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

5. Regionally Averaged Warm Season Precipitation Anomalies

6. PNW
SSJ
CRB GB
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
640.3
477.1
168.6
180.7
84.4
99.3
34.0
23.9
0.13
0.21
0.20
-0.42
0.12
-0.29
1.5
2.8
3.8
-0.1
594.3
488.1
190.8
185.3
126.2
141.9
50.8
41.4
0.29
0.27
0.22
0.46
4.2
2.4
-9.7
-5.1

7. PNW
SSJ
CRB GB
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
640.3
477.1
168.6
180.7
84.4
99.3
34.0
23.9
0.13
0.21
0.20
-0.42
0.12
-0.29
1.5
2.8
3.8
-0.1
594.3
488.1
190.8
185.3
126.2
141.9
50.8
41.4
0.29
0.27
0.22
0.46
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
Fig 7 Annual hydropower production has followed suit. These results suggest reduced opportunity for conjunctive management of the West’s hydropower resources in recent decades.

9. 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
(reconstructed from observed peak river stage)
(naturalized from observed monthly records)
CA:
Reconstructed combined annual flow in the Sacramento/San Joaquin basin from tree-ring records.
(Overlapping period )
(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.
(Woodhouse, 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)

10. All three metrics high together
Changes in Streamflow Variability from Combined Paleo Reconstructions (black) and VIC Simulations (red) of Annual Flow
All three metrics high together

11. Changes in Streamflow Variability from Combined Paleo Reconstructions (black) and VIC Simulations (red) of Annual Flow for the SSJ and Colorado Basins Only

12. 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?