Downscaling for Climate Change & Water Resources Mike Dettinger, Hugo Hidalgo & Dan Cayan

1. Downscaling for Climate Change & Water Resources Mike Dettinger, Hugo Hidalgo & Dan Cayan USGS @ SIO, La Jolla, CA Quick review of western trends & projections USGS downscaling needs New downscaled scenarios

2. Review of recent trends & projections: Mostly results from: USGS Hydroclimatology (Global Change Hydrology) Program-funded studies Funded by Office of Surface Water: Thanks to Harry Lins Bill Kirby Julie Kiang & San Francisco Bay PES efforts

3. 2. USGS downscaling needs Mostly lessons from: Efforts contributing to the 2006 California Climate Change Assessment activities Planning for 2008 California Climate Change Assessment http://meteora.ucsd.edu/cap/pdffiles/CA_climate_Scenarios.pdf

4. Sediment/ Geomorphology Climate Plankton Watershed Hydrodynamics Hydrology Invasive Contaminants Species Fish SF Bay PES & CASCaDE projects: 3. New downscaled scenarios: A product of Computational Assessments of Scenariosof Changein the Delta Ecosystem

5. 1. Review of recent trends & projections: The western states have been warming in recent decades.

6. Geographically, here is the pattern of warming. (Cayan et al., 2001) Annual-mean warming, 1979-2005 (IPCC 4th WG1 Ch. 4)

7. Warming already has driven hydroclimatic trends. Observed: Less spring snowpack TRENDS (1950-97) in April 1 snow-water content at western snow courses Observed: Less snow/more rain Knowles et al., 2006 Mote, 2003 -2.2 std devs LESS as snowfall +1 std dev MORE as snowfall Observed: Earlier greenup Observed: Earlier snowmelt runoff Cayan et al., 2001 Stewart et al., 2005

8. Greenhouse forcings 1900-2100 20 of 23 in this range Under projected greenhouse forcings, all climate models yield warmer futures for California and the West. Dettinger, 2005

9. SIERRA NEVADA CENTRAL VALLEY No snow left Most snow left Under even the coolest of these scenarios (+2.5ºC), observed hydrologic trends in most Western streams are projected to continue in 21st Century. Stewart et al., 2004 Center-of-Timing of Streamflow With coming addition of • increased flood risks in Pacific coast states • low baseflows • warm streams • drier summer soils & fuels • potentially large changes in mountain • recharge Knowles & Cayan, 2004 Knowles and Cayan 2002

10. Three Stages of Climate-Change Response • Is there a problem? • What should I do? • What are my risks and options?

11. Downscaled Original GCM values 2. USGS downscaling needs The downscaling problem Global Climate Model (actually, general circulation model)

12. Sediment/ Geomorphology Climate Plankton Watershed Hydrodynamics Hydrology Invasive Contaminants Species Fish USGS PES SF Bay (CASCADE) project needs: Downscaled series needed for cascade of hydrologic, hydrodynamic, geomorphologic, geochemical & ecological modelers: Daily max & min temperatures Daily precipitation ? San Francisco Monterey

13. Downscaling options: • Broad-brush T and %P changes • Scenario options: • Synthetic statistical • Describe the probability distributions of downscaled • variables conditional on statistical relations between GCM and downscaled variables, and resample • accordingly • Deterministic statistical * • Describe spatio-physical relations between GCM and • downscaled variables, and interpolate directly • Dynamical • Simulate same physical processes as in GCM but at higher resolution and (today) over smaller spatial • domains

14. Our downscaling wish list: -- Resolution: Daily time resolution, ~ 10-km spatial resolution, multiple climate variables -- Accuracy: Reproduction of high-resolution historical records -- Feasibility: Not too computationally burdensome (?) -- Synchronicity: Downscaled weather synchronized with climate model weather (just a Delta SLR thing?) -- Theoretical: Doesn't constrain future 'weather' to be same as historical -- Aesthetic: Climate (& trends) arise from entire weather field rather than being imposed GCM-grid cell by grid cell -- Practical: Ability to downscale to grids + stations at same time, maintaining internal consistency

15. The Delta • The Delta encompasses 2,177 km2 of agricultural land; Central Valley, including the Delta, yields $31B/yr (45% of the US produce) • 6 km3/yr of water is pumped from the Delta by State & Feds for agriculture & SoCal urban uses (22M people) Lower salinity inflows Seawater intrusion Higher salinity freshwater inflows Export to south

16. LEVEE STABILIZATION • 1800 km of levees provide the dry land for 57 manmade islands • Aging levees, sea-level rise, storm surges & flows, oxidizing island peats, failing ecosystems & earthquakes threaten uncontrolled breaches • Levee breaches continue, despite massive structural controls in the 20th Century Florsheim & Dettinger, 2005

17. Non-tidal Range (floods) Tidal range Sources of extreme water levels from San Francisco to Sacramento NORTH Sacramento San Joaquin R Sacramento R Delta Bay San Francisco Monterey

18. From 2006 California Climate Change Assessment: Projected extreme high sea-level stands (black) Extreme high sea-level stands coinciding with low-pressure systems (red) A distant proxy for measuring the coincidences of high seas + floods Cayan et al, in press, Clim. Chg PROBLEM: BCSD-based downscaling yields enhanced floods, but downscaled floods not synchronized with GCM storm surges

19. Downscaled Projected Trends in December Precipitation by Two Approaches (GFDL CM2.1, A2 emissions, 21st Century) BCSD Ours Bias correction & spatial downscaling, from Ed Maurer, SCU

20. Our approach towards downscaling climate change: • Downscale weather day by day • Downscale enough (daily) weather and you get downscaled climate • Downscale enough climate and you get downscaled climate change • Don't impose climate or climate change after the fact !

