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Hydrologic Science: Keeping Pace with Changing Values and Perceptions

George M. Hornberger

University of Virginia

Charlottesville, Virginia

ABSTRACT

Hydrology as a science has evolved from roots in very pragmatic concerns about water supply, irrigation, and hydropower toward a place as a distinct geoscience in its own right. This evolution has occurred by virtue of an expanding base of knowledge on complex interactions in large-scale natural systems involving water and also because of changing values and perceptions involving protection of the natural environment. In this paper some of the major ideas behind the evolution of research and education in hydrological science are sketched. Recent trends in hydrological science and in societal values/perceptions are used to illustrate how we might profitably attempt to shape education and research in the future.

INTRODUCTION

Science is organized knowledge.

Herbert Spencer

The knowledge base of hydrological science, as with any science, grows and changes as the science matures. The facts, the theories, and the unresolved questions that are part of water science arguably have accumulated at an accelerating rate over the past several decades. This changing knowledge base should influence educational programs and should inform decisions about



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Sustaining our Water Resources 3 Hydrologic Science: Keeping Pace with Changing Values and Perceptions George M. Hornberger University of Virginia Charlottesville, Virginia ABSTRACT Hydrology as a science has evolved from roots in very pragmatic concerns about water supply, irrigation, and hydropower toward a place as a distinct geoscience in its own right. This evolution has occurred by virtue of an expanding base of knowledge on complex interactions in large-scale natural systems involving water and also because of changing values and perceptions involving protection of the natural environment. In this paper some of the major ideas behind the evolution of research and education in hydrological science are sketched. Recent trends in hydrological science and in societal values/perceptions are used to illustrate how we might profitably attempt to shape education and research in the future. INTRODUCTION Science is organized knowledge. Herbert Spencer The knowledge base of hydrological science, as with any science, grows and changes as the science matures. The facts, the theories, and the unresolved questions that are part of water science arguably have accumulated at an accelerating rate over the past several decades. This changing knowledge base should influence educational programs and should inform decisions about

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Sustaining our Water Resources future research programs. The progress of hydrology occurs within a broad cultural context, however, and is consequently influenced by changes in the values and perceptions of society, as well as by the perceptions of scientists. Stressing issues related to water quality, I will present some ideas on recent trends in hydrological science and in societal values/perceptions to illustrate how we might profitably attempt to shape education and research in the future. THE EVOLUTION OF HYDROLOGY There are no such things as applied sciences, only applications of science. Louis Pasteur I shall not try to recount, even briefly, the history of hydrology here. (Accounts of varying length can be found in Biswas, 1972; Nace, 1974; and NRC, 1991.) Suffice it to say that, because of the very pragmatic concerns of water supply, transportation, irrigation, and the like, the roots of hydrology stretch back to the earliest civilizations. In fact, it can be argued that, until quite recently, pragmatic considerations dominated the approach to hydrology (see, e.g., Dooge, 1988). Although the existence of specialized textbooks implies that hydrology was recognized as a subject of modern scientific study by at least the dawn of the twentieth century, hydrology was not formally recognized as a distinct branch of geophysics until 1922, when the International Association of Hydrological Sciences (IAHS) was formed as a branch of the International Union of Geodesy and Geophysics. In 1930 the American Geophysical Union created its Section of Hydrology. Despite formal acceptance as a branch of the geophysical sciences by national and international organizations, hydrology has failed to establish a separate identity as a geoscience. This must change if the science is to continue to advance apace (NRC, 1991; Eagleson, 1991). How have our perceptions of hydrology changed? Hydrologic science can now be seen as a geoscience interactive on a wide range of space and time scales with the ocean, atmosphere, and solid earth sciences as well as with plant and animal sciences. The new perceptions concern the interaction of the components and the range of scales. Our perceptions of the necessary administrative boundaries also have changed. The ubiquity of water on the earth and its indispens-

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Sustaining our Water Resources ability to life do not make hydrologic science out of all geoscience and biology. Forging a separate identity for hydrologic science requires specifying and claiming its central elements, and locating its administrative boundaries as a flexible compromise between precedent and scientific completeness. (NRC, 1991) The above quote from the report of the Water Science and Technology Board's (WSTB) Committee on Opportunities in the Hydrologic Sciences (COHS) suggests that definition of hydrology as a distinct discipline within the geosciences is essential to the timely expansion of our knowledge base. The ''boundaries'' of hydrology identify it as distinct from but touching upon atmospheric and ocean sciences (Figure 3.1). The COHS went on to conclude that a fresh approach to hydrology was needed: Thus the science of hydrology has come to encompass a mix of natural and altered physical, chemical, and biological systems as well as to include important interactions with the engineering and social sciences. There is little doubt that coping with these issues will require Figure 3.1 Hydrologic science: a distinct geoscience (NRC, 1991).

