Consensus Study Report

NATIONAL ACADEMIES PRESS 500 Fifth Street, NW Washington, DC 20001

This study was supported by a contract between the National Academy of Sciences and the National Oceanic and Atmospheric Administration (contract number 1305M322DNRMA0003 / 1305M324F0183), and funding from the Interstate Council on Water Policy, the National Academy of Sciences Arthur L. Day Fund, and the National Academy of Sciences Maurice Ewing and Planetary Sciences Fund. 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: 978-0-309-99526-9
Library of Congress Control Number: 2026949259
Digital Object Identifier: https://doi.org/10.17226/29217

This publication is available from the National Academies Press, 500 Fifth Street, NW, Keck 360, Washington, DC 20001; (800) 624-6242; https://nap.nationalacademies.org.

The manufacturer’s authorized representative in the European Union for product safety is Authorised Rep Compliance Ltd., Ground Floor, 71 Lower Baggot Street, Dublin D02 P593 Ireland; www.arccompliance.com.

Copyright 2026 by the National Academy of Sciences. National Academies of Sciences, Engineering, and Medicine and National Academies Press and the graphical logos for each are all trademarks of the National Academy of Sciences. All rights reserved.

Printed in the United States of America.

Suggested citation: National Academies of Sciences, Engineering, and Medicine. 2026. Improving Future U.S. Drought Assessment. Washington, DC: National Academies Press. https://doi.org/10.17226/29217.

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. Tsu-Jae Liu 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.nationalacademies.org.

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.

Rapid Expert Consultations published by the National Academies of Sciences, Engineering, and Medicine are authored by subject-matter experts on narrowly focused topics that can be supported by a body of evidence. The discussions contained in rapid expert consultations are considered those of the authors and do not contain policy recommendations. Rapid expert consultations are reviewed by the institution before release.

For information about other products and activities of the National Academies, please visit www.nationalacademies.org/purpose.

COMMITTEE ON THE FUTURE OF DROUGHT IN THE UNITED STATES

JONATHAN T. OVERPECK (Chair), University of Michigan

AMIR AGHAKOUCHAK, University of California, Irvine

CLARA DESER, National Science Foundation - National Center for Atmospheric Research

TRENT W. FORD, Illinois State Water Survey, University of Illinois, Urbana-Champaign

KRISTIANA M. HANSEN, University of Wyoming

RICHARD R. HEIM, JR., National Oceanic and Atmospheric Administration (retired)

JENNIFER HENDERSON, Texas Tech University

ZACHARY H. HOYLMAN, Montana Climate Office, University of Montana

YUSUKE KUWAYAMA, University of Maryland, Baltimore County

VENKATARAMAN LAKSHMI, University of Virginia

JUSTIN S. MANKIN, Dartmouth College

KAREN A. MCKINNON, University of California, Los Angeles

Study Staff

STEVEN STICHTER, Senior Program Officer, Earth Systems and Resources Program Area, Study Director

DEBORAH GLICKSON, Senior Program Director, Earth Systems and Resources Program Area

CHARLES R. BURGIS, Program Officer, Earth Systems and Resources Program Area

DOMINIQUE JENKINS, Senior Program Assistant, Earth Systems and Resources Program Area

Reviewers

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:

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 JAMES S. (JAY) FAMIGLIETTI, Arizona State University, and

JAY R. LUND, University of California, Davis. 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.

Acknowledgments

The committee wishes to thank Britt Parker, Veva Deheza, Elizabeth Ossowski, Joel Lisonbee, and Andrew Hoell, from the National Oceanic and Atmospheric Administration (NOAA) National Integrated Drought Information System, who were liaisons to the study. They oversaw the contract and provided helpful information to the committee.

Additionally, the committee thanks Beth Callaway and Danny Johnson from the Interstate Council on Water Policy (ICWP), who were also liaisons to the study, providing information from the perspectives of ICWP member groups.

The study committee met as a full group four times in person and three times virtually over approximately 12 months to gather information for the study, deliberate on the charge in the statement of task, and write the report. The committee thanks the following individuals for presentations during these meetings and for submitted materials:

From State Climatology Offices: John Nielsen-Gammon, Texas; Laura Edwards, South Dakota; Russ Schumacher, Colorado

From Regional Climate Centers (RRCs) and Climate Adaptation Partnerships (CAPs): Beth Hall, Midwestern RCC; Tim Brown, Western RCC; Michael Crimmins, Southwest CLIMAS CAP; Rick Thoman, Alaska Center for Climate Assessment and Policy

From other organizations: Brian Fuchs, National Drought Mitigation Center, University of Nebraska–Lincoln; Chris Milly, U.S. Geological Survey, emeritus; Upmanu Lall, Arizona State University; Benjamin Cook, Columbia University; Sujay Kumar, NASA; Michael Dettinger, Scripps; Robert S. Webb, former NOAA Physical Sciences Lab; Andy Wood, Colorado School of Mines; Balaji Rajagopalan, University of Colorado; Mark Brusberg, retired U.S. Department of

Agriculture; Terry Fulp, former U.S. Bureau of Reclamation; Michael Anderson, California Department of Water Resources, and California State Climatologist; Elizabeth Kerby, Missouri Department of Natural Resources; Seth Shanahan, Southern Nevada Water Authority

Project Management

Steven Stichter managed the study and assisted the committee in drafting the report with support from Charles R. Burgis, under the guidance of Deborah Glickson. Dominique Jenkins provided administrative and logistical support. Lauren Everett managed the report review process.

