Debates Hypothesis testing in hydrology - Wiley Online Library

PUBLICATIONS Water Resources Research COMMENTARY 10.1002/2016WR020050 Key Points: Hydrologic hypotheses can contribute to advances in research depending upon the research environment Overarching hypotheses and hypotheses to explain unexpected observations are particularly useful Well-instrumented, long-term research programs can promote development of new hypotheses in hydrology

Correspondence to: D. M. McKnight, [email protected]

Citation: McKnight, D. M. (2017), Debates— Hypothesis testing in hydrology: A view from the field: The value of hydrologic hypotheses in designing field studies and interpreting the results to advance hydrology, Water Resour. Res., 53, 1779–1783, doi:10.1002/2016WR020050.

Debates—Hypothesis testing in hydrology: A view from the field: The value of hydrologic hypotheses in designing field studies and interpreting the results to advance hydrology Diane M. McKnight1 1 Department Civil, Environmental and Architectural Engineering and INSTAAR, University of Colorado, Boulder, Colorado, USA

Abstract Advances in hydrology are greatly needed and approaches that employ hypotheses to guide research have the potential to contribute to future advances. In this context, hypotheses can serve a range of purposes. Overarching hypotheses can provide a common integrating framework for collaborative research and can be revised as research progresses over time. Hypotheses that attempt to explain unexpected field observations or experimental results can provide a guide for designing further field studies. Focused testable hypotheses can facilitate effective presentation of proposed research, and clarify alternative hypotheses. Finally, the value of employing a hypothesis-based approach depends upon the research environment, which can act as an ‘‘environmental filter’’ in determining successful research outcomes.

1. Introduction Received 2 NOV 2016 Accepted 9 FEB 2017 Accepted article online 27 FEB 2017 Published online 29 MAR 2017

Published 2017. This article is a U.S. Government work and is in the public domain in the USA.

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I appreciate the opportunity to participate in this debate on the role of hypotheses in hydrology. I should begin by admitting that my initial reaction to the question posed for this debate was ‘‘but of course hypotheses are useful.’’ I recognize that my opinion on the topic of the value of hypotheses is influenced by my experience as a field scientist, especially at the beginning of my career. My perspective has been strongly influenced by discussions with my mentor and friend Robert Averett as a young research hydrologist with the U.S. Geological Survey. In the summer of 1979 during my first 3 months with the agency, I took a training class in Water Quality and Stream Ecology led by Dr. Averett, who was the Central Region Branch Chief for the National Research Program office of the Water Resources Division. We learned that for all water quality studies it was critical to start with a hypothesis. He advised us to envision a statement of the hypothesis in the eventual title of the report. He explained that rather than planning to write a hypothetical report entitled ‘‘A water quality study of Spring Creek’’ we should plan to write a report entitled ‘‘A study of the effect of road salt on the water quality of Spring Creek’’ or some other title reflecting our hypothesis, or our alternate hypothesis. He assured us that this vision would then lead to the design of a field study that would support such a report. Less than a year later, I joined a team of scientists working in the blast zone of Mount St. Helens, which had erupted on 18 May 1980. Our helicopter flew over a barren wasteland of ash and debris avalanche deposit and forests that had been flattened by the lateral blast from the volcano. We landed on an island of piled up logs in the center of Spirit Lake. The lake water we collected had a strange dark black color, a sulfurous smell, and a concentration of dissolved organic matter more than a 50 times greater than the concentration before the eruption. As I flew back to Denver with gallons of this water in coolers in my checked luggage and a tentative plan to return in the spring, I thought about what to do next. The advice to start with a hypothesis seemed particularly useful. I hypothesized that the dark black organic matter was being leached from the pyrolized soils and vegetation in the debris avalanche deposit. I also considered that in the coming winter the snow would surely cover this landscape and would melt in the spring. Thus, I hypothesized that not only would the snowmelt dilute the dissolved organic matter, but would also drive oxidation reactions that would change its chemistry. Both of these hypotheses turned out to be correct, but there were some surprises.

