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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2018. Review of the New York City Department of Environmental Protection Operations Support Tool for Water Supply. Washington, DC: The National Academies Press. doi: 10.17226/25218.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2018. Review of the New York City Department of Environmental Protection Operations Support Tool for Water Supply. Washington, DC: The National Academies Press. doi: 10.17226/25218.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2018. Review of the New York City Department of Environmental Protection Operations Support Tool for Water Supply. Washington, DC: The National Academies Press. doi: 10.17226/25218.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2018. Review of the New York City Department of Environmental Protection Operations Support Tool for Water Supply. Washington, DC: The National Academies Press. doi: 10.17226/25218.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2018. Review of the New York City Department of Environmental Protection Operations Support Tool for Water Supply. Washington, DC: The National Academies Press. doi: 10.17226/25218.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2018. Review of the New York City Department of Environmental Protection Operations Support Tool for Water Supply. Washington, DC: The National Academies Press. doi: 10.17226/25218.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2018. Review of the New York City Department of Environmental Protection Operations Support Tool for Water Supply. Washington, DC: The National Academies Press. doi: 10.17226/25218.
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Page viii Cite
Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2018. Review of the New York City Department of Environmental Protection Operations Support Tool for Water Supply. Washington, DC: The National Academies Press. doi: 10.17226/25218.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2018. Review of the New York City Department of Environmental Protection Operations Support Tool for Water Supply. Washington, DC: The National Academies Press. doi: 10.17226/25218.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2018. Review of the New York City Department of Environmental Protection Operations Support Tool for Water Supply. Washington, DC: The National Academies Press. doi: 10.17226/25218.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Review of the New York City Department of Environmental Protection Operations Support Tool for Water Supply Committee to Review the New York City Department of Environmental Protection Operations Support Tool for Water Supply Water Science and Technology Board Division on Earth and Life Studies This prepublication version of Review of the New York City Department of Environmental Protection  Operations Support Tool for Water Supply has been provided to the public to facilitate timely access to  the report. Although the substance of the report is final, editorial changes may be made throughout the  text and citations will be checked prior to publication. The final report will be available through the  National Academies Press later this year, 2018.  A Consensus Study Report of PREPUBLICATION COPY

THE NATIONAL ACADEMIES PRESS 500 Fifth Street, NW Washington, DC 20001 This activity was supported by the New York City Department of Environmental Protection under Contract No. CAT‐447. Any opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of any organization or agency that provided support for the project. International Standard Book Number‐13: International Standard Book Number‐10: Digital Object Identifier: https://doi.org/10.17226/25218 Additional copies of this publication are available for sale from the National Academies Press, 500 Fifth Street, NW, Keck 360, Washington, DC 20001; (800)624‐6242 or (202) 334‐3313; http://www.nap.edu. Copyright 2018 by the National Academy of Sciences. All rights reserved. Printed in the United States of America Suggested citation: National Academies of Sciences, Engineering, and Medicine. 2018. Review of the New York City Department of Environmental Protection Operations Support Tool for Water Supply. Washington, DC: The National Academies Press. doi: https://doi.org/10.17226/25218. PREPUBLICATION COPY

The National Academy of Sciences was established in 1863 by an Act of Congress, signed by President Lincoln, as a private, nongovernmental institution to advise the nation on issues related to science and technology. Members are elected by their peers for outstanding contributions to research. Dr. Marcia McNutt is president. The National Academy of Engineering was established in 1964 under the charter of the National Academy of Sciences to bring the practices of engineering to advising the nation. Members are elected by their peers for extraordinary contributions to engineering. Dr. C. D. Mote, Jr., is president. The National Academy of Medicine (formerly the Institute of Medicine) was established in 1970 under the charter of the National Academy of Sciences to advise the nation on medical and health issues. Members are elected by their peers for distinguished contributions to medicine and health. Dr. Victor J. Dzau is president. The three Academies work together as the National Academies of Sciences, Engineering, and Medicine to provide independent, objective analysis and advice to the nation and conduct other activities to solve complex problems and inform public policy decisions. The National Academies also encourage education and research, recognize outstanding contributions to knowledge, and increase public understanding in matters of science, engineering, and medicine. Learn more about the National Academies of Sciences, Engineering, and Medicine at www.national-academies.org. PREPUBLICATION COPY