21. The constructed-analogs method Hidalgo, H.G., Dettinger, M.D., and Cayan, D.R., in review, Downscaling using constructed analogues daily US precipitation and temperatures: J. Climate, 24 p.

22. Hidalgo, H.G., Dettinger, M.D., and Cayan, D.R., in review, Downscaling using constructed analogues daily US precipitation and temperatures: J. Climate, 24 p.

23. Hidalgo, H.G., Dettinger, M.D., and Cayan, D.R., in review, Downscaling using constructed analogues daily US precipitation and temperatures: J. Climate, 24 p.

25. Skill of downscaling as indicated by application of method to historical OBSERVATIONS Skill at monthly average scale

26. Distributions of daily precip at selected sites Application of method to historical OBSERVATIONS shows that even extremes are captured accurately Hidalgo, H.G., Dettinger, M.D., and Cayan, D.R., in review, Downscaling using constructed analogues daily US precipitation and temperatures: J. Climate, 24 p.

27. Our constructed analogs offer: -- Resolution -- done -- Accuracy -- just illustrated -- Computationally feasible -- yes -- Synchronized with GCM -- yes -- Doesn't constrain future 'weather' to be same as historical -- done -- Source of trends -- everything fitted to the whole field at daily level with no trends imposed externally -- Downscaling fields & stations -- direct extension

28. 3. A downscaling offer: Available by end of (calendar) year 2007: -- Daily downscaled Tmin, Tmax & Precip fields for 1950-2100 across CONUS on 12-km grid will be online and available from PES/CASCADE: • From GFDL CM2.1 climate model(one of the warmest models) under heavy emissions (A2) and moderate emissions (B1) scenarios From NCAR PCM climate model(one of the coolest models) under heavy emissions (A2) and moderate emissions (B1) scenarios Planned by beginning of summer 2008: -- Same for between 4 and 6 more models

29. Caveats with this approach -- If you don't like the PRISM temperature & precipitation interpolations in your area, you won't like the downscaled fields -- If the GCM's weather (historical or otherwise) is flakey, this downscaling won't fix it -- CA needs daily GCM fields and an archive of observed weather data (analogs) -- U of WA & USBR are hurrying to distribute another set of downscaled fields (BCSD), which may become the de facto default

30. What do our constructed-analogs downscaled climate changes look like? A2 Dettinger, 2005

31. One day in the 21st Century (GFDL A2)‏ Downscaled Original GCM values

32. Trends, December temperatures Trends, June-August temperatures

33. Trends, December precipitation Trends, June-August precipitation

34. A2 B1

35. A2 Tmin Trends B1 Tmin Trends

36. A2 Precip Trends B1 Precip Trends

37. TO SUMMARIZE re: clim.chg.… • Warming by +2 to +6ºC, • likely more so at higher altitudes • Uncertain precipitation changes, • maybe less over much of western mountains, mostly small declines • Significant changes in • rain-vs-snow storms * • snowpack amounts * • snowmelt timing * • flood risk • streamflow timing * • low flows • growing seasons * • recharge? • total flow (drought?) • just in response to temperatures * Already detected & PES/CASCADE is producing a new generation of downscaled scenarios for CONUS

39. REFERENCES Bales, R., Molotch, N., Painter, T., Dettinger, M., Rice, R., and Dozier, J., 2006, Mountain hydrology of the western United States: Water Resources Research, 42, W08432, doi:10.1029/2005WR004387, 13 p. Cayan, D.R., Kammerdiener, S., Dettinger, M.D., Caprio, J.M., and Peterson, D.H., 2001, Changes in the onset of spring in the western United States: Bulletin, American Meteorological Society, 82, 399-415. Dettinger, M.D., 2005, From climate-change spaghetti to climate-change distributions for 21st Century California: San Francisco Estuary and Watershed Science, 3(1), http://repositories.cdlib.org/jmie/sfews/ vol3/iss1/art4. Dettinger, M.D., and Cayan, D.R., 1995, Large-scale atmospheric forcing of recent trends toward early snowmelt in California: Journal of Climate 8(3), 606-623. Dettinger, M.D., Cayan, D.R., Meyer, M.K., and Jeton, A.E., 2004, Simulated hydrologic responses to climate variations and change in the Merced, Carson, and American River basins, Sierra Nevada, California, 1900-2099: Climatic Change, 62, 283-317. Hidalgo, H.G., Cayan, D.R., and Dettinger, M.D., in preparation, Variability of spring-summer drought in regions of high and low evaporative efficiency: for submission to J. Hydrometeorology Intergovernmental Panel on Climate Change, 2007, Climate change 2007, The physical science basis—Summary for Policymakers: IPCC Secretariat, http://www.ipcc.ch, 21 p. Knowles, N., and D. Cayan, 2002, Potential effects of global warming on the Sacramento/San Joaquin watershed and the San Francisco estuary: Geophys. Res. Lett., 29: 18-21. Knowles, N., and D. Cayan, 2004, Elevational Dependence of Projected Hydrologic Changes in the San Francisco Estuary and Watershed. Climatic Change, 62, 319-336. Knowles, N., Dettinger, M., and Cayan, D., 2006, Trends in snowfall versus rainfall for the Western United States: Journal of Climate, 19(18), 4545-4559. Mote, P.W., 2003, Trends in snow water equivalent in the Pacific Northwest and their climatic causes. Geophysical Research Letters, 30, DOI 10.1029/2003GL0172588. Stewart, I., Cayan, D., and Dettinger, M., 2005, Changes towards earlier streamflow timing across western North America: Journal of Climate,18, 1136-1155.