HYPOTHESES OF ONE KIND OR ANOTHER

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2. Hydrologic Hypotheses in a Changing World-Reframing the Question Hydrologic change is underway in watersheds around the world, from the Arctic to the Antarctic, and the consequences for many communities are profound. Scientists and engineers from many fields recognize that climate change presents a grand challenge for our time. Thus, advances in hydrology are greatly needed and consideration of different approaches to stimulate and advance hydrologic research is worthwhile. I would like to begin by reframing the question. The progress of hydrology as a science, as for many fields of science, is an inherently human endeavor. Furthermore, a key aspect of hydrology is that progress can be achieved through integration of concepts from different fields of study, e.g., geology, physics, chemistry, and ecology. Thus, the question becomes: Does the process of hydrologists and their colleagues developing and examining hypotheses lead to advances in hydrology? In the context of reframing the question, I would also like to argue that there are potentially many different types of hypotheses that may be useful in hydrology. Further, the definition provided of a hypothesis seems to be somewhat constrained or idealized. Using an analogy from ecology, the social, cultural, and resource constraints that influence the conduct of science in the present day may act as an ‘‘environmental filter’’ that determines which approaches to advancing science are successful, and thus more common. In this context of selective pressures, the value of a given type of hypothesis may not depend upon the extent to which that hypothesis meets the definition provided as a framework for this debate. Rather, hypotheses of one kind or another may be useful, depending on the ‘‘filters’’ acting within a given research environment. For hydrology, examples of effective ‘‘filters’’ are the constraints on acquiring funding for research, the relationships that may promote or discourage successful interdisciplinary collaboration, and the rapid pace of hydrologic change itself. In my contribution to this debate, I will focus on three types of hypotheses that my colleagues and I have found useful in studying the coupling of hydrologic and biogeochemical processes in streams, lakes, and watersheds. The first type is the classic overarching hypothesis that over time can help to focus long-term collaborative studies. The second type is the hypothesis to explain unexpected results from the field and can guide further studies, turning discoveries into new insight. Finally, I will discuss the intriguing hypothesis that can aid in garnering support for curiosity-driven research into important but seemingly wellunderstood processes. To illustrate the differences among these types of hypotheses, I have used examples from my own experience because I know the back story behind the findings that were eventually published.

3. Overarching Hypotheses Developing a framework for collaborative research in hydrology presents many challenges. The process of developing an ‘‘overarching’’ hypothesis can be effective for promoting collaboration among hydrologists and other scientists with different areas of expertise. An overarching hypothesis can lead to a study design in which different lines of evidence are obtained in a coordinated, coherent manner and brought together over time. In designing an interdisciplinary study, the agreed upon overarching hypothesis can aid in deploying limited resources. As a research team continues to collaborate, reconsideration of the overarching hypothesis can provide a buffer against the tendency for each individual scientist to drill down into the details of their own field of study. Although an overarching hypothesis may not be directly testable as in the definition provided for this debate, it may be more readily transferred to other watersheds and applied at different scales. Early in my career, I was fortunate to join a collaborative team led by Robert Wharton and focused on longterm study of ecological processes in the McMurdo Dry Valleys, Antarctica, an ice-free polar desert on the coast of McMurdo Sound. The landscape is comprised of large expanses of bare patterned ground, terminal, and alpine glaciers, and permanently ice-covered lakes in the valley floors, connected by meltwater streams that flow for 6–12 weeks each summer. Katabatic winds coming from the polar plateau can reach 140 miles per hour, transporting sediment on to the glacier surfaces during the winter. The abundant microbial mats in the streams are hotspots of primary productivity, initiating growth less than an hour after flow begins each summer. The first overarching hypothesis of the McMurdo Dry Valleys Long-Term Ecological Research (MCMLTER) project in 1992 was fairly straightforward: The structure and function of the McMurdo Dry Valleys

