1. Hydrologic extremes in a changing climate -- modeling and observations Dennis P. Lettenmaier Department of Civil and Environmental Engineering University of Washington Program on Climate Change Summer Institute The water cycle in a changing climate Friday Harbor September 16, 2011

2. A warmer climate, with its increased climate variability, will increase the risk of both floods and droughts IPCC WG2, 2007

3. Most climate scientists agree that global warming will result in an intensification, acceleration or enhancement of the global hydrologic cycle, and there is some observational evidence that this is already happening. UNESCO World Water Development Report Water in a Changing Climate, 2009

4. Total U.S. flood damages, 1934-2000 from Pielke et al., 2000

5. Extreme precipitation should be increasing as the climate warms

6. Is extreme precipitation increasing?

7. Trends in annual maximum daily (left column) and 5- day (right column) precipitation, 1951-99. Upper row from observations. from Min et al., Nature 2011

8. Trends in U.S. daily precipitation by frequency interval, 1910- 1996 From Karl and Knight, BAMS 1998

9. From Karl and Knight, 1998 Continental U.S. weighted contribution of upper 10 th percentile to annual precipitation, 1910-96

10. Trends in annual precipitation maxima in 100 largest U.S. urban areas, 1950-2009 from Mishra and Lettenmaier, GRL 2011

11. Study Locations

20. 7.Changes in design storms calculated for various return periods. +37% +30%

23. 23 Can RCMs reproduce the timing of precipitation maxima ? Winter Summer

24. 24 Can RCMs reproduce the timing of precipitation maxima ? Winter Summer

25. Are extreme floods increasing?

26. JANUARY FLOODS JANUARY 12, 2009 When disaster becomes routine Crisis repeats as nature’s buffers disappear Disaster Declarations Federal Emergency Management Agency disaster declarations in King County in connection with flooding: January 1990 November 1990 December 1990 November 1995 February 1996 December 1996 March 1997 November 2003 December 2006 December 2007 Mapes 2009

27. Pecos River flood frequency distribution (from Kochel et al, 1988) Issues in the historical record

28. What causes a flood? Heavy precipitation Antecedent soil moisture and/or snow Interaction of storm characteristics (geometry, duration, intensity) with catchment geometry and characteristics (topography, channel network density, geology/soils) Storm orientation and movement relative to catchment/channel orientation

29. Other important flood characteristics Hydrologists usually think in terms of the annual maximum flood, which is the series of the largest floods each year (usually their peak discharge) Bankfull capacity corresponds roughly to 2-year return period (median annual maximum flood), which also is very roughly the mean annual flood Damages due to “floods” below bankfull capacity usually are minimal; damages increase rapidly (sometimes characterized as a power law) above bankfull discharge Flood risk is usually estimated by fitting a probability distribution to the annual maximum series, this distribution may be extrapolated to the T-year (e.g., 100- year, often used for flood plain planning) flood The T-year return period precipitation event (of specified duration) generally doesn’t cause the T-year flood (due to factors indicated above)

30. Number of statistically significant increasing and decreasing trends in U.S. streamflow (of 395 stations) by quantile (from Lins and Slack, 1999)

31. About 10% of the 400 sites show an increase in annual maximum flow from 1941-71 to 1971-99 Maximum flow Increase No change Decrease Visual courtesy Bob Hirsch, figure from McCabe & Wolock, GRL, 2002

32. Tufts University Decadal Magnification Factors of Floods – Sites w/ no regulation 1,642 of 14,893 USGS Gage Sites with M>1 and p>0.9 From Yaindl and Vogel, 2009 visual courtesy Rich Vogel

33. Tufts University Decadal Flood Magnification Factors - HCDN Sites 208 of 1,588 HCDN Gage Sites with M>1 and p>0.9 From Yaindl and Vogel, 2011 visual courtesy Rich Vogel

34. Tufts University Decadal Flood Magnification Factors Sites With No Regulation From Yaindl and Vogel, 2011 visual courtesy Rich Vogel

35. Tufts University Results Decadal Flood Magnification Factors From Yaindl and Vogel, 2011 3 Groups of USGS Gages visual courtesy Rich Vogel

36. Paradox: Given increases in precipitation and runoff, why are there so few significant trends in floods? Visual courtesy Tim Cohn, USGS

37. [Lins and Cohn, 2002 ] Possible explanation Visual courtesy Tim Cohn, USGS

38. Predicting urban flooding in a future climate – Thornton Creek example

39. Global Climate Models ECHAM5 Developed at Max Planck Institute for Meteorology (Hamburg, Germany) Used to simulate the A1B scenario in our study CCSM3 Developed at National Center for Atmospheric Research (NCAR; Boulder, Colorado)Used to simulate the A2 scenario in our study

40. Global Climate Models Mote et al 2005 ECHAM5 CCSM3

41. Dynamical Downscaling Courtesy Eric Salathé Global Model Regional Model

42. Results of Future Analysis SeaTacSpokanePortland 1-hour+16%+10%+11% 24-hour+19%+4%+5% 1-hour-5%-7%+2% 24-hour+15%+22%+2% * Statistically significant for difference in means and distributions, and non-zero temporal trends ECHAM5/ WRF CCSM3/ WRF * * * * Changes in average annual maximum precipitation between 1970–2000 and 2020–2050:

43. Results of Bias Correction -- SeaTac Raw ChangeCorrected Change 1-hour+16%+14% 24-hour+19%+28% 1-hour-5%-6% 24-hour+15%+14% ECHAM5 CCSM3 Comparison of changes in average annual maximum between 1970–2000 and 2020–2050: * * Statistically significant for difference in means and distributions, and non-zero temporal trends * **

44. Thornton Creek

45. Changes in Average Streamflow Annual Maxima (1970-2000 to 2020-2050)

46. Results of Hydrologic Modeling Changes in average annual maxima streamflow at outlet of watershed between 1970-2000 and 2020-2050: Juanita CreekThornton Creek CCSM3+25%+55% ECHAM5+11%+28% * Statistically significant for difference in means **

47. The November Surprise JANFEBMARAPR MAYJUNJULAUG SEPOCTNOVDEC Courtesy Eric Salathé NOV

48. Drought trends

49. from Andreadis and Lettenmaier, GRL 2006 Reconstructed U.S. soil moisture trends, 1915-2003

50. Trends in U.S. drought duration, 2915-2003 from Andreadis and Lettenmaier, GRL 2006

51. Trends in U.S. drought severity, 1915-2003 from Andreadis and Lettenmaier, GRL 2006

52. Trends in number of global droughts, 1950-2000 from Sheffield and Wood, J Clim, 2008

53. Trends in global drought duration, 1950-2000

54. Soil moisture trends in China, 1950- 2006 from Wang et al., J Clim, 2011

55. Trends in soil moisture, area in drought, and drought severity over China, 1950- 2006 from Wang et al., J Clim, 2011

56. Trends in number of droughts by area covered over China, 1950-2006 from Wang et al., J Clim, 2011

57. Conclusions Over the last 50-100 years, based on instrumental record: Extreme precipitation seems to be increasing (although certainly not everywhere, not surprising given detectability issues) Evidence for increases in floods is much sketchier (trends in low flows are clearly pervasive across the U.S. however) Trends in droughts also not obvious -- U.S. has experienced a general wetting trend over the last century, and drought trends essentially represent a balance between increasing precipitation and increased evaporative demand – some evidence balance is towards increasing droughts in parts of the western U.S., decreasing elsewhere