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Sustaining our Water Resources a much-improved scientific understanding of the earth system and its component parts. Unified and coherent treatment of hydrologic science is central to this larger effort. (NRC, 1991) The evolution of hydrology, and its relation to the work of the WSTB over the past 10 years, can be illustrated by considering questions involving water quality. In 1969 William Ackerman presented a paper at the First International Symposium for Hydrology Professors entitled "Scientific Hydrology in the United States." Ackerman (1969) saw a deficiency in hydrology in the area of water quality: With regard to environmental quality, it seems to me that classical, as well as modern hydrology, are seriously weak in the almost exclusive attention to quantitative aspects. With the exception of sedimentation we have virtually excluded water quality as a parameter of water science. Although Ackerman's assertion might be challenged on the basis that hydrogeologists had been concerned with water quality in the broad scientific sense long before 1969, I think his view reflects the notion at the time that hydrology was primarily a field of engineering that had relegated issues of water quality to its sibling field of sanitary engineering. Certainly, there was no dearth of interest in or work on water treatment and wastewater treatment in the engineering community. Much of this work represented "scientific" accomplishments in support of the engineering objective of producing potable water. One of the early reports of the WSTB reflects this particular emphasis regarding water quality—a report on the operation, maintenance, and performance of an experimental water treatment plant on the Potomac estuary (NRC, 1984). The view of hydrology as a distinct earth science, as espoused by the COHS and endorsed by many scientists including me, explicitly includes water quality issues (see, e.g., Figure 3.1) with critical areas reflecting the change from concerns for technological solutions to relatively small-scale problems to concerns for protection of the natural environment at regional and global as well as local scales (NRC, 1991). For example, the COHS report notes several areas of current concern: a lack of understanding of global biogeochemical cycles; of geochemical processes on hillslopes; and of the fate and transport of contaminants in hydrological systems, particularly of contaminants released into soils and ground water. This increased attention to biogeochemical processes in the natural environment does not negate the continued importance of water treatment, of course. Rather, it indicates changes in the

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Sustaining our Water Resources awareness of hydrological scientists of the need to gain fundamental understanding of the functioning of natural systems and, in a more applied sense, to deal with such topics as the transport of contaminants by ground water. This increasing emphasis on the quality of ground water is actually reflected in the reports of the WSTB over its history—from a report on protection of the quality of ground water (NRC, 1986) to the report of a colloquium on science, policy, and public perception related to remediation of contaminated soil and ground water (NRC, 1990a) to a thorough review of the use of ground water models in both scientific and regulatory applications (NRC, 1990b). In fact, even the early report on the experimental treatment plant on the Potomac estuary (NRC, 1984) recognized that limitations on a full assessment of the feasibility of the engineering schemes were, in part, due to a lack of scientific understanding of the fate and transport of contaminants in the natural environment: "Accurate characterization of most nonconservative quality parameters is virtually impossible considering the multiplicity of complex, natural processes and factors controlling their degradation and transformation. The precise nature and kinetics of these reactions in the estuary are currently unknown" (NRC, 1984). Furthermore, the limitations of technological "fixes" to environmental problems came to be widely recognized (NRC, 1990a). Failure to recognize limitations can lead legislation- and regulation-driven solutions to ground water remediation to be less than cost effective (Freeze and Cherry, 1989). The linkages of the strictly physical, chemical, and biological aspects of scientific hydrology with the social science aspects of water resources planning and management become even more important under these conditions. THE EVOLUTION OF HYDROLOGICAL RESEARCH Where there is much desire to learn, there of necessity will be much arguing, much writing, many opinions; for opinion in good men is but knowledge in the making. John Milton Some of the impetus for the latest call for recognition of hydrology as a separate earth science discipline stems from the evolution of problems addressed in the science to greater and greater complexity and to ever-increasing time and space scales. Eaton (1969) pointed out that "characteristic of the current focus is that hydrology and hydrology research increasingly are involved in problems of great complexity because of both the magnitude of the undertaking—e.g., the Trans-Texas Canal—and also because of the interaction