Consensus Study Reports published by the National Academies of Sciences, Engineering, and Medicine document the evidence-based consensus on the study’s statement of task by an authoring committee of experts. Reports typically include findings, conclusions, and recommendations based on information gathered by the committee and the committee’s deliberations. Each report has been subjected to a rigorous and independent peer-review process and it represents the position of the National Academies on the statement of task. Proceedings published by the National Academies of Sciences, Engineering, and Medicine chronicle the presentations and discussions at a workshop, symposium, or other event convened by the National Academies. The statements and opinions contained in proceedings are those of the participants and are not endorsed by other participants, the planning committee, or the National Academies. For information about other products and activities of the National Academies, please visit www.nationalacademies.org/about/whatwedo.   PREPUBLICATION COPY

  COMMITTEE TO REVIEW THE NEW YORK CITY DEPARTMENT OF ENVIRONMENTAL PROTECTION OPERATIONS SUPPORT TOOL FOR WATER SUPPLY DEBRA S. KNOPMAN, Chair, RAND Corporation, Arlington, Virginia MONICA B. EMELKO, University of Waterloo, Waterloo, Canada PAUL L. FREEDMAN, LimnoTech, Ann Arbor, Michigan JEROME B. GILBERT, NAE, Consulting Engineer, Orinda, California ROBERT M. HIRSCH, U.S. Geological Survey, Reston, Virginia KIMBERLY L. JONES, Howard University, Washington, DC CYNTHIA E. ROSENZWEIG, Goddard Institute for Space Studies, New York, New York KAREN S. SKLENAR, The Cadmus Group, Inc., West Chester, Pennsylvania JOHN E. TOBIASON, University of Massachusetts, Amherst JAMES G. UBER, University of Cincinnati, Cincinnati, Ohio ERIC F. WOOD, NAE, Princeton University, Princeton, New Jersey National Academies Staff LAURA J. EHLERS, Study Director, Water Science and Technology Board BRENDAN McGOVERN, Senior Program/Research Assistant, Water Science and Technology Board ERIN M. MARKOVICH, Senior Program/Research Assistant, Board on Atmospheric Sciences and Climate PREPUBLICATION COPY v

WATER SCIENCE AND TECHNOLOGY BOARD CATHERINE L. KLING, NAS, Chair, Cornell University, Ithaca, New York DAVID A. DZOMBAK, NAE, Carnegie Mellon University, Pittsburgh, Pennsylvania WENDY D. GRAHAM, University of Florida, Gainesville ARTURO A. KELLER, University of California, Santa Barbara MARK W. LeCHEVALLIER, Dr. Water Consulting, LLC, Morrison, Colorado DINAH LOUDA, Veolia Institute, Paris, France MARGARET A. PALMER, University of Maryland, Annapolis STEPHEN POLASKY, NAS, University of Minnesota, St. Paul DAVID L. SEDLAK, NAE, University of California, Berkeley DAVID WEGNER, Retired, Water, Energy and Transportation Committee, Tucson, Arizona P. KAY WHITLOCK, Christopher B. Burke Engineering, Ltd., Rosemont, Illinois JAMES W. ZIGLAR, SR., Van Ness Feldman, Potomac, MD Staff ELIZABETH EIDE, Acting Director LAURA J. EHLERS, Senior Staff Officer STEPHANIE E. JOHNSON, Senior Staff Officer M. JEANNE AQUILINO, Financial/Administrative Associate COURTNEY DeVANE, Administrative Coordinator BRENDAN R. McGOVERN, Senior Project Assistant/Research Assistant CARLY BRODY, Senior Project Assistant PREPUBLICATION COPY vi

