1. Sensitivity of Snow-Dominated Hydrologic Regimes to Global Warming Dennis P. Lettenmaier 1, Jennifer C. Adam 1, Tim P. Barnett 2 1.Dept. of Civil and Environmental Engineering, University of Washington 2.Climate Research Division, Scripps Inst. of Oceanography European Geosciences Union General Assembly 2006 Thursday, April 6 Vienna, Austria

2. 1. Background “On a global scale, the largest changes in the hydrological cycle due to warming are predicted for the snow-dominated basins of mid- to higher latitudes …” (Barnett et al, 2005) Approximately one-sixth of the world’s population lives in river basins that are strongly affected by snowmelt, and for which reservoir storage is unable to substantially attenuate seasonal shifts in runoff. This region accounts for roughly one-quarter of the global gross domestic product. Reduction of snow affected area can roughly be estimated on the basis of movement of the snowline (lower boundary of transient rain-on-snow zone) by the psuedo-adiabatic lapse rate, or roughly 6 o C/km.

3. Typical hydrographs of snow, transient (rain and snow) and rain dominated watersheds in northwestern U.S.

4. Map of Snowmelt-Dominated Regions {Snowfall÷Runoff ≥ 50%} – {Basins with large storage} Basins with ≥ 50% Runoff Derived from Snowmelt- Dominated Regions Legend

5. Population includes approximately one-sixth of the global population

6. Gross Domestic Product includes roughly one-quarter of global GDP

7. Less Storage of Water in Snow pack (snow rain) Warming Earlier Onset of Snowmelt Earlier Peak Runoff Reduction in Peak Runoff Reduced Surface Water Availability During Summer/Autumn (seasons of peak demand) Mechanisms for shift in seasonal hydrographs in a warming climate

8. Mountainous Regions snowmelt dominated regions occupy regions pole-ward of 45° exceptions include mountainous areas (lower latitudes) and areas warmed by ocean waters (higher latitudes)

10. 2. Observational evidence

11. As the West warms, winter flows rise and summer flows drop I.T. Stewart, D.R. Cayan, M.D. Dettinger, 2004, Changes toward earlier streamflow timing across western North America, J. Climate (in review) Figure courtesy of Iris Stewart, Scripps Inst. of Oceanog. (UC San Diego)

15. March June Relative Trend (% per year) Trends in fraction of annual runoff 1947-2003 (cells > 50 mm of SWE on April 1) Figure courtesy of Alan Hamlet, U. Washington

16. 3. Hydrologic implications of climate change globally

17. Global Climate Change Selected Basins 1 MacKenzie 2 Mississippi 3 Amazon 4 Severnaya Dvina 5 Yenisei 6 Amur 7 Yellow 8 Xi 9 Mekong -90 -60 -30 0 30 60 90 -90 -60 -30 0 30 60 90 -150-120-90-60-300306090120 150 -150-120-90-60-300306090120150 1 2 3 4 5 6 7 8 9 from Nijssen et al, Climatic Change, 2001

18. Season in which change was imposed -5% -10% +5% +10% Runoff Sensitivity Change in Runoff as a result of change in Precipitation from Nijssen et al, Climatic Change, 2001

19. Runoff Sensitivity Change in Runoff as a result of change in Temperature from Nijssen et al, Climatic Change, 2001

20. 4. Western U.S. impact studies

21. Diminishing Sierra Snowpack % Remaining, Relative to 1961-1990 Total snow losses by the end of the century: 29–73% for the lower emissions scenario (3-7 MAF) 73–89% for higher emissions (7-9 MAF – 2 Lake Shastas) Dramatic losses under both scenarios Almost all snow gone by April 1 north of Yosemite under higher emissions Visual courtesy Ed Maurer

22. Future Spring Snowpack Remaining by Elevation as a % of 1961-1990 levels Losses greatest below 3,000 m: 37–79% for B1 81–94% for A1fi. Below 1800 m (~6000 ft) >80% April 1 snow loss under all simulations Below 2600 m (8500 ft) >75% loss for 3 of 4 simulations, both of high emissions scenarios Visual courtesy Ed Maurer

