Advisers to the Nation on Science, Engineering, and Medicine

National Academy of Sciences

National Academy of Engineering

Institute of Medicine

National Research Council

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 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 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 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 achievement 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 Sciences 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. Dr. Kenneth I. Shine 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. Bruce M. Alberts and Dr. Wm A. Wulf are chairman and vice chairman, respectively, of the National Research Council.

The Committee on Hydrologic Science (COHS) of the National Research Council (NRC) is engaged in studying the priorities and future strategies for hydrologic science. In order to involve a broad community representation, COHS is organizing workshops on priority topics in hydrologic science. These efforts will culminate in reports from the NRC on the individual workshops as well as a synthesis report on strategic directions in hydrologic science. The first workshop—Predictability and Limits-to-Prediction in Hydrologic Systems—was held at the National Center for Atmospheric Research in Boulder, Colorado, September 21–22, 2000. Fourteen technical presentations covered basic research and understanding, model formulations and behavior, observing strategies, and transition to operational predictions (see Appendix A for the agenda).

Understanding the limits of prediction directly impacts the activities and mission goals of federal, state, and local agencies, the relevance of research in the academic community, the engineering practices in industry, and the safety and awareness of the public at large when it comes to water issues. In many contexts (be it predicting the dispersion of a subsurface contaminant plume, the forecasting of a flash flood, the evolution of ecohydrologic systems, or the seasonal variations in stream water chemistry), data and models are used to gain insight to an event. The event may be in the future (classical forecasting), it may simply be downstream, or it may be the outcome of a particular perturbation of the system.

In all these cases, predictions should require (1) fundamental understanding of the dynamics of the system and propagation of perturbations or uncertainty through it, (2) adequate data to characterize the system states, and (3) procedures for producing the expected evolution of the system including interactions among its components. A prediction system additionally requires mechanisms for measuring the accuracy of predictions (often referred to as forecast skill), communicating the predictions to users in an effective manner, and using feedback from the users and the system’s performance to improve the prediction and prediction delivery systems. This brief report is directed to understanding the common features associated with these issues in hydrologic science.

The premise of the workshop was that meaningful and robust prediction systems are built on basic understanding of predictability. Predictability research is directed toward understanding (1) remote and local sources of variability, (2) propagation of uncertainty and variability in hydrologic systems, (3) strategies for identifying and characterizing memories, information pathways, and feedback in hydrologic systems, and (4) quantifying intrinsic and model-derived limits to prediction.

Preceding the workshop the participants were provided with a white paper on the topic that was prepared by COHS members. The white paper was designed to provoke thinking in several critical directions among the participants. In the paper, six main questions were introduced. These questions reemerged during the workshop in order to initiate discussions during three panel-discussion periods. The questions are:

1. Are there predictable aspects of terrestrial hydrology that can enhance atmospheric weather and climate predictability?

2. What are the stability and feedback characteristics of two-way coupled subsurface, surface, and atmospheric hydrologic systems? How do they impact predictability?

3. What are the conceptual and model frameworks required to define limits-to-prediction in hydrologic systems?

4. What are the data and records requirements for estimating the inherent limits-to-prediction directly from observations?

5. What are the opportunities for extending the lead time and accuracy of hydrologic predictions based on predictable weather and climate patterns so that the predictions meet the requirements of water resource and other applications?

6. What are the robustness and predictability criteria for models used in impact studies (e.g., hydrologic impacts of land use and global change)?

The workshop contained energetic and substantial discussions. It was evident that the topic resonated with the interests of the research community and the demands of federal agencies and international research programs. There are currently a number of U.S. agencies—e.g., the interagency U.S. Global Change Research Program (USGCRP)—and international research programs—e.g., World Climate Research Program’s (WCRP) GEWEX and CLIVAR—that identify predictability of hydrologic systems as being among their priorities. More specific agency examples include National Oceanic and Atmospheric Administration’s GEWEX American Prediction Project (GAPP), National Aeronautics and Space Administration’s Seasonal-to-Interannual Predictability Project (NSIPP), and National Weather Service’s Advanced Hydrologic Prediction System (AHPS). Representatives from these and other agencies supporting COHS opened the workshop by defining their program requirements and objectives in predictability science related to the water cycle.

The report is divided into 4 chapters. Chapter 1 gives details of the workshop and provides the motivation for this present report. The definitions of three different types of limits-to-prediction are given in Chapter 2. Chapter 3 builds on the workshop presentations and frames the challenges in predictability science. Chapter 4 provides conclusions based on the workshop presentations and committee discussions. These conclusions present some of the promising scientific directions that could provide a starting point for either understanding the predictability of hydrologic systems or identifying what the limits to prediction are in these systems. The hope of the committee is that both the research and user communities find the discussions of this report useful both for implementing research strategies and for identifying the

ways in which predictability research can be integrated more effectively into operational activities.

Dara Entekhabi, Chair

Committee on Hydrologic Science

This report has been reviewed 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 will assist the institution in making its published report as sound as possible and to ensure that the report meets 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 wish to thank the following individuals for their review of this report:

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 report before its release. The review of this report

was overseen by Eugenia Kalnay, University of Maryland. Appointed by the National Research Council, she 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 authoring committee and the institution.