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Global Change and Extreme Hydrology: Testing Conventional Wisdom Committee on Hydrologic Science     Water Science and Technology Board Division on Earth and Life Studies                       THE NATIONAL ACADEMIES PRESS Washington, D.C. www.nap.edu

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THE NATIONAL ACADEMIES PRESS 500 Fifth Street, N.W. Washington, DC 20001 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance. Support for this project was provided by the U.S. Nuclear Regulatory Commission Grant No. NRC- 04-09-153, National Oceanic and Atmospheric Administration Grant No. RA133R-09-SE-4232, and National Aeronautics and Space Administration Grant No. NNX10AK53G. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the organization or agency that provided support for the project. International Standard Book Number 13: 978-0-309-21768-2 International Standard Book Number 10: 0-309-21768-7 Cover photos: (top) National Oceanic and Atmospheric Administration, (bottom) California Department of Water Resources. Cover design by Sharon Martin. Additional copies of this report are available from the National Academies Press, 500 Fifth Street, N.W., Lockbox 285, Washington, DC 20055, (800) 624-6242 or (202) 334-3313 (in the Washington metropolitan area); Internet http://www.nap.edu. Copyright 2011 by the National Academy of Sciences. All rights reserved. Printed in the United States of America    

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    The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scho- lars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technic- al matters. Dr. Ralph J. Cicerone is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engi- neers. Dr. Charles M. Vest is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the ser- vices of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identi- fy issues of medical care, research, and education. Dr. Harvey V. Fineberg is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy’s purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Ralph J. Cicerone and Dr. Charles M. Vest are chair and vice chair, respectively, of the National Research Council. www.nationalacademies.org     

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  Committee on Hydrologic Science Charles J. Vörösmarty, Chair, City University of New York* Victor R. Baker, University of Arizona* Dennis P. Lettenmaier, University of Washington* Daniel P. Loucks, Cornell University* David Pimentel, Cornell University George F. Smith, Riverside Technologies, inc.* Emily H. Stanley, University of Wisconsin Chunmiao Zheng, The University of Alabama* Staff Laura J. Helsabeck, Study Director Anita Hall, Senior Program Associate *Workshop planning committee v   

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  Water Science and Technology Board Donald I. Siegel, Chair, Syracuse University, Syracuse, New York Lisa Alvarez-Cohen, University of California, Berkeley Edward J. Bouwer, Johns Hopkins University, Baltimore, Maryland Yu-Ping Chin, Ohio State University, Columbus Otto C. Doering III, Purdue University, West Lafayette, Indiana M. Siobhan Fennessy, Kenyon College, Gambier, Ohio Ben Grumbles, Clean Water America Alliance, Washington, D.C. George R. Hallberg, The Cadmus Group, Inc., Watertown, Massachusetts Kenneth R. Herd, Southwest Florida Water Management District, Brooksville George M. Hornberger, Vanderbilt University, Nashville, Tennessee Kimberly L. Jones, Howard University, Washington, D.C. Larry Larson, Association of State Floodplain Managers, Madison, Wisconsin David Moreau, University of North Carolina, Chapel Hill Dennis D. Murphy, University of Nevada, Reno Marylynn V. Yates, University of California, Riverside Staff Stephen D. Parker, Director Jeffrey Jacobs, Scholar Laura J. Ehlers, Senior Staff Officer Stephanie Johnson, Senior Staff Officer Laura J. Helsabeck, Staff Officer Jeanne Aquilino, Financial and Administrative Associate Anita Hall, Senior Program Associate Michael Stoever, Research Associate Sarah Brennan, Program Assistant vi   

