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GROUNDWATER Fl INTERFACES Committee on Hydrologic Science Water Science and Technology Board Board on Atmospheric Sciences and Climate Division on Earth Life Studies NATIONAL RESEARCH COUNCIL OF THE NATIONAL ACADEMIES THE NATIONAL ACADEMIES PRESS Washington, D.C. www.nap.edu

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TTIE NATIONAL ACADEMIES PRESS 500 Fifth Street, N.W. Washington,D.C. 20001 NOTICE: The project that is the subject of this report was approved by the Governing Board of the Na- tional Research Council, whose members are drawn from the councils of the National Academy of Sci- ences, the National Academy of Engineering, and the Institute of Medicine. The members of the commit- tee responsible for the report were chosen for their special competences and with regard for appropriate balance. Support for this project was provided by Be Amy Research Office, Bacons Aeronautics and Space A - niskation under Grant No. NAG5-865 I, National Oceanic and Atmospheric A~ninis~ation, National Science Foundation under Grant No. EAR-9986796, National Weaker Service, U.S. Environmental Protection Agency under Giant No. X- 2895301, and U.S. Geological Survey. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U. S. Government. International Standard Book Number 0-309-09 ~ ~ 3-6 (Book) International Standard Book Number 0-309-51962-4 (PDF) Groundwater Fluxes Across Interfaces is 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); Tnternet, http://www.nap.edu Cover photograph: Savica springs and waterfalls, Triglav National Park, SIovenia, provided by Bostjan Burger(2003~. http://www.burger.si/SlikaDneva/teme/walipaper_slapovi.htmI. Copyright 2004 by the National Academy of Sciences. All rights reserved. Printed in the United States of America

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THE NATIONAL ACADEMIES Advisers to the Nation on Science, Engineering, and Medirine The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and tech- nology 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 scien- tific and technical matters. Dr. Bruce M. Alberts is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Acad- emy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administra- tion 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 supe- rior achievements of engineers. Dr. Wm. A. Wulf 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 services 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 Sci- ences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Institute of Medicine. Dr. Harvey V. Fineberg is president of the The National Research Council was organized by the National Academy of Sciences in 1916 to associ- ate the broad community of science and technology with the Academy's purposes of furthering knowI- edge and advising the federal government. Functioning in accordance with general policies detennined 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. Bruce M. Alberts and Dr. Wm. A. Wulf are chair and vice chair, re- spectively, of the National Research Council. www.nationaI-acaclemies.org

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COMMITTEE ON HYDROLOGIC SCIENCE1 ERIC F. WOOD, Chair, Princeton University, Princeton, New Jersey MARY P. ANDERSON, University of Wisconsin, Madison VICTOR R. BAKER, University of Arizona, Tucson DARA ENTEKHABI, Massachusetts Institute of Technology, Cambridge (through December 3 I, 2002) NANCY B. GRIMM, Arizona State University, Tempe GEORGE M. HORNBERGER, University of Virginia, Charlottesville DENNIS P. LETTENMAIER, University of Washington, Seattle WIGWAM K. NUTTLE, Consultant, Ottawa, Ontario, Canada (through December 3 i, 2002) KENNETH W. POTTER, University of Wisconsin, Madison (through December 31, 2002) JOHN 0. ROADS, Scripps Institution of Oceanography, La Jolla, California (through December 3 I, 2002) JOHN L. WILSON, New Mexico Tech, Socorro, New Mexico NRC Staff WILLIAM S. LOGAN, Project Director, Water Science and Technology Board ANITA A. HALL, Senior Project Assistant, Water Science and Technology Board ~ The activities of the Committee on Hydrologic Science are overseen and supported by the NRC's Water Science and Technology Board and Board on Atmospheric Sciences and Climate (see Appendix D). v

