Alan F. Hamlet

1
Evaluating Recent 20th Century Changes in Cool
Season Precipitation and Hydropower Variability
in the Western U.S. in the Context of
Paleoclimatic Reconstructions

• Alan F. Hamlet
• Anthony L. Westerling
• Tim P. Barnett
• Dennis P. Lettenmaier
• JISAO/CSES Climate Impacts Group
• Dept. of Civil and Environmental Engineering
• University of Washington
• Scripps Institute of Oceanography
• School of Engineering, University of California,
  Merced

2
Cool Season Precipitation Explains Most of the
Variability in Annual Flow in the PNW and CA
R2 0.83
Relationship Between Annual Flow and Cool Season
Precip.
Columbia River
R2 0.91
Relationship Between Annual Flow and Cool Season
Precip.
Sacramento River
3
Cool Season Precip Explains Most of the
Variability in Annual Flow in the CRB, but the
Summer Monsoon Also Plays a Role
R2 0.56
Relationship Between Annual Flow and Cool Season
Precip.
Colorado River
R2 0.18
Relationship Between Annual Flow and Warm Season
Precip.
Colorado River
4
Regionally Averaged Cool Season Precipitation
Anomalies
PRECIP
5
Regionally Averaged Warm Season Precipitation
Anomalies
PRECIP
6
PNW SSJ CRB GB
1916-1946 mean (mm) 574.7 443.9 174.7 172.2
Std deviation 88.8 100.1 30.6 23.6
CV 0.15 0.23 0.17 0.14
lag 1auto corr -0.15 0.06 0.11 0.60
trend ( per decade) -1.1 6.9 -3.5 3.7

1947-1976 mean (mm) 640.3 477.1 168.6 180.7
Std deviation 84.4 99.3 34.0 23.9
CV 0.13 0.21 0.20 0.13
lag 1auto corr -0.42 0.12 -0.29 0.15
trend ( per decade) 1.5 2.8 3.8 -0.1

1977-2003 mean (mm) 594.3 488.1 190.8 185.3
Std deviation 126.2 141.9 50.8 41.4
CV 0.21 0.29 0.27 0.22
lag 1auto corr 0.22 0.12 0.15 0.46
trend ( per decade) 4.2 2.4 -9.7 -5.1

7
PNW SSJ CRB GB
1916-1946 mean (mm) 574.7 443.9 174.7 172.2
Std deviation 88.8 100.1 30.6 23.6
CV 0.15 0.23 0.17 0.14
lag 1auto corr -0.15 0.06 0.11 0.60
trend ( per decade) -1.1 6.9 -3.5 3.7

1947-1976 mean (mm) 640.3 477.1 168.6 180.7
Std deviation 84.4 99.3 34.0 23.9
CV 0.13 0.21 0.20 0.13
lag 1auto corr -0.42 0.12 -0.29 0.15
trend ( per decade) 1.5 2.8 3.8 -0.1

1977-2003 mean (mm) 594.3 488.1 190.8 185.3
Std deviation 126.2 141.9 50.8 41.4
CV 0.21 0.29 0.27 0.22
lag 1auto corr 0.22 0.12 0.15 0.46
trend ( per decade) 4.2 2.4 -9.7 -5.1

8
Simulated Changes in System Wide Energy
Production in the Western U.S.
Correlation CRB-SSJ 0.07 CRB-PNW
0.08 SSJ-PNW 0.36
Correlation CRB-SSJ 0.14 CRB-PNW
-0.14 SSJ-PNW 0.06
Correlation CRB-SSJ 0.73 CRB-PNW
0.51 SSJ-PNW 0.65
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Long-Term Comparison of Annual Flow Records from
Observations and Paleo Reconstructions
PNW Observed (naturalized) annual flow in the
Columbia River at The Dalles, OR 1858-1877
(reconstructed from observed peak river
stage) 1878-2003 (naturalized from observed
monthly records) CA Reconstructed combined
annual flow in the Sacramento/San Joaquin basin
from tree-ring records. (Overlapping period
1858-1977) (Meko, D.M., 2001 Reconstructed
Sacramento River System Runoff From Tree Rings,
Report prepared for the California Department of
Water Resources, July) Colorado River
Basin Reconstructed annual flow in the Colorado
River at Lees Ferry, AZ from tree ring
records. (Overlapping period 1858-1977) (Woodhou
se, C.A., S.T. Gray, and D.M. Meko, 2006 Updated
Streamflow Reconstructions for the Upper Colorado
River Basin, Water Resources Research, Vol. 42,
W05415)
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Changes in Streamflow Variability from Combined
Paleo Reconstructions (black) and VIC Simulations
(red) of Annual Flow
All three metrics high together
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Changes in Streamflow Variability from Combined
Paleo Reconstructions (black) and VIC Simulations
(red) of Annual Flow for the SSJ and Colorado
Basins Only
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Conclusions

• Cool season precipitation is a major driver of
  annual river flow and hydropower production in
  the Western U.S.
• Substantial and persistent changes in cool season
  precipitation variability have emerged over the
  West since about 1975, including increased CV,
  within-region persistence, and inter-regional
  correlation.
• Long-term streamflow reconstructions show that
  the current changes in variability are very
  unusual in the context of natural variations over
  the last 150 years or so, but have probably
  occurred before in about 1580, and again in about
  1780 (almost exactly once every 200 years!)
• Is there a component of the most recent long-term
  droughts that is also related to global warming?