1. Example: Application of the Variable Infiltration Capacity model to climate impact assessment in the Colorado River basin Dennis P. Lettenmaier Department of Civil and Environmental Engineering University of Washington for presentation at Dividing the waters: Science for Judges Workshop IV Workshop on Climate Change Modeling: General Circulation Models and Hydrometeorologic Models Hotel Boulderado Boulder, Colorado May 13 – 15, 2007

2. Outline of this talk 1)Climate variability and change context 2)Prediction and assessment approach 3)Hydrology and water management implications for Colorado River basin -- Accelerated Climate Prediction Initiative (ACPI) 4)Postmortem – Milly et al (2005); Seager et al (2007); Christensen and Lettenmaier (2007)

3. 1) Climate variability and change context

4. Temperature trends in the PNW over the instrumental record Almost every station shows warming (filled circles) Urbanization not a major source of warming

5. Trends in timing of spring snowmelt (1948-2000) Courtesy of Mike Dettinger, Iris Stewart, Dan Cayan +20d later –20d earlier

7. Source: Mote et al, 2005

8. Trends in snowpack

9. 2) Prediction and assessment approach

10. Climate Scenarios Global climate simulations, next ~100 yrs Downscaling Delta Precip, Temp Hydrologic Model (VIC) Natural Streamflow Reservoir Model DamReleases, Regulated Streamflow Performance Measures Reliability of System Objectives

11. Reservoir Model Hydrology Model Coupled Land- Atmosphere-Ocean General Circulation Model

12. Accelerated Climate Prediction Initiative (ACPI) – NCAR/DOE Parallel Climate Model (PCM) grid over western U.S.

13. Bias Correction and Downscaling Approach climate model scenario meteorological outputs  hydrologic model inputs  snowpack runoff streamflow 1/8-1/4 degree resolution daily P, Tmin, Tmax 2.8 (T42)/0.5 degree resolution monthly total P, avg. T

14. Bias Correction from NCDC observations from PCM historical runraw climate scenario bias-corrected climate scenario month m Note: future scenario temperature trend (relative to control run) removed before, and replaced after, bias-correction step.

15. Downscaling observed mean fields (1/8-1/4 degree) monthly PCM anomaly (T42) VIC-scale monthly simulation interpolated to VIC scale

17. Dam Operations in ColSim Storage Dams Run-of-River Dams VirginRegulated Flow In=Flow out + Energy H

18. Inflow Run of River Reservoirs (inflow=outflow + energy) Inflow Storage Reservoirs Releases Depend on: Storage and Inflow Rule Curves (streamflow forecasts) Flood Control Requirements Energy Requirements Minimum Flow Requirements System Flow Requirements System Checkpoint Consumptive use Inflow + C ol S im

19. 3) Accelerated Climate Prediction Initiative (ACPI)

20. GCM grid mesh over western U.S. (NCAR/DOE Parallel Climate Model at ~ 2.8 degrees lat-long)

21. Climate Change Scenarios Historical B06.22 (greenhouse CO 2 +aerosols forcing) 1870-2000 Climate Control B06.45 (CO 2 +aerosols at 1995 levels) 1995-2048 Climate Change B06.44 (BAU6, future scenario forcing) 1995-2099 Climate Change B06.46 (BAU6, future scenario forcing) 1995-2099 Climate Change B06.47 (BAU6, future scenario forcing) 1995-2099 Climate Control B06.45 derived-subset 1995-2015 Climate Change B06.44 derived-subset 2040-2060 PCM Simulations (~ 3 degrees lat-long) PNNL Regional Climate Model (RCM) Simulations (~ ¾ degree lat-long)

22. Future streamflows 3 ensembles averaged summarized into 3 periods; »Period 1 2010 - 2039 »Period 22040 - 2070 »Period 3 2070 - 2098

23. 4a) Hydrology and water management implications: Colorado River basin

24. Timeseries Annual Average Period 1 2010-2039 Period 2 2040-2069 Period 3 2070-2098 hist. avg. ctrl. avg. PCM Projected Colorado R. Temperature

25. hist. avg. ctrl. avg. PCM Projected Colorado R. Precipitation Timeseries Annual Average Period 1 2010-2039 Period 2 2040-2069 Period 3 2070-2098

26. Annual Average Hydrograph Simulated Historic (1950-1999)Period 1 (2010-2039) Control (static 1995 climate)Period 2 (2040-2069) Period 3 (2070-2098)

27. Projected Spatial Change in Runoff 90 % 86 % 82 % 83 %

28. April 1 Snow Water Equivalent

29. Natural Flow at Lee Ferry, AZ Currently used 16.3 BCM allocated 20.3 BCM

31. Storage Reservoirs Run of River Reservoirs CRRM Basin storage aggregated into 4 storage reservoirs –Lake Powell and Lake Mead have 85% of basin storage Reservoir evaporation = f(reservoir surface area, mean monthly temperature) Hydropower = f(release, reservoir elevation) Monthly timestep Historic Streamflows to Validate Projected Inflows to assess future performance of system

32. Water Management Model (CRRM) Multi Species Conservation Program year 2000 demands – upper basin 5.4 BCM – lower basin 9.3 BCM – Mexico 1.8 BCM Minimum Annual Release from Glen Canyon Dam of 10.8 BCM Minimum Annual Release from Imperial Dam of 1.8 BCM

33. Total Basin Storage

34. Annual Releases to the Lower Basin target release

35. Annual Releases to Mexico target release

36. Annual Hydropower Production

37. Uncontrolled Spills

38. Deliveries to CAP & MWD

39. Postmortem: Christensen and Lettenmaier (HESSD, 2007) – multimodel ensemble analysis with 11 IPCC AR4 models (downscaled as in C&L, 2004)

40. Magnitude and Consistency of Model-Projected Changes in Annual Runoff by Water Resources Region, 2041-2060 Median change in annual runoff from 24 numerical experiments (color scale) and fraction of 24 experiments producing common direction of change (inset numerical values). +25% +10% +5% +2% -2% -5% -10% -25% Decrease Increase (After Milly, P.C.D., K.A. Dunne, A.V. Vecchia, Global pattern of trends in streamflow and water availability in a changing climate, Nature, 438, 347-350, 2005.) 96% 75% 67% 62% 87% 71% 67% 62% 58% 67% 62% 58% 67% 100%

41. from Seager et al, Science Express, 2007

49. 5) Conclusions and Comparative analysis 1) Columbia River reservoir system primarily provides within-year storage (total storage/mean flow ~ 0.3). California is intermediate (~ 0.3), Colorado is an over-year system (~4) 2) Climate sensitivities in Columbia basin are dominated by seasonality shifts in streamflow, and may even be beneficial for hydropower. However, fish flow targets would be difficult to meet under altered climate, and mitigation by altered operation is essentially impossible. 3) California system operation is dominated by water supply (mostly ag), reliability of which would be reduced significantly by a combination of seaonality shifts and reduced (annual) volumes. Partial mitigation by altered operations is possible, but complicated by flood issues. 4) Colorado system is sensitive primarily to annual streamflow volumes. Low runoff ratio makes the system highly sensitive to modest changes in precipitation (in winter, esp, in headwaters). Sensitivity to altered operations is modest, and mitigation possibilities by increased storage are nil (even if otherwise feasible).