Hydrologic Sciences: Taking Stock and Looking Ahead (1998)

In 1991, the Water Science and Technology Board's Committee on Opportunities in the Hydrologic Sciences (hereafter referred to as "the committee"), completed its report, Opportunities in the Hydrologic Sciences (National Academy Press, Washington, D.C.). Under the leadership of Peter S. Eagleson, a group of our colleagues crafted a report both sophisticated and engaging—a thoughtful reflection on hydrologic science as a geoscience; an articulation of a vision of research, education, and institutional support in the hydrologic sciences; and a rousing call to action. Over the past six years this report, sometimes known as the "Blue Book," has indeed stimulated actions and a great deal of discussion. It has resulted in the creation of a Hydrologic Sciences Program within the Earth Sciences Directorate of the National Science Foundation and has been cited as a key component in the founding of several hydrologic sciences research and teaching programs in U.S. universities.

At its heart, Opportunities in the Hydrologic Sciences articulates a vision of hydrologic science as a distinct geoscience with a crucial societal role. A key objective of the committee in writing the report was to delineate a "puzzle-driven" science underlying, and complementary to, the "problem-driven" application of that science and attendant technology to problems of water resources. Although the discipline does overlap and "interact" with ocean, atmospheric, and solid earth sciences, as well as ecosystem sciences, the committee views hydrologic science as distinct and unique among the sciences in its focus on continental water processes and the global water balance. This focus leads to a set of fundamental concepts that form a cornerstone of the science and that are central to understanding all of the diverse processes at work in the global water balance.

These ideas include the concepts of fluxes, reservoirs, pathways, and residence times, along with the inevitability of dealing with an extraordinary range of spatial and temporal scales.

In Opportunities in the Hydrologic Sciences this vision is developed, illustrated, and supplemented by presenting representative critical and emerging areas of hydrologic research; by explicitly considering the role of data collection; by considering the nature of an education in hydrologic science; and finally by analyzing priorities, resources, and strategies important to the development of this fledgling geoscience. The report's systematic discussion of research areas includes some that indeed were very active in 1991 and other key lines of research that at the time were in need of nurturing and development. Because hydrologic science is inherently an observational science, the committee emphasizes the central role of data collection, distribution, and analysis in all of the critical and emerging areas of research. The committee also makes a strong case that a distinct hydrologic science cannot exist without an appropriate educational infrastructure. The report presents an approach to hydrologic science education that instills a sense of the internally driven intellectual pursuit of scientific understanding for its own sake and for that of society. Hydrologic education must be multidisciplinary, with programs building on a solid base of education in mathematics and the physical and life sciences at the graduate, undergraduate, and K-12 levels. Finally, the report articulates a set of priority problems that dominate hydrologic science at present. It also makes clear that resources must be redeployed and reorganized if the committee's recommendations are to be achieved.

One recommendation was to review ''in five years the progress toward achieving the goals elaborated in this report, assessing the vitality of the field, surveying the changes that have occurred, and making recommendations for further action'' (p. 16). Beginning in 1995 the Water Science and Technology Board began to consider whether and how to implement this recommendation. The board concluded that a review of the scope envisioned in the committee's recommendation was somewhat premature, since many of the recommendations are just being implemented and the results of research inspired by the committee are still sparse. Further, it is clear that the substantive recommendations of the report have not become outdated. Instead, the board decided to use the occasion of the seventh Abel Wolman Distinguished Lecture to direct the hydrologic community's attention to the vitality of the hydrologic sciences. The board attempted to do this in two ways: first, by challenging the Wolman Lecturer to use the bully pulpit of the Wolman Lecture to reflect freely and personally on the intellectual vitality of the hydrologic sciences; and second, by organizing a one-day symposium that would build on the Wolman Lecture by expounding on some

of the current efforts in the hydrologic sciences in celebration of the sixth anniversary of the Blue Book.

It seems particularly apropos to use the Wolman Lecture for this purpose. Abel Wolman (1892–1989) is the subject of the very first biographical vignette

included in Opportunities in the Hydrologic Sciences. The vignette begins by stating that "in the course of an exceptionally long and active career, Abel Wolman may have done more than any single person to bring the benefits of hydrologic science to the people of the world." Dr. Wolman blended a sophisticated knowledge and curiosity about hydrologic and environmental sciences with an abiding concern for the institutions necessary to secure adequate and safe supplies of water for society. (The complete vignette from Opportunities in the Hydrologic Sciences is included here as a sidebar on p. 3) It seems an appropriate tribute to a great scientist and a stimulating and sophisticated report to bring them together for a day of reflection and review.

