Colorado Water Dec 2003

• Year: 2003
• Title: Colorado Water
• CWCB Section: Administration
• Author: Water Center of Colorado State University
• Description: December 2003 Issue
• Publications - Doc Type: Newsletter

become partners in the project, including the Northern Colorado Water Conservancy District (NCWCD), Denver Water,
and the U.S. Bureau of Reclamation, Aspinall Unit (Gunnison Basin).
The physical basis for the reconstructions
Tree growth is primarily controlled by climate conditions during the year prior to and including the growing season. At
lower elevations in Colorado, variations in tree growth closely reflect the amount of soil moisture at the onset of the grow-
ing season, which is controlled by variations in precipitation, and, to a lesser degree, temperature, humidity, and evapora-
tion. Since annual streamflow integrates both precipitation and temperature over the course of the previous seasons (simi-
lar to tree rings), it is closely correlated with annual tree growth. Trees that provide the best information about streamflow
variability —those particularly sensitive to variations in moisture — include species such as ponderosa pine, pinyon pine,
and Douglas -fir, growing in open stands on dry and rocky sites where soil moisture storage is minimal (Figure 2). Trees
growing in these sites are also less likely to be subject to non - climatic disturbances, such as fires and insect infestation, and
the effects of competition from nearby trees. In addition, the oldest individuals (up to 900 years old) of these species tend
to be found on these sites.
Developing the reconstructions
Tree -ring reconstructions of streamflow are developed from multiple
tree -ring chronologies. A tree -ring chronology is a time - 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 (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 cor-
rect year. Then all rings are measured to the nearest 0.001mm using a
computer- assisted measuring device. After age - related trends in growth
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. The complete series of ring -width indices from a site is called a
tree -ring chronology.
Once a gaged natural flow record of interest is selected for reconstruc-
tion, a set of chronologies from the region near 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 testing it on a portion of the gage data that
was withheld from the calibration process.
What the reconstructions show
So far, we have developed
reconstructions, extending back
to the 1400s to 1600s, of the 15
stream gages that comprise the
water supply systems for Denver
Water, NCWCD, and the city of
Boulder (Figure 1). Our tree -
ring reconstructions match the
variability in the gaged records
of streamflow very well (Figure
3). Statistically speaking, 60%
to 75 % of the variance in the
gaged records is explained by the
0 400000
w
d
u 300000
0
E 200000
E
L
V) 100000
3
C
0
a
Figure 2. Sampling a 400 - year -old ponderosa pine
near Deckers, Colorado
1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
Figure 3. Blue River at Dillon Reservoir undepleted gaged annual flow,1916 -2002 (light) and
tree -ring reconstructed flow (dark). Note the close fit of the two time series (M = 0.63), especially
for the most extreme low -flow events (1954, 2002).
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