Subsequent
Efforts (1980s and 1990s)
In the 1980s
and 1990s, three studies, described below, built on the work of
Stockton and Jacoby. The first two studies entailed new tree-ring
reconstructions of streamflow at Lees Ferry. In the third, the
Stockton and Jacoby Lees Ferry reconstruction was used to construct
drought scenarios for a major multidisciplinary assessment of
drought impacts in the Southwest.
In the late 1980s, researchers at the University
of California headed by Joel Michaelsen developed a set of tree-ring
based reconstructions of streamflow for the California Department
of Water Resources. Among the reconstructions was one for the
Colorado River at Lees Ferry. This study shared some of the same
tree-ring data as used by Stockton and Jacoby, but also included
tree-ring chronologies from a broader geographic region. As in
Stockton and Jacoby’s study, the common end date of the
tree-ring data was in the early 1960s. The reconstruction, extending
from 1568-1962, had a long-term mean of 13.8 MAF and shared many
of the features of the Stockton and Jacoby reconstruction, including
the early 20th century period of high flows and the sustained
drought in the late 16th century. Two subsequent papers also describe
and discuss the Michelsen et al. reconstruction: Loaiciga
et al. (1992) and Tarboton
(1994).
Hugo
Hidalgo, Thomas Piechota, and John Dracup of University of Nevada-Las
Vegas developed a tree-ring reconstruction of annual flow at Lees
Ferry in which they used virtually the same tree-ring dataset
as Stockton and Jacoby, but a different principal components analysis
(PCA) modeling technique to calibrate those data with the natural
flow record. The resulting reconstruction had a lower long-term
mean (1493-1962, 13.1 MAF) than Stockton and Jacoby, and the low-flow
periods were even lower than the difference in means would suggest.
For example, the lowest 20-year running mean during the late 1500s
drought was only 9 MAF in the Hidalgo et al. reconstruction, compared
to 11 MAF in Stockton and Jacoby. The Hidalgo et al. reconstruction
demonstrated the sensitivity of reconstructions to choices made
in the modeling process, a subject treated in more detail later.
The
Severe Sustained Drought Study (late 1980s and early 1990's)
In
the late 1980s, a major interdisciplinary study began on the impacts
of a severe, sustained drought in the Southwest. Two drought scenarios
derived from Stockton and Jacoby's Lees Ferry reconstruction were
used as the basis for assessing social, political, legal, and economic
impacts of severe, sustained drought. One drought scenario was Stockton
and Jacoby's reconstructed flows for 1579-1600, and the other was
a re-ordering of those 22 years so that flows progressively declined
over the period of the drought.
The Severe
Sustained Drought Study was novel in that a drought reconstructed
from tree-rings was used to represent a future drought scenario.
It attempted to answer the question, what would happen if the
late 15th century drought seen in the tree-ring record (or a variation
of it) were to recur today? Unfortunately, the study was largely
ignored by the water resource management community, perhaps in
part because water supplies in the Southwest had rebounded after
a drought from 1988-1992, and the year the study was published
(1995) saw well above average flows on the Colorado.
On
to...The latest reconstructions (2006-2007)