This, then, was the starting point for the 1997 Wolman Lecture and Symposium on the Hydrologic Sciences and the papers contained in this volume. These papers both explicitly and implicitly assess progress on a number of issues from the perspective of particular "puzzle areas" in hydrologic science. Some issues cross-cut all of these puzzle areas and so receive attention from all of the authors, whereas other issues are more focused and are addressed by a subset of the authors.

The board was honored that Thomas Dunne of the School of Environmental Science and Management and the Department of Geological Sciences and Geography at the University of California, Santa Barbara, agreed to present the 1997 Abel Wolman Distinguished Lecture. There are few hydrologists at work today who have not been influenced in one way or another by Dunne's thinking and writing. A member of the National Research Council's Committee on Opportunities in the Hydrologic Sciences, Professor Dunne presented a fascinating and thoughtful critique, the text of which is included here.

The symposium featured four particularly active areas of hydrologic inquiry. These areas were identified as emerging and critical in Opportunities in the Hydrologic Sciences and also have been characterized by significant progress over the past decade. These topical areas, as identified by the titles of papers presented at the symposium, are: "Aquatic Ecosystems: Defined by Hydrology," by Diane M. McKnight; "Hydrologic Measurements and Observations: An Assessment of Needs," by Eric F. Wood; "Ground Water Dating and Isotope Chemistry," by Fred M. Phillips; and "Streamflow Prediction: Capabilities, Opportunities, and Challenges,'' by Stephen J. Burges. The goal in choosing these four topics and speakers was not, of course, to be comprehensive or to suggest that these topics are in any sense of highest priority, either in intellectual value or in centrality to hydrologic science. Rather, it was hoped that an assessment of these topics would illustrate the state of the hydrologic sciences half a decade after the publication of Opportunities in the Hydrologic Sciences. To add more depth and breadth to discussions about the vitality of the hydrologic sciences, four discussants were invited to reflect on these primary papers. For these discussants the board enlisted several younger scientists who have already had an impact on the hydrologic sciences and who will likely be intellectual leaders in

the field many years into the future: Kaye Brubaker, Dara Entekhabi, David Genereux, and Efi Foufoula-Georgiou. These short informal reflections are not included here but they stimulated considerable discussion at the symposium and have informed this overview.

Professor Dunne in his lecture provides a fairly explicit assessment of progress toward achieving the Committee on Opportunities in the Hydrologic Sciences' vision of hydrology as a distinct geoscience. His perceptive analysis and eloquent discussion provides insight into the nature of hydrologic science, along with a framework from which to contemplate the contributions of the other four symposium speakers. Professor Dunne's analysis and vision seem thoroughly consistent with that of the committee.

Dunne makes a persuasive case that, while hydrology has recently become recognized as a key element of the Earth sciences, the organizational and research infrastructure, educational institutions, and available funding have not caught up with appreciation for the science. Drawing examples from the increasingly important fields of planetary-scale hydrology and landscape hydrology, Dunne identifies and assesses four key elements essential to the sustenance of hydrologic science:

• the need to construct a coherent body of transferable theory and create an intellectual center for the science;
• the need for communication across multiple disciplines, backgrounds, and approaches;
• the need for appropriate measurements and observations; and
• the need for some level of central guidance and assessment.

The proliferating diversity of approaches to hydrologic puzzles, as opposed to a convergence on a consistent set of theoretical constructs, indicates to Dunne that hydrologic science has not yet reached maturity as a geoscience. He finds further support for this conclusion in the continuing lack of communication, and even intellectual respect, across the artificial boundaries of experimentalist versus theorist, modeler versus field observer, engineer versus earth scientist. The promise and excitement of the growing number of new data sources available to hydrologists are tempered by the recognition that hydrologists have not yet found a way to participate fully in the design of data collection campaigns so that they may contribute directly to the advancement of fundamental hydrologic science. Finally, Dunne contemplates the value of oversight and an institutional focal point for the convergence and continuity that would mark hydrologic science as a distinct and mature geoscience.

