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:
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)