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Sustaining our Water Resources of hydrologic and nonhydrologic factors—e.g., regulation of Everglades water levels." Freeze and Back (1983), in the preface of their compendium of classic papers in physical hydrogeology, argue that there have been three "revolutions" (in the sense used by Kuhn in his 1962 book) in physical hydrogeology and that the scale and complexity of problems addressed have grown with each one. The first revolution grew from Darcy's work (published in 1856) and focused work on the column scale (application to sand filters). The second stemmed from the work of Theis (published in 1935) and led to aquifer testing. The third revolution was a result of the introduction of digital computing to the field in the early 1960s and produced an ability to consider large-scale regional systems. The COHS report carries this argument forward, noting that the global scale must now be very seriously considered as important to hydrology. This realization of the importance of water to the earth system at geophysical space and time scales has profound implications for the research and educational infrastructure of hydrologic science. We cannot build the necessary scientific understanding of hydrology at the global scale from the traditional research and educational programs that have been designed to serve the pragmatic needs of the engineering community. (NRC, 1991) How does this evolution toward ever-increasing complexity and spatial/temporal scale influence research? Our inability to deal with complexity of process and heterogeneity of geological materials at a wide range of spatial scales led Beven (1987) to conclude that hydrology faced an imminent crisis and was ripe for a scientific revolution (again in the sense used by Kuhn in his 1962 book): Hydrology in the future will require a macroscale theory that deals explicitly with the problems posed by spatial integration of heterogeneous nonlinear interacting processes (including the effects of preferential flow pathways ...) to provide a rigorous basis for both "lumped" and "physically-based" predictions. Such a theory will be inherently stochastic and will deal with the value of observations and qualitative knowledge in reducing predictive uncertainty; the interactions between parameterizations and uncertainty; and the changes in hydrological response to be expected as spatial scale increases. Such a theoretical framework should initiate new lines of thought, and innovative methods of measurement, analysis, and hypothesis testing to be developed during a future period of "normal'' science in hydrology.

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Sustaining our Water Resources Work on a research program to define a rigorous stochastic theoretical basis for hydrology has been ongoing for several years now, but, as Beven suggests, there is not yet even near-universal agreement that this is the most fruitful course of action. Furthermore, there are "costs" associated with a transition to a stochastic basis. A stochastic theory is probably most advanced in ground water hydrology where great progress has been made in the past 15 years or so on describing the flow of water and the transport of conservative solutes through heterogeneous media (see, e.g., review by Yeh, 1992). The approach has also been extended to include transport of chemically reactive species by ground water (Lynn Gelhar, Massachusetts Institute of Technology, personal communication, August 1992). But a very significant problem arises: the data requirements and the (necessarily large-scale) field experiments to explore the implications and test stochastic theories are considerable (NRC, 1990b; Gelhar et al., 1992). Therefore, the cost of the necessary field research in hydrogeology can be expected to be substantial. This same conclusion holds true for other areas of hydrology if we are to realize Beven's goal of a scientific revolution. For example, the COHS report suggests several priority categories for research, one of which can be read to require a strongly interdisciplinary effort and a commitment to improved laboratory and field experimentation. How does hydrological science stand in regard to funding the important research activities that are necessary to push the frontiers? Evans and Harshbarger (1969) summarized data for expenditures on research in water resources in 1966 and on projected expenditures for five years in the future from 1966, that is, 1971 (Table 3.1). The COHS collected information from federal agencies on expenditures for research in hydrological science (Table 3.2). Although it may be patently unfair to compare Tables 3.1 and 3.2, these data do seem to confirm the nagging suspicion that the progress of hydrology as a science, especially vis-à-vis other sciences, may not have been what it should. The data in Table 3.2 also suggest that there has indeed been an increasing emphasis on research involving water quality. This reflects the changes within our society toward a greater valuation of environmental quality. A good part of this is associated with concern for remediation of contaminated soils and ground waters. It is interesting to note that the level of research funding for basic hydrology (e.g., Table 3.2) is hardly noticeable in terms of federal budgets for environmental remediation. Russell et al. (1991) place the price tag for hazardous waste remediation in this country at some $7.5 × 1011 and indicate that the price could be as high as $1.6 × 1012! While it may be argued that the expenditure of large sums of money on ground water remediation will unavoidably benefit research in hydrogeology through spinoff effects, there should be some concern among the community