Dunne also introduces a number of key themes that appear throughout the papers of the symposium speakers and the comments of the discussants. These

include the central role of scaling in developing understanding across the vast array of spatial and temporal scales of interest to hydrologic science; the importance of basing hydrologic science on an understanding of fundamental physical, chemical, and biological processes; the interpenetration of hydrology with the other Earth and life sciences; and the dominance of climatological and meteorological uncertainties over process uncertainties, particularly at the larger spatial scales of importance in regional, continental, and global hydrology. The four symposium papers in this volume examine these and other issues from several different perspectives.

Use of the hydrologic cycle as a useful framework for interpreting and understanding biological and ecological processes is a recurrent theme throughout Opportunities in the Hydrologic Sciences. One of the committee's Critical and Emerging Areas is entitled "Hydrology and Living Communities." Further, the role of hydrology as a framework for aquatic ecology runs through all five prioritized research areas in the report's chapter on scientific priorities. Ecological processes either play a key role along with hydrologic processes in the research area or the research area is motivated by the role of hydrology in defining aquatic ecosystems. It is clear that no assessment of the state of hydrologic science can ignore the important role of aquatic ecosystems and ecological processes.

Diane McKnight is emphatic in declaring the important role of hydrologic science as an underpinning of aquatic ecosystem science, titling her talk "Aquatic Ecosystems: Defined by Hydrology." She notes that fluxes of water, mass, energy, and organisms do indeed define aquatic ecosystems. Analysis and prediction of these fluxes play a dominant role in understanding aquatic ecosystems, and hydrologic processes control many of these rates. In addition, many important integrative concepts in ecosystem science derive from the topology and directionality of hydrologic systems. "Upstream" and "downstream" matter in aquatic ecology just as much as they matter in hydrology. Finally, although not emphasized by McKnight, it is equally clear that ecological processes play much more than a passive role in controlling hydrologic processes.

In assessing hydrologic science vis-à-vis aquatic ecosystems, McKnight emphasizes the complexities of spatial and temporal scales and of scale mismatches, in both space and time. For example, she notes that scale mismatches can lead to hydrologic data collection designs which miss or fail to resolve key ecological fluxes and processes. Also, microscopic processes can play key roles in macroscopic behavior, either hydrological or ecological, with all the attendant challenges of scaling and parameterization.

McKnight's consideration of the role of hydrologic science in ecosystem science raises another important issue, one that threads its way through all of the papers in this volume: the joy and difficulty of multidisciplinarity. The complex-

ity and scope of the hydrologic sciences, along with ecosystem sciences, provide ample opportunity for contributions from many different disciplines. The joy rests in the synergism of multiple perspectives, approaches, and backgrounds. Difficulties arise, however, because educational institutions, research funding entities, professional societies, and other relevant institutions are often organized along more traditional disciplinary lines.

Eric Wood returns to and focuses on a theme introduced by Tom Dunne that underlies all of the symposium papers as well as the Blue Book—the role of observation and measurement as the foundation of hydrology. This topic was considered important enough to merit its own chapter in the Blue Book, despite its role in most of the other chapters. Similarly it was afforded a key role at the symposium, despite its appearance in all of the other papers.

In his paper Wood demonstrates the centrality of measurements and observations to hydrologic science, using examples drawn from surface water hydrology, while attempting to understand why observations and measurements have become the "stepchild" of the science. His assessment of the "data health" of hydrologic sciences carries several themes. Despite a broad and general recognition of the centrality of measurements and observations in hydrology, specific quality and quantity criteria still do not exist against which to evaluate data collection efforts. In Wood's paper are echoes of Dunne's concern over the lack of convergence on fundamental principles, which remains a problem in hydrologic science. Echoing McKnight, but in the context of land surface-atmosphere interactions, Wood highlights the mismatches that can occur between data collected for one science (atmospheric science in this case) and the needs of another (hydrologic science). Finally, he finds that collaboration between data collectors/observers and theoreticians/model developers has improved but must continue to be nurtured.

Wood's conclusion is mixed. Large quantities of new data have become available in the last decade, but there is still no coherent program of hydrologic data collection driven by the needs of hydrologic science. As discussant Dara Entekhabi stated at the symposium, hydrology has "transitioned from an era characterized by data starvation to [one characterized by] data confusion."

Environmental tracers received relatively little direct attention in Opportunities in the Hydrologic Sciences but played an important role there nonetheless. Environmental isotopes were identified as a key tool in studying subsurface

hydrology, and careful reading reveals that tracers are implicitly an important component of a majority of the Categories of Scientific Opportunity described in Chapter 6 of the Blue Book.