Contents

ACKNOWLEDGMENTS

PREFACE

SUMMARY

The Nation’s Drought Assessment Enterprise

The Changing Nature of Drought

Designing a Two-Pronged Approach for Incorporating Nonstationarity

Understanding and Assessing the Drivers of Nonstationary Drought

Drought Assessment Data, Indicators, and Indices

Drought Impacts and Impact Data

Artificial Intelligence and Machine Learning Applications in Drought Assessment

Vision for Drought Assessment under Nonstationarity

1 INTRODUCTION

Need and Opportunity for Improved U.S. Drought Assessment

Building on a Strong Foundation

Committee Charge and Statement of Task

Audiences for the Report

A Two-Pronged Strategy for Characterizing and Assessing Drought and for Supporting Drought Decision-making

Risk and Decision Framing in Drought Assessment

Structure, Transparency, and Reproducibility in Drought Assessment

Report Roadmap

2 DEFINING DROUGHT MONITORING AND ASSESSMENT

Defining Drought

Drought Assessment

3 DRIVERS OF DROUGHT IN A CHANGING CLIMATE

Physical Factors That Drive Drought

Spatial and Seasonal Diversity in Drought

Drivers of Drought Trends

Are We Heading to a Drier or Wetter Future … Or Both?

4 NONSTATIONARITY, ARIDIFICATION, AND DROUGHT

Overview of Drought Nonstationarity

Aridification and Nonstationarity

Nonstationarity and Decision-making

Existing and Emerging Approaches for Drought Nonstationarity

Limitations of Existing Methods

5 DROUGHT ASSESSMENT DATA SOURCES AND NONSTATIONARITY

Data Sources and Requirements for Understanding Drought Under Nonstationarity

Data Needs for Decision-making

Evaluation Frameworks for Drought Products

6 DROUGHT INDICATORS, INDICES, AND NONSTATIONARITY

Drought Indicators and Indices

Analytical Choices Under Nonstationarity

Drought Severity Thresholds

From Indicators to Indices: The Role of Standardization

Convergence of Evidence

System-Specific Perspectives

Uncertainty in Drought Indices

Opportunities to Leverage AI/ML in Drought Science

7 DROUGHT IMPACTS, IMPACT DATA, AND NONSTATIONARITY

Drought Impacts

Risk Framework for Impacts

Drought Impact Data, Collection, and Reporting

Incorporating Impact Data into Drought Assessment

8 FRAMEWORK FOR INCORPORATING NONSTATIONARITY INTO DROUGHT ASSESSMENT

Vision

Implementation

Challenges and Unanswered Questions

Summary

9 FRAMEWORK IN ACTION

Short-term Operational Drought Assessment

Long-term Drought Planning

Implications of the Framework for Drought Assessment and Policy

Conclusion

REFERENCES

Appendix A Committee Member and Staff Biographical Sketches

Appendix B Report Conclusions and Recommendations

Appendix C Code Used for Generating Selected Graphics

Boxes, Figures, and Tables

BOXES

S-1 The U.S. Drought Monitor

1-1 Study Statement of Task

2-1 Types of Drought

2-2 Case Study: The Ongoing U.S. Megadrought

2-3 Case Study: The Expanding Impact of U.S. Flash Drought (2012 and 2017 Examples)

2-4 Case Study: Persistent Drought Risk in an Increasingly Wet Climate. The Southeast U.S. Drought of 2011–2012

2-5 Case Study: Drought in Alaska

2-6 The U.S. Drought Monitor

8-1 Considerations for a National Drought Assessment Framework

FIGURES

S-1 Elements of the proposed two-pronged assessment framework

2-1 A schematic showing the translation of precipitation deficiencies during a hypothetical 4-year period through other components of the hydrologic cycle over time

2-2 Classical definitions of drought and the associated processes

2-3 Impact of anthropogenic climate change on the current U.S. megadrought

2-4 Illustration of drought onset and intensification seen with the 2012 U.S. flash drought

2-5 Extent of 2017 flash drought rainfall deficit

2-6 U.S. Drought Monitor assessment of Southeast drought on June 21, 2011

2-7 Matrix of the core components of drought assessment, including their primary purpose(s), existing examples, and needs of each