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Preface This report is a product of the Committee on Hydrologic Science (COHS), which was established in the NRC in late 1998. The committee exists for two separate but related purposes. These are: (1) to advance hydrologic science through the identification of research and educational opportunities in hydrologic sciences, including data aspects, and (2) to provide advice to U.S. government agencies and interagency efforts on program strategy with respect to hydrologic content and research priorities, and to advise on U.S. involvement in international pro- grams such as the World Climate Research Program (WCRP) and the International Geosphere-Biosphere Pro- gram (TGBP). ~ its relatively short history, the COHS has published three reports. The first was Hydrologic Science Priorities for the U.S. Global Change Research Program: An Initial Assessment MARC, 1999~. The second report, Report of a Workshop on Predictability and Limits-to-Prediction in Hydrologic Systems MARC, 2002a), was based on a workshop on this topic held in September 2000 in Boulder, CO. The Bird report, Review of USGCRP Plan for a New Science Initiative on the Global Water Cycle MARC, 2002b), was a fastback review of a report commissioned by the USGCRP. This report, like the Predictability report, was based on a workshop. This workshop, titled "Ground- water Fluxes Across Interfaces," was organized and convened by COHS members Mary P. Anderson and John L. Wilson, and was held in Egg Harbor, WI on May 12-14, 2002. The participants were asked to provide wnt- ten material prior to meeting and to present brief prepared oral statements during the workshop. They were asked to examine and assess currently used and proposed new methods to estimate recharge and discharge rates to identify methods that hold the most promise for addressing three general issues related to groundwater fluxes. These were: diffuse vs. focused fluxes, climate feedback functions, and spatial and temporal scales. The participants were divided into three subgroups and as a stardng point for discussion, were asked to con- sider the following issues and associated questions. Participants, however, were encouraged to redefine issues and develop new questions as the workshop progressed. Subgroup on Diffuse and Focused Recharge/Discharge drogeologic setting? What is the relative importance of diffuse versus focused recharge/discharge in any specific hy- . . v''

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. ~ . V111 oceans? Preface Is fresh groundwater discharge a significant source of fresh water recycling to estuaries and the What is the role of landscape vegetation cover (including phreatophytes) on spatial and temporal groundwater recharge/discharge patterns? patterns? What are the landscape indicators and big-indicators of groundwater recharge/-discharge? What are the effects of human activities on spatial and temporal groundwater recharge/discharge Subgroup on Climate Feedback Functions water balances? mate? Are fluxes to and from groundwater reservoirs important components of continental and global Do groundwater recharge and discharge processes provide feedback mechanisms that affect cTi- How do spatial patterns of groundwater recharge/discharge change seasonally/-annually? What are the important time scales for groundwater reservoirs affecting continental and global water balances, and how are they controlled by fluxes and storage? What is the magnitude of the effect of fluctuations of sea level and levels of large lakes (e.g., the Great Lakes) on groundwater recharge/discharge? Subgroup on Spatial and Temporal Scales How do estimates of groundwater recharge/discharge aggregate when averaged over different scales and what implications does this have for measurement scale? At the scale of a representative elementary volume, does groundwater recharge/discharge occur along preferential pathways? If so, when is it important to measure fluxes at this scale? How accurately can recharge/discharge patterns/rates be mapped over the contiguous U.S., and how does uncertainty in these patterns/rates vary with spatial and temporal scale and geographic location? In addressing the three broad sets of issues outlined above, the group was also asked to consider the following questions: 1. What are the most promising techniques for measuring/estimating groundwater recharge? What are the most promising techniques for measuring/estimat~ng groundwater discharge? What are the potential errors associated with applying these techniques at various spatial scales (i.e., is it possible to extrapolate point measurements of groundwater flux in order to obtain accurate estimates over broad geographical areas?) 2. What sorts of monitoring networks, instrumentation, and analyses are required to produce maps of seasonal recharge and discharge? 3. What scales of measurement and measurement techniques are most appropriate for assessing bio- geochem~cal processes occurring within the groundwater-surface water interface and the ejects of groundwa- ter discharge on ecosystems? The agenda for the workshop is in Appendix A. The hydrogeology of the Door Peninsula region of Wisconsin, based on a workshop presentation by Kenneth R. Bradbury of the Wisconsin Geological and Natu- ral History Survey, is summarized in Append~x B.

