1. Long-term climate and water cycle variability and change Dennis P. Lettenmaier Department of Civil and Environmental Engineering University of Washington U.S. - Japan Workshop on Global Change Climate and Water January 14, 2003 Irvine, CA

2. Outline of this talk 1)What are the key modes of variability in the (land surface branch of the) water cycle? 2)How is the land surface water cycle changing? 3)What are the implications of possible future changes in the water cycle for human use and water management?

3. What are the key modes of variability in the land surface branch of the water cycle?

4. Seasonal distribution of precipitation across the U.S. (from Linsley et al, 1975)

5. Mean Diurnal Cycle of precipitation from observations and NCAR/DOE PCM Summer Winter Observed PCM

6. days Log(1-P) days Roosevelt Dam, AZ Snoqualmie Falls, WA Log survivor functions for winter daily precipitation (from Foufoula-Georgiou, 1985)

7. Variation of standard deviation of storm precipitation with aggregation scale for Darwin storm of 12/24/98 and Kwajelin storm of 12/4/98 Figure courtesy of Efi Foufoula-Georgiou

8. Cross-correlation of January precipitation for selected west coast sites (from Leytham, 1982)

9. Clearwater River flood frequency distribution (from Linsley et al 1975)

10. Pecos River flood frequency distribution (from Kochel et al, 1988)

13. Perth metropolitan water supply total system inflow, 1910-2001

14. Rio Bravo Basin

15. Rio Bravo/Rio Conchos total naturalized reservoir inflow, 1954-2001 4978Mm 3 (1955 to 1992 avg) 2542 Mm 3 (1993 to 2001 avg) Note: does not include some upstream inflows due to lack of extended data

17. Annual number of extreme precipitation events in Tijuana over ~50 years (slide courtesy of Tereza Cavazos)

18. Effects of the PDO and ENSO on Columbia River Summer Streamflows Cool Warm

19. Naturalized Summer Streamflow at The Dalles

20. Annual Flow at The Dalles 1858-1998

21. How is the land surface water cycle changing?

22. Global precipitation trends 1900-99 by season (from IPCC, 2001)

23. Flow Quantile Number of Gages Number of U.S. gages, of 395, reporting significantly significant trends (alpha = 0.05) during 1944-93 (from Lins and Slack, 1999)

24. Trends in Streamflow and Precipitation, Columbia River basin

25. Annual Flow at The Dalles 1858-1998

26. U.S. trends in pan evaporation (from Lawrimore and Peterson, 2000)

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

28. Trends in snowpack

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

30. Trends in snowpack

31. North Vancouver Intensity-duration regression analysis = statistical outliers (from Catherine Denault and Rob Millar, UBC)

32. What are the implications for society?

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

35. Typical projected effect of climate change on upper Columbia River discharge, mid-21 st Century

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

39. Annual Average Hydrographs, Colorado River Simulated Historic (1950-1999) Period 1 (2010-2039) Control (static 1995 climate) Period 2 (2040-2069) Period 3 (2070-2098)

41. Total Basin Storage

42. Annual Releases to the Lower Basin target release

43. Annual Releases to Mexico target release

44. Conclusions Water cycle variability takes many forms, we are only beginning to understand the interaction of climate and land surface hydrologic variability Jury is still out as to whether (and/or which) observed changes in land surface variables are a manifestation of “acceleration of the hyrologic cycle”, or are simply manifestations of natural variability If climate change projections are accurate (even as to general direction) hydrologists will be faced with important challenges understanding the interaction of climate and hydrology at the intersection of variability and change (stated otherwise, the time- honored paradigm for hydrologic risk analysis and design must be re-evaluated).