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ecosystem is controlled by physical constraints. Because the availability of liquid water is such an important constraint for life, we designed a stream gauging network as part of our overall monitoring program. By 2010, our overarching hypothesis had evolved to: Climate warming in the McMurdo Dry Valley ecosystem will amplify connectivity among landscape units leading to enhanced coupling of nutrient cycles across landscapes, and increased biodiversity and productivity within the ecosystem. While the stream gauging network supports the study of hydrologic transport of nutrients and organisms across the landscape, to address this new overarching hypothesis we recognized that we needed to improve our measurements of aeolian transport of sediment and biota. Within the framework of an overarching hypothesis, several subhypotheses can be developed each of which may be more readily tested in the strict definition provided. Even so, the value of these subhypotheses may derive from the extent to which the examination of these subhypotheses through field and laboratory studies or modeling support or challenge the overarching hypothesis. In the example of the first overarching hypothesis of the MCMLTER project, one subhypothesis was that microbial mats required sustained flow to become established and we proposed to reroute flow into an abandoned channel to directly test this hypothesis. When we carried out the experimental diversion of flow into a channel that we estimated (based on previous aerial photography) had not experienced flow for several decades, we observed an unexpected result. The microbial mats that had been dormant on the streambed, covered with a fine layer of aeolian sediment, began growing with in a week of the initiation of flow, growing more rapidly than microbial mats in other streams as the pulse of freshly mobilized nutrients moved downstream.

4. Hypotheses to Explain the Unexpected As alluded to above, one of the great values of field data and experiments is the discovery of the unexpected response or pattern. In this context, another motivation for developing a hypothesis is to explain otherwise perplexing field observations that are discordant with well-established concepts. These hypotheses for surprising results may integrate several different puzzling observations. Such hypotheses can also be particularly useful in deciding what to measure next or which process might be added to an existing hydrologic model. One example of an unexpected result arose in studies of acid mine drainage (AMD) streams that my colleagues and I conducted through the USGS Toxic Substances Hydrology Program. AMD streams are common in Colorado Rocky Mountains and can be recognized by the distinctive reddish-orange ferric iron oxide deposits on the streambed. In the summer of 1983, in order to quantify the solubility of these oxides under varying pH, Ken Bencala and I conducted a tracer experiment in which we added sulfuric acid to the stream to lower the pH from about 4.2 to 3.2, anticipating that the low pH would cause a pulse of dissolved ferric iron to develop downstream. To be sure of the chemical form of the expected iron pulse, I made colorometric measurements to distinguish between ferric and ferrous iron at the lowermost sampling site. The pronounced dissolved iron pulse did occur. But, when the sample immediately turned bright pink with the addition of the colorometric reagent, without needing to add the reducing reagent, I realized that the iron pulse was composed of predominantly ferrous iron, not ferric iron. At that point, all we knew was that we did not know what the reductant might be. After discussions with leading geochemists and additional measurements of the day/night changes in ferrous and ferric iron in the stream, we developed the hypothesis that sunlight was the reductant, i.e., that photochemistry is an important process controlling the transport of iron in these streams and other aquatic systems. This hypothesis was the basis for further experimental and modeling research conducted by the Toxic Substances program that have advanced understanding of acid mine drainage impacts and remediation alternatives. This example illustrates the importance of robust, complete field observations. How else will we overcome our assumptions and find out what we do not understand? Development of this kind of hypothesis becomes a deductive process, in which we gain more insight into the whole system through observation of details. It is also worthwhile to note that in presenting the results of a field study, these hypotheses developed to explain unexpected results can be employed retrospectively. It is possible to write the introduction to the paper presenting the results as if the investigators started with the correct hypothesis or assumptions. There is no immediate benefit in confusing the reader with a convoluted narrative of the stages of puzzlement that may have occurred over the course of the research.

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In the example of the experiment described above, the paper analyzing the results of that particular experiment was published in 1989, after the publication of two other papers based on field studies conducted in subsequent field seasons. At that point, we did not present the importance of iron photochemistry as a hypothesis, but simply stated in the first sentence of the Introduction that ‘‘The transport of dissolved and particulate iron in aquatic environments may be affected by many processes, including: . . ., (2) oxidation/ reduction reactions, such as microbial oxidation of ferrous iron and photoreduction of ferric iron,. . .,’’ and noted in the Site Description that ‘‘iron concentrations in the stream vary on a diel basis . . . as a result of photoreduction of hydrous iron oxides, oxidation of ferrous iron and other processes.’’ The establishment of well-designed monitoring programs and long-term research sites is conducive to such discovery-based hypotheses. In a well-instrumented and monitored field site, it can be less possible to dodge the perplexing observation with the argument that ‘‘it’s complicated’’ or by invoking a process which was not measured. This approach may become more important as the pace of change in watersheds has accelerates with land use change and climate change. Since there may not be enough time for empirical relationships to provide a basis for adaptation, hydrologists may benefit by also developing process-based hypotheses that seek to explain emerging responses.