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Preface IX Following the formal presentations, participants engaged in group discussions and group writing ses- sions. The written material provided by the participants formed the nexus of the first draft of this report. Some participants also contributed written materials at a later time. We Bust that this report will be useful to scientists and managers who work in groundwater, surface water, and land surface hydrology, as well as others interested in the cycling of water and its dissolved con- stituents. Eric F. Wood, Chair Committee on Hydrologic Science Mary P. Anderson and John L. Wilson, co-chairs Workshop on Groundwater Fluxes Across Interfaces

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Xl Acknowledgments This report could not have been written without many days of writing and discussion on the part of the workshop participants. Those noted below contributed written materials prior to the workshop, helped in the synthesis of these materials, participated in the workshop discussions, and contributed written material to the report itself either at or following the workshop. We are gratefi~l to all of these participants for their ideas, their time, their energy and their fortitude. We hope that they also derived benefit from what turned out to be a highly productive and stimulating experience. Jean M. Bahr, University of Wisconsin, Madison Kenneth R. Bradbury, Wisconsin Geological & Natural History Survey, Madison Michael D. Dettinger, U. S. Geological Survey, San Diego Christopher I. DuffY, Pennsylvania State University Hendrik M. Haitjema, Indiana University Stacy E. How~ngton, U.S. Army ERDC Randall J. Hunt, U. S. Geological Survey, Milton, W! P. C. D. (Chris) Milly, U. S. Geological Survey, Princeton, N] Shiomo P. Neuman, University of Arizona, Tucson Matth~ ew Rodell, NASA Goddard, GreenbeIt, MD Guido D. Salvucci, Boston University Bridget R. Scanion, Texas Bureau of Economic Geology, Austin John S. Selker, Oregon State University, Corvallis Donald I. Siegel, Univ. of Syracuse, NY l:~esTie Smith, Univ. of British Columbia, Vancouver, Canada William W. Woessner, University of Montana, Missoula Alan Flint, U. S. Geological Survey, Sacramento, kindly contnbuted material for Box 4-2. Additional insights into this topic were provided by the participants in Mary Anderson's graduate seminar at the Univer- sity of Wisconsin-Madison held dunng the fall semester, 2002. This report has been reviewed in drain form by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the NRC's Report Review Committee. The purpose of this independent review is to provide candid and critical comments that wall assist the institution in making its published report as sound as possible and to ensure that the report meets institutional standards for i

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. . Xll Acknowledgments objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript re- main confidential to protect the integrity of the deliberative process. We wish to thank the following individuals for their review of this report: Peter G Cook CSTRO Land and Water, Australia Paul A. Ferre, University of Arizona Efi Foutoula-Georg~ou, University of Minnesota, Minneapolis Glendon Gee, Pacific Northwest National Laboratory, RichIand, Washington F. Edwin Harvey, University of Nebraska-Lincoin Mark Person, Indiana University Mary W. Stoertz, Ohio University, Athens Although the reviewers listed above have provided many constructive comments and suggestions, they were not asked to endorse the conclusions or recommendations nor did they see the final draft of the re- port before its release. The review of this report was overseen by Dr. Lynn W. GeThar, Massachusetts Institute of Technology. Appointed by the National Research Council, Dr. Gelhar was responsible for making certain that an independent examination of this report was earned 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 authoring committee and the institution.

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Contents EXECUTIVE SUMMARY INTRODUCTION Statement of Task, ~ Basic Concepts Related to Groundwater Recharge/Discharge, 11 Overview of Methods for Estimating Recharge and Discharge, 12 Scope, 15 DIFFUSE AND FOCUSED RECHARGE AND DISCHARGE Experimental Benchmark Sites, 17 rNTERACTIONS OF GROUNDWATER WITH CLIMATE Influence of Climate on Groundwater, 33 Influence of Groundwater on Climate, 38 Effects on Atmospheric Composition, 40 4 SPATIAL AND TEMPORAL SCALES OF RECHARGE AND DISCHARGE Nexus of Technology and Need, 42 Defining of Scaling, 46 Expected Scaling Behavior of Recharge/Discharge Fluxes, 49 Strategy to Address the Issue of Scale, 49 Relation of Infiltration to Groundwater Recharge, 53 Variation of Focused and Diffuse Discharge with Measurement Scale, 54 Use of Remotely SensedlMapped Parameters as Surrogates for Recharge/Discharge, 55 FINDINGS AND RECOMMENDATIONS . . . Xlll s 16 32 42 56

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XIV REFERENCES APPENDIX A WORKSHOP AGENDA APPENDIX B HYDROGEOLOGY OF DOOR COUNTY APPENDIX C AGENCY INTEREST IN GROUNDWATER FLUXES APPENDIX D NATIONAL RESEARCH COUNCIL BOARD MEMBERSHIP AND STAFF APPENDIX E BIOGRAPHICAL SKETCHES OF MEMBERS OF THE COMMITTEE ON HYDROLOGIC SCIENCE Contents 59 69 71 74 79 83