9645

• UCREFRP Catalog Number: 9645
• Author: Woodhouse, C. A., S. T. Gray and D. M. Meko
• Title: Updated Streamflow Reconstructions for the Upper Colorado River Basin
• USFW Year: 2006
• USFW - Doc Type: Water Resources Research
• Copyright Material: YES

W05415

WOODHOUSE ET AL.: UPDATED COLORADO RIVER RECONSTRUCTIONS

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2.2
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1500 1550 1600

1700 1750 1800
Ending Year

Figure 10. Twenty-year running means of four alternative reconstructions of the annual flow of the
Colorado River at Lees Ferry for common period 1520-1961. Lees-A is our updated reconstruction from
residual chronologies. Lees-D is our updated reconstruction from PCs of standard chronologies (see text).
SJ 1976 is the mean of two reconstructions generated by equations 2 and 3 of Stockton and Jacoby [1976,
p. 24). HPD2000 is the PC-based reconstruction of Hidalgo et al. [2000). The horizontal lines are the
1906-2004 observed mean (solid line) and the lowest observed 20-year running mean of the 1906-2004
period (dash-dotted line).

1650

different chronologies were used as predictors. Two of
Stockton and Jacoby's [1976] original sites were recollected,
but it is not evident that any of the same trees were sampled.
Gauge data were different as well, and both the tree ring
data and gauge data in previous efforts resulted in a
calibration period nearly half the length of the calibration
period used in this study. Differences could also result from
data processing and decisions in detrending the raw ring
widths. SJ\976 and HDP2000 used standard chronologies,
and models with PCs of lagged chronologies as predictors.
Over the common period 1906-1961, the SJ\976 recon-
struction showed a lag 1 autocorrelation of 0.36 and
HDP2000 0.41, which are similar to the 0.22 and 0.31 for
the models from this study that were based on standard
chronologies. All of these lag 1 values are also consistent
with values for the gauge record (0.2S). The inclusion of
lagged predictors may have the effect of enhancing the
persistence in the extreme low flow years. The reliance on
just seven tree ring chronologies to sample the runoff
variations over the entire Upper Colorado River Basin, as
with updated reconstruction Lees-A, might also present a
case of potential undersampling of the watershed. However,
we note that the Lees-A is closely tracked by Lees-D, a
PC-based reconstruction with weights on 31 chronologies
distributed over the watershed (Figure 4).

Table 9. Comparative Statistics of Lees Ferry Reconstructions
Calibrationb

1850

1950

2000

1900

[40] We can rule out the choice of calibration period as a
major source of differences among the reconstructions;
recalibrating our model Lees-A on 1914-1961 (following
SJl976) instead of 1906-1995 did not appreciably affect
the inferred magnitudes of past droughts. The accuracy of
the naturalized flow values is clearly important to the
estimated severity of reconstructed droughts: SJl976
reported important differences in drought severity and in
long-term mean reconstructed flow depending on the ver-
sion of the natural flow record (several existed at that time)
used to calibrate their reconstruction model.

5. Recent Drought (2000-2004) in a
Multicentury Perspective
[41] To assess the long-term standing of the most recent
drought on the Colorado River, the observed natural flows
at Lees Ferry averaged over the heart of the recent drought
(water years 2000-2004) can be compared with 5-year
running means of the Lees Ferry reconstruction. Because of
the unexplained variance in the regression however, we
must allow for the possibility that the true 5-year mean for
any reconstructed period may have been lower than the
reconstructed 5-year mean. For this assessment, error bars
were placed on the reconstructed 5-year running means.

ReconstmctionC

Model" Period Nc R2 Lowest 20-year Mean Long-Term Mean
Lees- A 1906-1995 7 0.81 15.6 I 1.2 BCM (12.6 I 0.9 MAF) 18.1 I 0.2 BCM (14.7 I 0.2 MAF)
Lees-D 1906-1995 30 0.77 12.8 "' 1.1 BCM (10.4 I 0.9 MAF) 17.6 I 0.2 BCM (14.3 "' 0.2 MAF)
SJl976 1914-1961 17 0.78d 13.5 BCM (10.9 MAF) 16.7 BCM (13.5 MAF)
HPD2000 1914-1961 6 0.82 11.2 I 1.0 BCM (9.1 "' 0.8 MAF) 16.3 "'0.2 BCM (13.2 "' 0.2 MAF)

aReconstruction model (see text).
bCalibration period, number of contributing chronologies, and proportion of variance explained by regression model.
CStatistics of long-term reconstruction. expressed in units ofbiJlion cubic meters and million acre-feet along with 95% contidence interval estimated from
the cross-validation root-mean square error (see tex~; statistics for common period 1520-1961.
dSJl 976 is a mean of two reconstmctions with R- values of 0.78 and 0.87 [Stockton and Jacoby, 1976]; no cross-validation was performed for these
models.

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