| 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 (see figure, below left). 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, University
of Arizona |
Climatologist Mark Losleben sampling
an old ponderosa pine in central Colorado. Note that the
tree is growing directly out of the granite bedrock.
Photo
by Connie Woodhouse |
|
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 (see photo, above right). 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. |
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.
For a more information on the reconstruction process,
see the resources on the Tree-Ring
Info page. |
On to Pioneering work in the
Colorado River basin (1940s)
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