  Preface New York City’s water system reliably delivers 1.1 billion gallons of water per day to more than 8 million residents, about half of the population of New York State. To meet its mission, the New York City Department of Environmental Protection’s (NYC DEP’s) Bureau of Water Supply operates a system of 19 reservoirs and three controlled lakes that drain a nearly 2,000-square-mile watershed that extends 125 miles north and west of New York City. To reliably manage an engineered system of this geographic extent and complexity, the NYC DEP depends on a constant influx of timely and accurate data to guide its operations, along with mathematical modeling tools to support its decision making. To this end, NYC DEP embarked on the development of the Operations Support Tool (OST), to not only enhance operational decisions but specifically to help better address operations under conditions of high turbidity from the Catskill system. OST is a combined water quantity and water quality mathematical simulation model that probabilistically predicts future storage and water quality in the City’s reservoir system by accounting for dozens of variables such as weather forecasts, current demand for water, and myriad changes to the operation of the water supply system. One of the goals of OST is to provide a technical basis for allowing NYC DEP to divert or release water from Schoharie and Ashokan reservoirs in the Catskill system at the most opportune times to moderate the use of alum, a chemical additive widely used to reduce the turbidity of delivered water. Thus, NYC DEP’s use of OST can help ensure delivery of the lowest-turbidity water, while also benefitting downstream communities by managing water releases and enhancing the health of local streams and reducing the risk of flooding. NYC DEP anticipates that OST could have uses well beyond turbidity control throughout the water supply system, to address both water quality and water quantity concerns. To further develop and plan for improvements of OST, the NYC DEP asked the National Academies of Sciences, Engineering, and Medicine to:  Review the City’s use of OST for water supply operations, including managing elevated turbidity, and consider potential ways in which the City can more effectively use OST.  Evaluate the performance measures the City uses to assess the efficacy of the Catskill Turbidity Control Program, and make recommendations for additional performance measures, if necessary.  Review the City’s plan for use of OST in evaluating proposed modifications to the Catalum State Pollutant Discharge Elimination System permit, as well as alternatives to be considered in the associated environmental review.  Review NYC DEP’s existing studies of the potential effects of climate change on the City’s water supply to help identify and enhance understanding of areas of potential future concern with regard to the use of OST. PREPUBLICATION COPY vii

viii Review of the NYC DEP Operations Support Tool for Water Supply The Committee has focused its efforts on addressing these four tasks, and by extension, made several decisions about issues that it would not address. First, the Committee considered OST’s performance in the context of the geographic boundaries and physical processes of the model as currently configured. As a consequence, this report does not consider ecological impacts along the Lower Esopus Creek or the health of the aquatic and benthic communities of the Kensico Reservoir. Similarly, because this report evaluates OST and its performance based on the model’s current configuration, the Committee did not evaluate the efficacy of any possible plans for model enhancements. Second, the Committee took no position on the relative draws from the Delaware and Catskill systems and associated downstream impacts. Although these issues were considered to be outside the scope of this study, they are important as NYC DEP continues to build better relationships with communities affected by how it operates the water supply. Although a number of activities and programs make up the Catskill Turbidity Control Program, evaluation of NYC DEP’s Stream Management Program fell outside the scope of this study. It will, however, be considered during the National Academies’ more comprehensive review of the City’s Water Protection Program that will commence in 2018. Finally, reviewing past decisions made by NYC DEP, with or without the benefit of insight from OST, is also outside of the scope of this report. This Consensus Study Report was reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise. The purpose of this independent review is to provide candid and critical comments that will assist the National Academies of Sciences, Engineering, and Medicine in making each published report as sound as possible and to ensure that it meets the institutional standards for quality, 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: Peter M. Huck, University of Waterloo; Eugene J. LeBoeuf, Vanderbilt University; Jay R. Lund, University of California, Davis; David R. Maidment, University of Texas, Austin; Richard N. Palmer, University of Massachusetts, Amherst; Adam Parris, Brooklyn College City University of New York; Cherie Schultz, Interstate Commission on the Potomac River Basin; and Jery R. Stedinger, Cornell University. Although the reviewers listed above provided many constructive comments and suggestions, they were not asked to endorse the conclusions or recommendations of this report nor did they see the final draft before its release. The review of this report was overseen by John J. Boland, Johns Hopkins University, and Philip C. Singer, University of North Carolina, Chapel Hill. They were responsible for making certain that an independent examination of this report was carried out in accordance with the standards of the National Academies and that all review comments were carefully considered. Responsibility for the final content rests entirely with the authoring committee and the National Academies. Debra Knopman, Chair PREPUBLICATION COPY