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  Preface The National Research Council (NRC) Committee on Hydrologic Science (COHS) held a workshop on January 5-6, 2010, that examined how climate warming translates into hydrologic extremes like floods and droughts. This issue represents a chief concern of scientists studying the societal implications of climate change. The event probed the “conventional wisdom” that climate change will “accelerate” the hydrologic cycle, fuel more evaporation, and generate more precipitation, based on an increased capacity of a warmer atmosphere to hold more water vapor. Associated with these theoretical expectations are increases in the frequency and severity of climate and weather extremes relative to present-day conditions, most notably severe floods and droughts. The workshop, titled Global Change and Extreme Hydrologic Events: Testing Con- ventional Wisdom, brought together three groups of experts. The first two groups consisted of atmospheric scientists and hydrologists focused on the scientific underpinnings and empirical evidence linking climate variability to hydrologic extremes. The third group consisted of water managers and decision-makers charged with the design and operation of water systems that in the future must be made resilient in light of a changing climate and an environment of hydrologic extremes. Although the workshop attendees represented a diversity of perspectives from the scientific and engineering communities, including from researchers and decision-makers, not all perspectives related to this issue were represented. The workshop, focused on floods on day 1 and droughts on day 2, was organized by the climatological, hydrologic, and water management perspectives and featured presentations by invited experts (see Appendixes B-D for workshop agenda, speaker abstracts, and a summary of the presentations, respectively). Breakout sessions were convened each afternoon for focused discussion among participants, speakers, and committee members. We thank the following speakers for sharing their perspectives: Gerry Galloway, University of Maryland; Pavel Groisman, National Oceanic and Atmospheric Administration; Mike Hayes, University of Nebraska-Lincoln; Katie Hirschboeck, University of Arizona; Tom Huntington, U.S. Geological Survey; Harry Lins, U.S. Geological Survey; Mark Person, New Mexico Tech; Siegfried Schubert, NASA Goddard; Richard Seager, Columbia University; Kevin Trenberth, National Center for Atmospheric Research; and Richard Vogel, Tufts University. The abstracts from the workshop presentations (Appendix C) contain the vii   

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viii   Preface    opinions expressed by the speakers. Although the committee relied on these experts’ opinions to identify and synthesize its findings, sole responsibility for the report findings rests with the committee. The topic of global change and extreme hydrologic events is complex, involving a variety of dimensions and associated questions. This report does not attempt to “test” a hypothesis but instead presents an overview of the current state of the science in terms of climate change and extreme hydrologic events, drawing heavily from the workshop discussions. The report includes descriptions of the changes in frequency and severity of extremes, the ability (or inability) to model these changes, and the problem of communicating the best science to water resources practitioners in useful forums. As noted later in this report, differing perspectives were evident across the three contributing groups (i.e., hydrology, atmospheric sciences, and water management). The COHS hopes that researchers will become aware of these differences and will be inspired to craft more coherent and unified linkages among climate-hydrology-water management issues. In this context, there is a special role for hydrologic sciences that will be articulated throughout the report. This report was reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise in accordance with the procedures approved by the NRC’s Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the NRC in making its published report as sound as possible, and to ensure that the report meets NRC institutional standards for objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We thank the following individuals for their review of this report: Gerald E. Galloway, Jr., University of Maryland, College Park; William Gutowski, Iowa State University; Mike Hayes, University of Nebraska- Lincoln; Tom Huntington, U.S. Geological Survey; Lee W. Larson, Hydrologist, retired, NOAA’s Missouri Basin River Forecast Center; and Kevin Trenberth, National Center for Atmospheric Research. Although these reviewers provided many constructive comments and suggestions, they did not see the final draft of the report before its release. The review of this report was overseen by David T. Ford, David Ford Consulting Engineers, Inc. Appointed by the NRC Division on Earth and Life Studies, he was responsible for making certain that an independent examination of this report was carried out in accordance with institutional procedures and that all review comments were carefully considered. Responsibility for the final content of this report rests entirely with the authors and the institution. Charles Vörösmarty, Chair Planning Committee for the Workshop on Extreme Hydrologic Events: Testing Conventional Wisdom    

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  Contents SUMMARY……………………………………………………………………………………….1 INTRODUCTION………………………………………………………………………………...3 FINDINGS………………………………………………………………………………………..6 REFERENCES…………………………………………………………………………………..15 APPENDIXES Appendix A………………………………………………………………………………………21 Appendix B………………………………………………………………………………………22 Appendix C………………………………………………………………………………………24 Appendix D………………………………………………………………………………………31 Appendix E………………………………………………………………………………………33 ix   

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