Implications of Changing 20th Century Precipitation Variability for Water Management in the Western U.S.

1. Implications of Changing 20th Century Precipitation Variability for Water Management in the Western U.S. • Alan F. Hamlet, • Philip W. Mote, • Dennis P. Lettenmaier • JISAO/CSES Climate Impacts Group • Dept. of Civil and Environmental Engineering • University of Washington

2. Cool Season Climate of the Western U.S. PNW GB CA CRB DJF Temp (°C) NDJFM Precip (mm)

3. Result: Daily Precipitation, Tmax, Tmin 1915-2003

4. Evaluation of Streamflow Simulations of the Colorado River at Lee’s Ferry, AZ

5. A Time Series of Temporally Smoothed, Regionally Averaged Met Data for the West

6. Differences in cool and warm season precipitation trends suggest different mechanisms (large-scale advective storms vs. smaller scale convective storms) and differing sensitivity to regional warming. Trends in warm season precipitation in the CRB are very different than the other regions and may function more like cool season precipitation (e.g. related to circulation rather than locally generated storms)

7. Regionally Averaged Cool Season Precipitation Anomalies PRECIP

8. Cool Season Precipitation CV and Lag 1 Autocorrelation Have Increased Across the West since the Mid-1970s.

9. Regional Synchronicity of Cool Season Precipitation is Increasing

10. Water Resources Implications

11. Some Water Supply Implications of Increased CV and Autocorrelation and Regional Synchronicity of Cool Season Precipitation • Increased drought severity and persistence • (higher risk of severe multi-year droughts) • Increased streamflow forecasting uncertainty • (increased variance) • Increased forecast lead time • (increased autocorrelation) • Decreased ability to conjunctively manage water supply systems between regions (increased synchronicity) • Need for increased management flexibility to cope with persistent wet and dry periods at different times • (e.g. CRB in the 1980s vs the late 1990s and 2000s)

12. 20-year Flood for “1973-2003” Compared to “1916-2003” for a Constant Late 20th Century Temperature Regime DJF Avg Temp (C) X20 ’73-’03 / X20 ’16-’03 X20 ’73-’03 / X20 ’16-’03

13. Annual hydropower production in the West has become more variable and more regionally synchronous in the period 1977-2002 in comparison with the rest of the 20th century. 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

14. Simulated Changes in Natural Runoff Timing in the Naches River Basin Associated with 2 C Warming • Impacts: • Increased winter flow • Earlier and reduced peak flows • Reduced summer flow volume • Reduced late summer low flow

15. Simulated Trends in Sept 1 Soil Moisture (1947-2003) are Sensitive to Precip Changes. Temp and Precip Effects DJF Temp (°C) Temp Effects Alone Precip Effects Alone Trend %/yr

16. Conclusions Persistent changes in cool season precipitation variance, autocorrelation, and regional synchronicity have emerged in the West after about 1973. These changes have many important implications for water management including increased intensity and duration of extremes (both droughts and floods), increased water supply forecast uncertainty, decreased ability to conjunctively manage water systems. Changes in warm season precipitation may be a mitigating factor in temperature-related losses of late summer streamflow. Are these changes in precipitation related to global warming? Are they systematic like temperature changes or simply one mode of observed variability that has been unusual in the 20th Century?