  Contents SUMMARY 1 1 INTRODUCTION 11 Description of the New York City Water Supply, 11 Regulatory Framework, 20 Overview of System Operations, 28 Origin of the Study, 31 References, 35 2 DESCRIPTION OF THE OPERATIONS SUPPORT TOOL 37 Overview of the Operations Support Tool, 37 OST Data Flow and Outputs, 44 Models Used in OST, 48 How OST Is Used by NYC DEP, 59 OST Data Input, 64 Validation of OST, 76 Conclusions and Recommendations, 78 References, 80 3 METRICS FOR THE CATSKILL TURBIDITY CONTROL PROGRAM 83 Background, 83 Catskill Turbidity Control Study, 86 Catskill Turbidity Control Program, 88 Performance Measures, 89 Conclusions and Recommendations, 95 References, 96 PREPUBLICATION COPY ix

x Review of the NYC DEP Operations Support Tool for Water Supply 4 USE OF OST WITHIN THE ENVIRONMENTAL IMPACT STATEMENT FOR MODIFICATIONS TO THE CATALUM SPDES PERMIT 97 Background on Environmental Impact Statements, 97 EIS and the Catalum SPDES Permit, 98 Use of OST in the EIS, 103 Conclusions and Recommendations, 109 References, 110 5 USE OF OST IN A CHANGING CLIMATE 111 Observations of a Changing Climate: Impacts on Catskill and Delaware Watersheds in Context, 111 Global- and Regional-Scale Projections of Climate Change, 116 Improving the Utility of OST for Continued Use as an Operational Decision Support Tool, 119 Using OST in Simulation Mode to Better Plan for Impacts of Climate Change on the NYC Water Supply, 123 Coordination Beyond NYC DEP, 130 Conclusions and Recommendations, 131 References, 132 6 ENHANCEMENTS AND FUTURE USES OF OST 137 Use of OST to Capture Existing Staff Knowledge and Expertise, 137 Use of OST as a Training Tool, 138 Long-Term Planning Under a Range of Future Conditions, 139 Use of OST to Support Compliance with the Filtration Avoidance Determination, 140 Use of OST to Manage Other Water Quality Metrics, 140 Use of OST to Help Illuminate and Frame Research Questions, 141 Communication About OST and How It Is Used, 142 References, 142 APPENDIXES A Observed Hydrologic Trends 145 B Use of Climate and Hydrologic Models for Projecting Future Water Resources 155 C Glossary of Terms 165 D Biographical Sketches of Committee Members and Staff 169 PREPUBLICATION COPY

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New York City’s water supply system is one of the oldest, largest, and most complex in the nation. It delivers more than 1.1 billion gallons of water each day from three upstate watersheds (Croton, Catskill, and Delaware) to meet the needs of more than eight million people in the City, one million people in Westchester, Putnam, Orange, and Ulster counties, and millions of commuters and tourists who visit the City throughout the year. The Catskill and Delaware portions, which make up about 90 percent of the supply, receive no filtration or treatment other than disinfection, except for rare instances of high turbidity when a coagulant is added to increase deposition of suspended solids. The remaining 10 percent of the supply comes from the Croton watershed and receives treatment via filtration.

The drinking water supply is managed by the Bureau of Water Supply within the New York City Department of Environmental Protection (NYC DEP). To continue to avoid filtration of the Catskill/Delaware portion of the water supply, in 2007, NYC DEP reexamined its control of turbidity in the Catskill portion of the water supply, including both structural improvements to the system and operational changes. The Operations Support Tool (OST) was developed as part of these efforts. OST couples models of reservoir operations and water quality; it uses real-time data on streamflow, snow pack, water quality, reservoir levels, diversions, and releases; and it incorporates streamflow forecasts—all in order to predict future reservoir levels, water delivery to customers, and water quality within the system. These predictions inform the system operators, who then make decisions based on the most current data and forecasts.

This report reviews the use of OST in current and future reservoir operations. It considers potential ways in which the City can more effectively use OST, makes recommendations for additional performance measures, and reviews the potential effects of climate change on the City’s water supply to help identify and enhance understanding of areas of potential future concern with regard to the use of OST.

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