3-1 A schematic demonstrating land-atmosphere feedbacks that can amplify drought

4-1 Drought nonstationarity and its consequences for drought monitoring and assessment

4-2 USDM climatology and evidence for drought nonstationarity

4-3 The development and attribution of the most severe and persistent droughts in Southwestern North America (SWNA) since 800 CE

4-4 Aridification and drought convergence across the continental United States

5-1 Data source availability for drought assessment under nonstationarity: Contiguous United States (CONUS) Hydrological Observing Systems (1785–2025)

6-1 Nonstationary drought trends in Blaine County, Idaho, based on moving rolling 30-year climatologies reference periods

6-2 Nonstationary drought trends in Palm Beach County, Florida, based on moving 30-year climatologies

6-3 Synthetic examples illustrating how nonstationarity influences drought characterization under a percentile-based classification framework

6-4 The computation procedure of nonparametric ESMI and parametric SMI indices

6-5 Application of monthly (a) ESMI, (b) CHIRPS, and (c) CHIRTS datasets for the July 2012 drought over North America

7-1 Examples of drought indicators, indices, and impacts for different drought types

7-2 Attributes of a system that can properly incorporate impact data into drought assessment

8-1 Conceptual illustration of baseline choice in drought assessment

9-1 Maps of 90-day Standardized Precipitation Index (SPI) on March 18, 2026, showing SPI-based drought categories across the Mid-South United States

9-2 Maps of 90-day Standardized Precipitation Evapotranspiration Index (SPEI) on March 18, 2026, showing SPEI-based drought categories across Nebraska

9-3 Thirty-day SPEI for the period ending June 30, 2021, evaluated against contemporary and future GEV distributions under SSP3-7.0 (EC-Earth3) using the GAMLSS approach

9-4 Scenario planning example from the Intermountain West

9-5 Elements of the proposed two-pronged assessment framework

TABLES

S-1 USDM Drought Monitor Categories

1-1 Location of Core Text Addressing Statement of Task Components

1-2 Core Report Terms

2-1 USDM Drought Monitor Categories

4-1 Diverse Approaches to Diagnosing Aridity and Its Underlying PET Estimates

5-1 Characteristics of Representative Data Sources for Drought Monitoring and Analysis

6-1 A Modified Drought Classification Schema for the USDM Drought Categories

Preface

Droughts of the twenty-first century have been unprecedented, less a reflection of what has come before than a harbinger of what is ahead. Reflected in record low spring snowpack and record high temperatures, the United States is now 27 years into the first multidecadal “megadrought” in the nation’s history. In less than three decades, the megadrought has highlighted how warming temperatures across the West can intensify soil moisture declines, forest mortality, and wildfire impacts, including wildfires that destroyed lives and livelihoods. During the same twenty-first century period, the United States has seen other unusually devastating droughts across the Great Plains, in the Southeast, and in Hawaii. Even my home state of Michigan has seen typical summer dry spells become persistent and costly droughts.

These intensifying drought conditions spurred this report, which was commissioned by federal and state leaders who work hard to provide society’s decision-makers with assessments of ongoing droughts and information for dealing with these and future droughts, even as the job gets more challenging due to nonstationarity. Their work includes collaboration with scientists, modelers, observing systems, and perhaps most visibly, the individuals and organizations across the country and continent who produce regular updates to the U.S. Drought Monitor, the North American Drought Monitor, and other regular products essential for decision-making in every state of the country. The job of this National Academies study team and this report is to recommend how to build on the solid foundation of drought assessment that already exists in the United States, and in doing so increase our nation’s resilience to nonstationarity as it responds to worsening drought and its impacts across the country.

The present report does two things. First, it shows the ways in which current drought assessment practices can be impacted by nonstationarity, including in ways that might make droughts more difficult to manage, and thus more costly. Second, this report provides tractable strategies to update the nation’s current strong foundation of

drought assessment to become more robust and helpful in the context of nonstationarity. We are on the cusp of a new era of drought assessment innovation and resilience that we hope will be sparked by this report and involve the collaborative efforts of federal and state experts, the broader scientific research community, and all those in society who have a stake in lessening the impacts and costs associated with the increasing challenge of drought.

This report is the work of a dozen scientific experts who volunteered their time and expertise to study how to help this country and its people become more resilient to the worsening impacts of drought under a changing climate—to these colleagues I wish to extend my deepest thanks. I also want to thank those from around the country who answered the committee’s call for information on the execution and use of drought assessments. And importantly, on behalf of the whole team, I want to extend my great appreciation to the excellent professional staff of the National Academy of Sciences, particularly Steven Stichter, Charles Burgis, and Dominique Jenkins, as well as to all those who helped with the peer review of this work. This report was a team effort, and we were lucky to have had such a great team.

Jonathan T. Overpeck, Chair
Committee on the Future of Drought in the United States
June 2026

Acronyms and Abbreviations