23. Impacts on Ski Season Warmer temperatures result in: Less precipitation falling as snow in winter Earlier melt of accumulated snow These combine to shorten the ski season Photo: SwissRe Visual courtesy Ed Maurer

24. Length of Ski Season 28-41 days (4-6 weeks) shorter for B1 scenario 39-44 days (6 weeks) shorter for A1fi Retreat of season start: 5-14 days (losing end of November and early December) This is at midpoint year of 2035 – in our lifetimes. Visual courtesy Ed Maurer

25. Length of Ski Season 49-106 days (7-15 week) shorter for B1 scenario 103 days shorter (15 week) to zero day ski season for A1fi Retreat of season start: at least 22 days This is at midpoint year of 2085 – in our childrens’ and grandchildrens’ lifetimes. Minimum ski conditions never attained

26. 5. Water resources implications in the western U.S.: The Accelerated Climate Prediction Initiative (see Climatic Change special issue, Jan-Feb. 2004, for details)

27. PCM Business-as-Usual scenarios Columbia River Basin (Basin Averages) control (2000-2048) historical (1950-99) BAU 3-run average

30. PCM Business-as-Usual scenarios California (Basin Average) control (2000-2048) historical (1950-99) BAU 3-run average

31. PCM Business-as-Usual Scenarios Snowpack Changes California April 1 SWE

33. Storage Decreases Sacramento Range: 5 - 10 % Mean: 8 % San Joaquin Range: 7 - 14 % Mean: 11 % Current Climate vs. Projected Climate

34. Hydropower Losses Central Valley Range: 3 - 18 % Mean: 9 % Sacramento System Range: 3 – 19 % Mean: 9% San Joaquin System Range: 16 – 63 % Mean: 28%

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

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

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

38. Total Basin Storage

39. Annual Releases to the Lower Basin target release

40. Annual Releases to Mexico target release

41. Annual Hydropower Production

42. Summary of ACPI results Columbia and California reservoir systems primarily provide within-year storage (total storage/mean flow ~ 0.3 – 0.5), whereas Colorado is an over-year system (~4) Climate sensitivities in Columbia basin and California 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. 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. 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); temperature changes are much less important.

43. Conclusions Impacts of climate change on the hydrology of snowmelt dominated rivers (of which mountainous watersheds are a particularly important subset) are among the most predictable impacts of climate change Transient snow domains are most “at risk”, but impacts will be felt in all ephemeral snow domains Changes over the last century are detectable, and have already impacted the reliability of water supply systems in the western U.S. Planning methods that incorporate ongoing and future climate change are urgently needed as operating agencies begin to recognize the problems and issues

44. Rhine River (Middelkoop et al. 2001) Aare River at Brugg Rhine River at Rheinfelden H. Middelkoop et al., Impact of climate change on hydrological regimes and water resources management in the Rhine Basin, Clim. Change, 49: 105-128, 2001. (Image: Ultrecht Univ., Netherlands) Discharge, m 3 /s

45. Rhine River (Middelkoop et al. 2001) H. Middelkoop et al., Impact of climate change on hydrological regimes and water resources management in the Rhine Basin, Clim. Change, 49: 105-128, 2001. (Image: Ultrecht Univ., Netherlands) Rhine River at Rees Discharge, m 3 /s Some Implications: reduction of water availability during season of peak demand increase in number of low-flow days (affects ship transport) decrease in level of flood protection decrease in annual hydropower production (some sub-basins)

46. Canadian Prairies (de Loë et al. 2001) R. de Loë et al., Adaptation options for the near term: climate change and the Canadian water sector, Global Env. Change, 11, 231-245, 2001. agriculture sensitive to drought (irrigation derived primarily from surface waters) predictions include: decrease in snow-pack, earlier peak runoff, and lower summer soil moistures implications: agriculture more at risk in a warming climate; and heightened competition with other water needs (aquatic habitat and down-stream requirements)

47. Glaciers… Recession of Grinnell Glacier, Glacier National Park (1911 and 2000)