5. Hypotheses for Planning and Proposing A focused hypothesis may be valuable for providing a framework for studying an inherently important or interesting phenomenon. In hydrology, there are phenomenon that are regularly observed and seem to be well-understood, for which greater knowledge or resolution of rates and variability would be of great value for applications or development of improved models. These studies can be framed to address questions, with goals and objectives. But, if these studies are framed in this manner, they may not appear to be interesting to other hydrologists or scientists from other fields of study. The statement that not much is known about a particular phenomenon may be an insufficient justification for obtaining support for the investigation, or for a doctoral student’s committee approving a thesis proposal. Taking a pragmatic approach, these studies can be framed to test specific hypotheses that meet the criteria of being falsifiable and reproducible. Thus, restructuring a curiosity-driven study to test an intriguing hypothesis, and possibly an alternative hypothesis, may strengthen the case for the proposed research by engaging the curiosity of potential reviewers or committee members. There are also constant advances in hydrologic instrumentation that allow for more measurements that are more detailed in space or time. In pursuing these technological advances, there may be an underlying rationale as to which patterns may emerge and what the implications would be. In this context, explaining how such better resolved data can be used to distinguish among potential scenarios, is similar to a hypothesisbased approach. These scenarios can meet the criteria of being falsifiable and transferrable. One example in my own experience that serves to illustrate this type of hypothesis is the one my colleagues Yan Zheng, Diana Nemergut, Natalie Mladenov, and I put forward in our proposal to study interactions involving microbial communities, dissolved organic matter, and arsenic in a seasonally variable ground water system in Bangladesh. Exposure of communities to toxic concentrations of dissolved arsenic in their drinking water through mobilization of geogenic arsenic in groundwater is one of the most important water quality problems on the global scale. Estimates of the number of people affected range up to 100 million worldwide. Over the past decade, numerous research groups have made major contributions to understanding different aspects of the underlying hydrology and biogeochemistry of this problem. Nonetheless, much remains to be understood and greater understanding would indeed have great value. Our project has the intent of contributing to the advances on the topic by applying our expertise in characterizing microbial communities and the chemistry of different fractions of dissolved organic matter that interact with arsenic in different ways. In our proposal, we presented a detailed hypothesis that provided a structure to our proposed work, i.e., that ‘‘the chemistry of the seasonally variable recharge structures microbial communities, which in turn regulate local rates of Fe reduction through electron shuttling by humics, exerting control on aresenic release to the aquifer.’’ Such a detailed hypothesis may have increased the chances for success in the review process, but more importantly it also continues to be useful as we have carried out the field and laboratory experiments. I would also argue that a similarly detailed hypothesis would be useful in designing a well-instrumented long-term research site or monitoring program in Bangladesh or

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Cambodia that would include measurements at high temporal resolution variations in dissolved arsenic, dissolved organic matter, and related constituents.

6. Concluding Remarks In my contribution to this debate, I have described three types of hypotheses: the classic overarching hypothesis, the hypothesis to explain unexpected results, and the intriguing hypothesis. I fully expect that there may be other approaches for differentiating among hypotheses and for incorporating hypotheses into hydrologic research. I would like to end by emphasizing that to be useful, any successful approach to employing hypotheses in hydrologic research will entail some creativity and knowledge of past results on the part of the investigators. Postscript: I read my colleagues contributions to this debate with great interest and especially appreciate the overall historical and philosophical context provided by Dr. Baker. While my fellow debaters and I have taken quite different approaches to addressing the topic of hypothesis testing in hydrology, there are common themes that have emerged. For example, all four contributions comment on the relationships between hypothesis testing and continuing model development in advancing hydrology. Other common themes may be the importance of surprises and the impact that advances in monitoring technology will have in the future. Hopefully, the readers of WRR will find among this range of perspectives some useful insights for their future endeavors.

Acknowledgments I am indebted to many colleagues for stimulating discussions and to research support provided through the USGS Toxic Substances Hydrology program, the McMurdo Dry Valleys Long-term Ecological Research project (NSF1115245), the Niwot Ridge Long-term Ecological Research project (NSF1027341), the Boulder Creek Critical Zone Observatory (NSF0724960), and other research projects supported by the National Science Foundation. I also acknowledge support for independent research while a program officer at the National Science Foundation.

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