trees will 'get by' with far less water than optimal conditions would provide.

Perhaps one should think of one's average annual rainfall.

1 Acre-inch is 27152 gallons.  If we get about 50 inches of rain a year, that would be 1,358,000 gallons per acre.  For a mature forest, with say 100 trees per acre, one gets 13576 gallons per tree, per year, or about 37 gallons a day.  Hmmm, quite a bit short of the 800 gallons a day.  But, our rain varies significantly here too, with much more rain in the winter than the summer.

How is the 800 gallons a day calculated?
I can't imagine very many century old trees with 4-foot trunks are being grown in laboratory settings.  And, one may not be able to scale up from a small 5 yr old tree to a mature 100 year old tree.  Transpiration might give one an estimate, but could it be overestimating the actual water consumption?  Also, here in Oregon, the optimal transpiration would occur mid-summer when the water supply is shortest.  The winter water supply is more abundant, but the lower temperatures would also reduce the transpiration.

[damocles]

In semi-arid regions, trees and the water they use are a vital part of the system. If trees are removed, particularly in an irrigation area, the water table in the soil rises, and salt is brought to the surface, killing crops, and having a negative effect on soil drainage. Salt will concentrate in the soil region where the water table moves up and down, and where evaporation of the groundwater is taking place. It is important to keep this at least 2 metres below the surface, and trees can help achieve that. We are having an increasing problem with soil salinization associated with irrigation in South-Eastern and South-Western Australia , remarkably similar to problems faced by the Sumerian civilisation several millennia ago.