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Sustaining our Water Resources TABLE 3.1 Research Expenditures for 1966 (Estimated) and for 1971 (Projected) as Summarized from a Report by the Federal Council for Science and Technology   Water Research Categories $ Millions     1966 1971 I. Nature of water 2.8 3.9 II. Water cycle 15.3 24.8 III. Water supply and conservation 22.9 26.9 IV. Water quantity management/control 4.1 8.5 V. Water quantity management/protection 12.2 53.8 VI. Water resources planning 3.0 13.5 VII. Resources data 2.3 4.0 VIII. Engineering works 4.5 10.3 IX. Manpower, grants, and facilities 24.7 54.0   Total 91.9 199.3   SOURCE: Evans and Harshbarger (1969) interested in promoting hydrological science because this might be yet another example of the discipline being driven by problem solving rather than puzzle solving. Davis (1992), in a paper presented at the Remson Symposium at the 1992 American Geophysical Union Fall Meeting, summarizes the concern: The uncertain balance between practical work and flights of the intellect continues, but our modern flights seem to be unreasonably tethered to practical demands, particularly to regulatory-driven requirements. Even our funding agencies have concocted a unique series of hurdles related to practical relevancy with precognition as the first incredible step. That is, research if funded only in practical results can be seen in a crystal ball.

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Sustaining our Water Resources TABLE 3.2 Federal Funding for Research in the Hydrological Sciences for Fiscal Year 1988 (from NRC, 1991)   Water Research Categories $ Millions I. Earth crust 22.7 II. Landforms 17.5 III. Climatic processes 6.2 IV. Weather processes 7.7 V. Surficial processes 7.4 VI. Living communities 25.2 VII. Chemical processes 38.3 VIII. Additional topics 2.0   Total 127.0 What about the future? One very promising step has been the creation of a program in hydrological science within the National Science Foundation (NSF). This program appears to have as a goal furthering the agenda outlined in the COHS report (NRC, 1991). Nevertheless, the program, in and of itself, cannot satisfy the total need for hydrological research. I have argued that hydrological research must progress to include more field observation and experimentation and must increasingly do so at large spatial scales. The stimulation of research in scientific hydrology with the essential costly field components will have to be done very carefully by the NSF given the modest budget available. THE EVOLUTION OF HYDROLOGICAL EDUCATION 'Tis Education forms the common mind, Just as the twig is bent, the tree's inclin'd. Alexander Pope The modern evolution of hydrological education has paralleled the evolution of the science as I have outlined above. Prior to the formation of the IAHS, not only was hydrology not recognized as a separate discipline,

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Sustaining our Water Resources but there were relatively few courses taught specifically on the subject. As Ackerman (1969) wrote: It was my good fortune to study hydrology at the University of Wisconsin about 1934 when, I suppose, a course by this name was offered at not more than three universities in the country. Since the 1930s, the teaching of hydrology in the United States has evolved within the problem-solving context in which the discipline was viewed. A 1974 UNESCO report on the teaching of hydrology worldwide summarized the status at that time: The science of hydrology has developed within many different fields of study, including civil engineering, meteorology, geology, physical geography, and geophysics and it has thus been difficult for it to emerge as a separate branch. In fact hydrology has been established as a separate field of study in only a few of the largest and most highly developed countries. The "most highly developed countries" have made precious little progress since the 1974 UNESCO report marking the end of the International Hydrological Decade, however, and it is becoming ever more clear that these countries, including the United States, need to assume the leadership role to change matters. As I have indicated, a major push for reform of hydrology education along the lines of recognizing hydrology as a distinct earth science discipline came through the efforts of the WSTB (NRC, 1991; Eagleson, 1991). But there are other voices as well. In 1989, when Vit Klemes was president of IAHS, he established a panel to address the intellectual content and context of hydrology education and to make recommendations accordingly. The panel suggested that hydrology had not progressed adequately over the past several decades and concluded that a revamping of educational systems was in order. The challenges of hydrology can be met only through a conscious and concerted effort to consolidate and develop hydrology intensively as a coherent geoscience and as a technology resting on a sound scientific basis. Education is central to the required process of change and improvement. The present structure of hydrological education, generally tailored to the needs of specialized nonhydrological disciplines, is ill-fitted to cope with present and future requirements. (Nash et al., 1990)

