| Pioneering
work in the Colorado River basin (1940s)
The modern science of dendrochronology (tree-ring research)
was developed by Andrew Ellicott Douglass (1867-1962), who spent
most of his career at the University of Arizona in Tucson. An
astronomer who became interested in the impact of solar cycles
on climate, Douglass realized that the rings of ponderosa pine
trees in Arizona recorded the variation in rainfall from one
year to the next. He used the patterns of wide and narrow rings,
consistent among trees across the region, to infer past climate
conditions and also to date wood from numerous archaeological
sites across the Southwest.
| 
Edmund Schulman using a razor blade to
prepare the surface of a tree core.
Photo
courtesy of LTRR, University of Arizona |
One of Douglass's foremost students was Edmund Schulman (1908-1958). Schulman
conducted extensive fieldwork across western North America
to find long-lived conifers whose rings contained the
most sensitive and faithful climate records. His most
famous discovery was of the incredible longevity of the
bristlecone pines (Pinus longaeva) in the Great
Basin, over 4000 years old. Back at the Douglass-initiated
Laboratory of Tree-Ring Research (LTRR) in Tucson, Schulman
made many advances in using these sensitive tree-ring
records to reconstruct past climate, and laid the foundation
for the science of dendroclimatology.
Schulman recognized that the moisture-sensitive trees
of the Southwest could be used as a proxy for annual streamflow
as well as precipitation. In 1942, he issued a report
for the Los Angeles Bureau of Power and Light entitled
"A
tree-ring history of runoff of the Colorado River, 1366-1941".
He expanded on this work for his Ph.D.
dissertation (1944), and a monograph, "Tree-Ring
Hydrology of the Colorado Basin", published by
the University of Arizona in 1945. |
Schulman's analyses were limited by the lack of computers and
less sophisticated statistical techniques at that time. He also
sampled many fewer trees at each site than is typically done
today. His "history of runoff" was not statistically
calibrated and validated against the gage-based streamflow record;
instead, he used a tree-ring growth index as a direct proxy
for streamflow. These limitations aside, Schulman's pioneering
work captured the important features of later streamflow reconstructions
of the Colorado River. He also began what has become a long,
if sporadic, collaboration between tree-ring scientists and
water managers to better understand the variability of the Colorado
River.
Schulman's
Colorado River basin Douglas-fir tree-ring index (1300-1945),
with annual values in green and 10-year running mean
in black. (From Schulman 1945)
|
The graph above shows the regional tree-ring index developed
by Schulman from several Douglas-fir sites in the Colorado River
basin. The long-term mean is set at 100, with values above 100
indicating wider rings (higher streamflow) and value below 100
indicating narrower rings (lower streamflow). The correlation
between Schulman's tree-ring index and the Lees Ferry natural
flow record from 1906-1945 is fairly high, suggesting that the
tree-ring index is indeed a good proxy for streamflow.
Several features of Schulman's tree-ring record are worth noting,
as they are seen in later Colorado River streamflow reconstructions.
First, the first two decades of 1900s appears to be the longest
period of sustained above-average flows since at least the early
1600s. Second, the late 1500s appears to be the most severe
and sustained period of below-average flows (that is, drought)
in the entire record. Overall, the 20th century (at least through
1945) does not capture the full range of variability--particularly
the low flows--seen in the previous six centuries.
On to...The first streamflow
reconstruction for Lees Ferry (1976)
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