How much water do trees keep in the sky?

Past studies have suggested that trees can intercept and hold around
two and a half millimeters of rainwater—even in a modest sized rain
event, in which 10mm of rain might fall, that is still one-quarter of
the rain. When rain is caught by leaves, it may not have a chance to
provide water to other plants, as it rapidly evaporates. This process
returns water to the atmosphere for another rain storm that may take
place the next day, miles and miles away. Water that reached the soil
below the tree is returned to the atmosphere over a period of several
days, and thus has a significantly different impact on the timing of
future rain events. This is important not just for weather forecasts,
as studies have shown that land-atmosphere interactions play a
significant role in future climate projections as well.

Current
weather and climate models include a rough estimation of canopy
interception, but they are rough because few good measurements of that
interception are available. A recent study in the journal Water
Resources Research aims to change that. No, they did not brush water
droplets off leaves one at a time; instead, they measured all of them
at once. While past studies have measured changes in canopy water by
weighing the entire plant over time, this is difficult when the plants
you are interested in are large trees. So Jan Friesen and colleagues
at Delft University of Technology figured they would use the tree
itself as the scale. A tree trunk is compressible, much like a giant,
if very stiff, spring. The more water you put into the canopy, the
more the tree gets compressed. So they measured that compression with
linear potentiometers capable of recording changes in distance of
0.04µm. The potentiometers were anchored to two points, one meter
apart, on a 0.15m diameter linden tree. They calculated that, based on
the expected elasticity of the tree, they should be able to detect
0.4kg of water (0.4 liters) added to the tree canopy, and they
calibrated the system by adding 5, 10, and 20kg weights to the tree.
Of course, when you are measuring micrometers of movement, the
slightest breeze, or even a change in temperature, could destroy your
measurement. To compensate for these problems, they minimized
temperature changes by heating the tree trunk but, every time the wind
picked up, the tree would bend in one direction or another. They
corrected this problem by placing three potentiometers at 120 degree
intervals around the tree and averaging. When all was said and done,
they had a system that could measure canopy water interception to
within five kilograms. They then measured canopy water content on
another tree during a 5.5mm rain storm; the tree collected over 200kg
of water! The foliage covered an area around 31m2, so 200kg
corresponds to 6.5mm of water… anyone else see a problem? The team
attributes this mystery excess water to a combination of errors in
their calibration and errors in estimating the actual area covered by
the tree. Interestingly, they were also able to record the loss of
this water after the rain stopped. Over a period of about five hours,
this weight all evaporated or dripped out of the canopy. Clearly, they
need to do a little more work to calibrate their system under larger
loads, but the technique is very promising. It’s possible that it
could be used to record diurnal changes in tree mass as the tree pulls
water up from the soil following a rain event, although it is unclear
how that excess water in the trunk will affect their
calibration—there’s lots of fun work to be done here.
ttp://www.agu.org/pubs/crossref/2008/2008WR007074.shtml

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