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Sustaining our Water Resources The COHS report (NRC, 1991) offers some prescriptive advice on how educational efforts in hydrology should be structured. Although not everyone will agree with the COHS report (e.g., Back, 1991), it is, in my opinion, an excellent point for getting started. I would like to stress two particular points related to education in hydrology: the need to balance the orientation toward computation with a coequal emphasis toward experimentation and the need to keep striving to encourage women and underrepresented minorities in the field. With regard to the first of these, as early as the 1969 seminar for hydrology professors, Hall (1969) expressed some concern about the potential imbalance in hydrology toward computer exercises: Some basic work on the physical science is being accomplished ..., but day-by-day the emphasis has shifted visibly from fundamental physics to fundamental mathematics. Even in hydrogeology, where numerical simulation has (arguably) seen some of its greatest successes in all of the earth sciences, our computational abilities have outpaced our abilities to gather and interpret data in the field. The next major level of improvement in ground water simulation models will not arise from improved numerical procedures; rather, a greater investment must be made in obtaining more accurate descriptions of aquifer properties and their variability. . . . [O]n the whole we need more geology in hydrogeology. (Konikow, 1987) Perhaps the warning of Truesdell (1984)—"Preponderance of computing discourages critical analysis, creative thought, and the training of thinkers"—is too extreme to apply to hydrology, but I do think we need to heed the warning of the recent IAHS panel: One urgent educational problem, which has reached crisis proportions in many universities, is the lack of field and laboratory experience. This is a problem at all levels and in many disciplines and has existed long enough to be self-perpetuating through the next generation of faculty. The consequences in hydrology are both profound and disturbing especially with the current emphasis on conceptual modeling. Although such models constitute useful tools in the investigation of the physical world, exclusive or undue reliance on them may tend to separate students from the realities they are supposed to study. In the absence of appropriate testing, models

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Sustaining our Water Resources take on an aura of reality in the minds of their users and become a source of unsound science and practice. (Nash et al., 1990) The second issue is the recruitment and retention of women and underrepresented minorities into the hydrological sciences. The COHS report notes that statistics suggest that hydrology does about as poorly as other physical sciences—about 10 percent of the profession are women, for example. I noted a somewhat disturbing correlation in data from an NRC report as summarized in Physics Today several months ago (Fehrs and Czujko, 1992). Countries in which a higher proportion of the physics faculty members are women graduate a higher proportion of women Ph.D.s. We should be aware of a "catch-22" aspect of the problem, if there is cause-and-effect relationship here, and be prepared to do what is necessary to break any vicious cycle. The COHS reports the prescriptive advice for changing the current imbalance. The WSTB should make sure that this agenda remains at the top of everyone's list! HYDROLOGY AND WATER RESOURCES What is all knowledge too but recorded experience, and a product of history; of which, therefore, reasoning and belief, no less than action and passion, are essential materials? Thomas Carlyle My comments in this paper have dealt almost exclusively with hydrology as a science. The complexities and difficulties facing us in devising research and educational policies and institutions to keep pace with the changing knowledge base in scientific hydrology are perhaps quite enough for a single presentation. Nevertheless, I would be remiss if I failed to point out that at least as much attention needs to be paid to the interface between scientific hydrology and the broader issues of water resources. The issue is how to promote close collaboration between natural scientists and social scientists over the long term so that the problems, issues, and policies in water resources can be adequately addressed (Evans and Harshbarger, 1969). One fact made clear during this committee's oversight of the San Joaquin Valley Drainage Program is that finding a solution to the valley's drainage problem, and any such situation anywhere in the West or in the world, is not a purely technical question. Indeed, the more difficult issues are often political, social, and economic (NRC, 1989).

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Sustaining our Water Resources There are no clear boundaries conveniently separating the ''disciplines" of hydrology and water resources (Figure 3.2), so collaboration among diverse groups is the only sensible course of action. This type of collaboration occurs on an ad hoc basis within many institutions. Whether it should be formalized in educational and research programs remains a question for future study. REFERENCES Ackerman, W.C. 1969. Scientific hydrology in the United States. Pp. 50–60 in The Progress of Hydrology, Proceedings of the First International Seminar for Hydrology Professors, University of Illinois, Urbana. Back, W. 1991. Review of "Opportunities in the Hydrologic Sciences." EOS 72. November. Beven, K. 1987. Towards a new paradigm in hydrology. IAHS Pub. 164:393–403. Biswas, A. K. 1972. History of Hydrology. North-Holland Publishing Co., Amsterdam. Figure 3.2 Knowledge overlap between hydrology and water resources (from Evans and Harshbarger, 1969).

