Tree
rings and streamflow
One
might think that trees growing right along a river would act as
natural stream gages, providing the best information about streamflow.
But in fact the growth of these riparian trees is usually insensitive
to variation in streamflow, since soil
there is usually saturated, even during drought.
The
trees that do provide good proxy streamflow data are typically
found on slopes well above the river channel.
The relationship between streamflow and the growth of these trees
is indirect yet strong. The
same climate factors, primarily precipitation and evapotranspiration
(loss of moisture from plants and soils), control both the growth
of moisture-limited trees and the amount of water that reaches
the stream. One can think of the tree as a "dipstick"
recording the overall water balance in a river basin--which is
eventually expressed as streamflow.
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The soil moisture around
an individual tree reflects the overall water balance of
a river basin (precipitation minus evapotranspiration) and
thus the amount of streamflow produced by the basin.
(Image courtesy of David Meko, LTRR) |
The
trees that provide the very best information about streamflow
variability in the Colorado River basin—those particularly
sensitive to fluctuations in moisture—are species
such as ponderosa pine, pinyon pine, and Douglas-fir, growing
on dry and rocky sites where soil moisture storage is minimal.
Trees growing in these sites are also less likely to be
subject to non-climate disturbances, such as fires and insect
infestation, and the effects of competition from nearby
trees. In addition, the oldest individuals of these species
tend to be found on these sites.
Trees
don't necessarily have to be located within the stream basin
in question. Because the atmospheric flows of moisture that
affect both tree growth and streamflow do cross drainage
divides, sometimes trees located a great distance away can
be helpful in reconstructing streamflow at a particular
gage.
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A
paleoclimatologist sampling an old ponderosa pine in central
Colorado. Note that the tree is growing directly out of
the granite bedrock.
(Photo
by Connie Woodhouse) |
Generating
a streamflow reconstruction
The extension,
or reconstruction, of a streamflow record using tree rings begins
with the collection and development of tree-ring chronologies.
A chronology is a series of annual values derived from the ring-width
measurements of 10 or more trees of the same species at a single
site. To create a tree-ring chronology, cores from the sampled
trees at each site are crossdated (that is, patterns of narrow
and wide rings are matched from tree to tree) to account for missing
or false rings, so that every annual ring is absolutely dated
to the correct year. Then all rings are measured using a computer-assisted
measuring device. After growth-related trends (that
is, unrelated to climate)
are statistically removed, the ring-width values from all sampled
trees for each year are averaged to create a time series of annual
ring-width indices, called the tree-ring chronology.
Once a gaged
flow record (such as the Lees Ferry natural flow record) is selected
for reconstruction, a set of tree-ring chronologies from the region
around the gage is calibrated with the gage record to form a reconstruction
model. A statistical technique called multiple linear regression
is commonly used. The reconstruction is evaluated by comparing
the observed gage values with the reconstructed values and assessing
the amount of variance in the gage record that is explained by
the reconstruction. The reconstruction model is then validated
by either testing it on a portion of the gage data that was withheld
from the calibration process, or testing the ability of the chronologies
used in the model to estimate streamflow in different subsets
of the data. Once the reconstruction model is validated, the model
is applied to the tree-ring data for all available years to generate
the full reconstruction.
On
to Pioneering work in the Colorado River
basin (1940s)
