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Water and Climate: What's Changing, and Does It Matter to Water Managers?

This article explores the need for understanding hydrologic change and its implications for water managers in the context of water and climate. It discusses the challenges in hydrology and the importance of addressing these challenges to mitigate the impacts of floods, droughts, and contamination. The article also highlights the need for a systematic understanding of water dynamics at continental scales and the links between water and nutrient cycles.

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Water and Climate: What's Changing, and Does It Matter to Water Managers?

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  1. Water and Climate: What's Changing, and Does It Matter to Water Managers? Dennis P. Lettenmaier Department of Civil and Environmental Engineering University of Washington for 2009 AAAS Annual Meeting Session on 21st Century Water: Friend or Foe? Chicago February 14, 2009

  2. What are the “grand challenges” in hydrology? From Science (2006) 125th Anniversary issue (of eight in Environmental Sciences): Hydrologic forecasting – floods, droughts, and contamination From the CUAHSI Science and Implementation Plan (2007): … a more comprehensive and … systematic understanding of continental water dynamics … From the USGCRP Water Cycle Study Group, 2001 (Hornberger Report):[understanding] the causes of water cycle variations on global and regional scales, to what extent [they] are predictable, [and] how … water and nutrient cycles [are] linked?

  3. Important problems all, but I will argue instead (in addition) that understanding hydrologic change should rise to the level of a grand challenge to the community.

  4. From Stewart et al, 2005

  5. Magnitude and Consistency of Model-Projected Changesin 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 (insetnumerical values). +25% 58% +10% 67% Increase 62% +5% 58% 87% 96% +2% 62% 62% 71% 87% -2% 75% 67% 67% 67% -5% 100% Decrease -10% -25% (After Milly, P.C.D., K.A. Dunne, A.V. Vecchia, Global pattern of trends in streamflow andwater availability in a changing climate, Nature, 438, 347-350, 2005.)

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

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

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

  9. Natural Flow at Lee Ferry, AZ allocated20.3 BCM Currently used 16.3 BCM

  10. Total Basin Storage

  11. Annual Releases to the Lower Basin target release

  12. Annual Releases to Mexico target release

  13. Annual Hydropower Production

  14. Case study 1: Yakima River Basin • Irrigated crops largest agriculture value in the state • Precipitation (fall-winter), growing season (spring-summer) • Five USBR reservoirs with storage capacity of ~1 million acre-ft, ~30% unregulated annual runoff • Snowpack sixth reservoir • Water-short years impact water entitlements

  15. Yakima River Basin 2020s 2080s historical • Basin shifts from snow to more rain dominant • Water prorating, junior water users receive 75% of allocation • Junior irrigators less than 75% prorating (current operations): 14% historically 32% in 2020s A1B (15% to 54% range of ensemble members) 36% in 2040s A1B 77% in 2080s A1B

  16. Crop Model - Apple Yields • Yields decline from historic by 20% to 25% (2020s) and 40% to 50% (2080s)

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

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

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

  20. Current Climate vs. Projected Climate • Hydropower Losses • Central Valley • Range: 3 - 18 % • Mean: 9 % • Sacramento System • Range: 3 – 19 % • Mean: 9% • San Joaquin System • Range: 16 – 63 % • Mean: 28%

  21. Stationarity—the idea that natural systems fluctuate within an unchanging envelope of variability—is a foundational concept that permeates training and practice in water-resource engineering. In view of the magnitude and ubiquity of the hydroclimatic change apparently now under way, however, we assert that stationarity is dead and should no longer serve as a central, default assumption in water-resource risk assessment and planning.

  22. How can the water management community respond? Central methodological problem: While water managers are used to dealing with risk, they mostly use methods that are heavily linked to the historical record

  23. “Synthetic hydrology” c. 1970 Figure adapted from Mandelbrot and Wallis (1969)

  24. Ensembles of Colorado River (Lees Ferry) temperature, precipitation, and discharge for IPCC A2 and B1 scenarios (left), and 50-year segments of tree ring reconstructions of Colorado Discharge (from Woodhouse et al, 2006)

  25. Hybrid Climate Change Perturbations New time series value = 19000 Objective: Combine the time series behavior of an observed precipitation, temperature, or streamflow record with changes in probability distributions associated with climate change. Value from observed time series = 10000

  26. Observed and Climate Change Adjusted Naturalized Streamflow Time Series for the Snake River at Ice Harbor KAF KAF Blue = Observed time series Red = Climate change time series

  27. Other implications of nonstationarity • Hydrologic network design (station discontinuance algorithms won’t work) • Need for stability in the evolution of climate scenarios (while recognizing that they will almost certainly change over time)

  28. Another complication: Water resources research has died in the U.S. • No federal agency has a competitive research program dedicated to water resources research (e.g., equivalent to the old OWRT) • As a result, very few Ph.D. students (and hence young faculty) have entered the area • And in turn, the research that would identify alternatives to classic stationarity assumptions is not being done See Lettenmaier, “Have we dropped the ball on water resources”, ASCE JWRPM editorial, to appear Nov., 2008

  29. Conclusions • Ample evidence that stationarity assumption is no longer defensible for water planning (especially in the western U.S.) • What to replace it with remains an open question • A key element though will have to be weaning practitioners from critical period analysis, to risk based approaches (not a new idea!!) • Support for the basic research needed to develop alternative methods (a new Harvard Water Program?) is lacking

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