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Sustaining our Water Resources Darcy, H. 1856. Les Fontaines Publiques de Ville de Dijon. Victor Dalmont, Paris. 647 pp. Davis, S. N. 1992. Practice and theory in hydrogeology, 900 A.D. – 1900 A.D. Paper presented at the Remson Symposium, American Geophysical Union Fall Meeting, December (Abstract). EOS 73:177. Dooge, J. C. I. 1988. Hydrology in perspective. Hydrological Sciences Journal. 33:61–85. Eagleson, P.S. 1991. Hydrologic science: A distinct geoscience. Reviews of Geophysics 29:237–248. Eaton, E.D. 1969. Comments on some recent trends in hydrology research. Pp. 974–993 in The Progress of Hydrology, Proceedings of the First International Seminar for Hydrology Professors, University of Illinois, Urbana. Evans, D. D., and J. W. Harshbarger. 1969. Curriculum development in hydrology. Pp. 1024–1043 in The Progress of Hydrology, Proceedings of the First International Seminar for Hydrology Professors, University of Illinois, Urbana. Fehrs, M., and R. Czujko. 1992. Women in physics: Reversing the exclusion. Physics Today. August:33–40. Freeze, R. A., and W. Back. 1983. Physical Hydrogeology. Hutchinson Ross Publishing Co., Stroudsburg, Pa. Freeze, R. A., and J. A. Cherry. 1989. What has gone wrong? Ground Water 27:458–464. Gelhar, L. W., C. Welty, and K. R. Rehfeldt. 1992. A critical review of data on field-scale dispersion in aquifers. Water Resources Research 28:1955–1974. Hall, W.A. 1969. The computer and hydrology. Pp. 226–232 in The Progress of Hydrology, Proceedings of the First International Seminar for Hydrology Professors, University of Illinois, Urbana.

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Sustaining our Water Resources Konikow, L. F. 1987. Predictive modeling of ground water. The Hydrogeologist. Newsletter of the Hydrogeology Section of the Geological Society of America. P. 4. Kuhn, T. S. 1962. The Structure of Scientific Revolutions. University of Chicago Press, Chicago. Nace, R. 1974. General evolution of the concept of the hydrological cycle. In Three Centuries of Scientific Hydrology. UNESCO-WMO-IAHS. UNESCO, IAMS, Paris and WMO, Geneva. Nash, J. E., P.S. Eagleson, J. R. Philip, and W. H. Van der Molen. 1990. The education of hydrologists. Hydrological Sciences Journal. 35:597–607. National Research Council (NRC). 1984. The Potomac Estuary Experimental Water Treatment Plant. National Academy Press, Washington, D.C. National Research Council (NRC). 1986. Ground Water Quality Protection: State and Local Strategies. National Academy Press, Washington, D.C. National Research Council (NRC). 1989. Irrigation-Induced Water Quality Problems. National Academy Press, Washington, D.C. National Research Council (NRC). 1990a. Ground Water and Soil Contamination Remediation: Toward Compatible Science, Policy, and Public Perception. National Academy Press, Washington, D.C. National Research Council (NRC). 1990b. Ground Water Models: Scientific and Regulatory Applications. National Academy Press, Washington, D.C. National Research Council (NRC). 1991. Opportunities in the Hydrologic Sciences. National Academy Press, Washington, D.C. Russell, M., E. W. Colglazier, and M. R. English. 1991. Hazardous Waste Remediation: The Task Ahead. Waste Management Research and Education Institute, University of Tennessee. Truesdell, C. 1984. The computer: Ruin of science and threat to mankind. An Idiot's Fugitive Essays on Science. Springer-Verlag, New York. Pp. 594–631.

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Sustaining our Water Resources UNESCO. 1974. The Teaching of Hydrology. The UNESCO Press, Paris. Yeh, T.-C. J. 1992. Stochastic modeling of ground water flow and solute transport in aquifers. Hydrological Processes 6:369–395.