In his paper Fred Phillips demonstrates nicely how germane the development of environmental tracers, particularly in subsurface hydrology, has been to the development of hydrology as a unique earth science. Environmental tracers, with applications to the unsaturated subsurface, shallow aquifers, deep aquifers, and regional flow systems, as well as surface-subsurface interactions, have achieved new prominence as much of the focus of hydrologic applications has shifted from water supply to water quality. Once again, central issues are characterizing fluxes, reservoirs, and change, and the broad range of spatial and temporal scales over which the physico-chemical processes controlling tracer movement occur.

In assessing ground water dating and isotope chemistry as components of the hydrologic sciences, Phillips, like his fellow authors, finds that multidisciplinarity plays a key role. In this case, other sciences, especially chemistry and geology, may drive the development of instrumentation and methodology, but the application to hydrologic problems becomes a key element of progress in the hydrologic sciences, leading to important hydrologic insights.

Progress in the development and application of environmental tracers to hydrologic problems is driven by instrumentation, funding, curiosity, practical need, and, not insignificantly, individual circumstances. Phillips illustrates this by describing a technique with both applied and basic applications (CFC tracers) that withered for a protracted period of time after initial discovery and development and an idea that developed purely out of scientific curiosity (³⁶Cl in fossil rat urine) but that almost immediately became applied to an important societal problem. The state of the science clearly depends on the scientific infrastructure and the development and maintenance of laboratory and experimental arts. Long-term, instrumentation-intensive funding is required. Phillips concludes that environmental tracer research and application are flourishing, playing a unique role in the advancement of hydrologic science. At least part of the credit for that, according to Phillips, is due to Opportunities in the Hydrologic Sciences, where the employment of modern geochemical techniques to trace water pathways was noted as an important emerging opportunity.

As has been demonstrated many times, successful hydrologic prediction does not require complete scientific understanding. However, a necessary criterion of scientific understanding is the ability to predict. Therefore, the words "prediction" and "forecasting" appear repeatedly throughout Opportunities in the Hydrologic Sciences. Certainly runoff and stremflow prediction remains central

to hydrologic science, fundamental as it is to, for example, geomorphic processes and landscape evolution, local and regional water balances, biogeochemical cycling, and ecological systems. Furthermore, many of the applications that drive public interest in and support the hydrologic sciences rest on the ability to predict streamflow.

In his paper Stephen Burges tackles the scientific and pragmatic challenge of streamflow prediction. While clearly articulating the societal need and hydrologic understanding necessary for successful streamflow prediction, Burges identifies and echoes a number of recurrent themes. Once again, we learn of the central role of the huge range of relevant spatial and temporal scales over which hydrologic processes act. Although much progress has been made in understanding how to apply knowledge across varying embedded scales, we are far from having an understanding that is adequate to allow successful streamflow prediction. Returning to a point noted by both Dunne and Wood, Burges emphasizes the importance of precipitation prediction to streamflow prediction. He suggests that understanding precipitation is the weak link in the problem of runoff prediction or, at the very least, that our understanding of terrestrial hydrologic processes cannot be truly tested predictively until precipitation forecasting uncertainty no longer dominates streamflow prediction uncertainty. Burges also reemphasizes the essential nature of observation and data. For streamflow prediction, precipitation data become central. Finally, streamflow prediction provides yet another example of the multidomain nature of hydrologic science. The atmosphere, oceans, hydrosphere, and biosphere all play important roles in continental water processes and the global water balance. Successful streamflow prediction is impossible without knowledge from all of these domains, and the "grand challenge" of hydrologic science is the coherent coupling of knowledge in all of these domains across a full range of spatial and temporal scales.

Taken together, the Wolman Lecturer and the symposium speakers and discussants provide a consistent diagnosis of the vitality of the hydrologic sciences; the science is indeed a vital, intellectually challenging geoscience. However, it remains a young science, in need of greater coherence and struggling to cope with its multidisciplinary, multidomain nature. Diligence and vigilance in nurturing our science are essential. The vision of the Committee on Opportunities in the Hydrologic Sciences has stood the test of the past seven years, and Opportunities in the Hydrologic Sciences continues to function as a touchstone for the future of hydrologic science.