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

2. R 2 = 0.83 R 2 = 0.91 Columbia River Sacramento River Cool Season Precipitation Explains Most of the Variability in Annual Flow in the PNW and CA Relationship Between Annual Flow and Cool Season Precip. Relationship Between Annual Flow and Cool Season Precip.

3. R 2 = 0.56 Colorado River R 2 = 0.18 Colorado River Cool Season Precip Explains Most of the Variability in Annual Flow in the CRB, but the Summer Monsoon Also Plays a Role Relationship Between Annual Flow and Cool Season Precip. Relationship Between Annual Flow and Warm Season Precip.

4. Regionally Averaged Cool Season Precipitation Anomalies PRECIP

5. Regionally Averaged Warm Season Precipitation Anomalies PRECIP

6. PNWSSJCRB GB 1916-1946mean (mm)574.7443.9174.7172.2 Std deviation88.8100.130.623.6 CV0.150.230.170.14 lag 1auto corr-0.150.060.110.60 trend (% per decade)-1.16.9-3.53.7 1947-1976mean (mm)640.3477.1168.6180.7 Std deviation84.499.334.023.9 CV0.130.210.200.13 lag 1auto corr-0.420.12-0.290.15 trend (% per decade)1.52.83.8-0.1 1977-2003mean (mm)594.3488.1190.8185.3 Std deviation126.2141.950.841.4 CV0.210.290.270.22 lag 1auto corr0.220.120.150.46 trend (% per decade)4.22.4-9.7-5.1

7. PNWSSJCRB GB 1916-1946mean (mm)574.7443.9174.7172.2 Std deviation88.8100.130.623.6 CV0.150.230.170.14 lag 1auto corr-0.150.060.110.60 trend (% per decade)-1.16.9-3.53.7 1947-1976mean (mm)640.3477.1168.6180.7 Std deviation84.499.334.023.9 CV0.130.210.200.13 lag 1auto corr-0.420.12-0.290.15 trend (% per decade)1.52.83.8-0.1 1977-2003mean (mm)594.3488.1190.8185.3 Std deviation126.2141.950.841.4 CV0.210.290.270.22 lag 1auto corr0.220.120.150.46 trend (% per decade)4.22.4-9.7-5.1

8. 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 Simulated Changes in System Wide Energy Production in the Western U.S.

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 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) (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. 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?