Dennis P. Lettenmaier was awarded the 2018 Robert E. Horton Medal at the AGU Fall Meeting Honors Ceremony, held on 12 December 2018 in Washington, D. C. The medal is for “outstanding contributions to hydrology.”  

Over the past 3 decades, Dr. Lettenmaier has grown into the foremost pioneer in hydrologic land surface modeling. His key overall contribution in this evolution has been to take the lead in the macroscale parameterization of land surface variability, as it controls runoff production, integrated with a full land surface energy balance, including the effects of vegetation. With newly available global data sets, this combination of energetics appropriate for different scales has represented the critical step toward the study of continental and global hydrology, a challenge made by Eagleson in the 1980s to the hydrologic community. The foundational references by him and his collaborators have been widely used by scientists from as many as 54 different countries.

In parallel, Dr. Lettenmaier led major breakthroughs regarding the effects of climate and climate change on the hydrologic cycle. This work indicated how climate change issues can result in vastly different responses in the timing and amount of surface runoff, allowing the development of strategies for threatened arid-region basins. In systematic analyses of climate change in cold regions, he and his team developed a quantitative basis for estimating the fraction of snowmelt in the annual runoff of major river basins, and they explored the relative controls of temperature and precipitation changes on mountainous snow packs, causality of flood frequency changes, snowpack-associated runoff, evaporation, and, most recently, novel algorithms for frozen soils and permafrost representations and for snow interception and blowing snow.

Beside these two main lines of enquiry, Dr. Lettenmaier has made successful explorations of other areas as well. For instance, regarding the hydrologic aspects of the Anthropocene, he investigated and evaluated the relative impacts of climate change and land cover changes in coastal lowlands. In a major improvement in drought assessment, with his associates he put forth a severity-area-duration method, allowing unbiased comparison of envelope droughts over large continental areas.

These scientific contributions are widely acknowledged, and Dr. Lettenmaier has already been recognized with several prestigious awards. In addition, he has distinguished himself by his unselfish cooperation and leadership initiatives, as borne out by the numerous committee and editorial responsibilities he has assumed over the years. The Horton Medal is awarded in recognition for “outstanding contributions to hydrology.” Unquestionably, Dr. Lettenmaier is a perfect embodiment of this ideal: His superior contributions through his intense dedication in scholarship and service place him among the very best in our hydrologic community.

—Wilfried H. Brutsaert, Cornell University, Ithaca, N.Y.

I want to thank my nominator, Wilf Brutsaert, for his kind words, and Dan Cayan, Randy Koster, and Soroosh Sorooshian, who wrote in support of Wilf’s nomination. I also thank the many students, postdocs, and staff who’ve worked with me over the years. This award is really about them. I was lucky that my career coincided with a major transformation in hydrology and that it came about at an opportune time for me. In the mid-1980s, I was asked to participate in Environmental Protection Agency’s (EPA) report to Congress on the effects of climate change on the United States. EPA wanted a study of climate change effects on California’s water resources. I thought we could investigate a single reservoir to which we would perturb inputs on the basis of assumed changes in precipitation and evaporative demand. The response was “not good enough, we need you to do all of California.” I really had no idea how, but with postdoc Thian Yew Gan doing the hydrologic modeling and Dan Sheer the reservoir operations, we glommed something together. The exercise made clear to me that we simply didn’t have the tools to do large-scale hydrologic prediction at that point. Shortly thereafter, discussions with Eric Wood led to conceptualization of the VIC model, followed shortly thereafter by Xu Liang’s dissertation work that developed the first version of the model. That in turn led to further development work by many students and postdocs at the University of Washington and Princeton University. It also led to a host of studies on topics we (or at least I) hadn’t begun to think of in those early days—drought work (Kostas Andreadis and Kingtse Mo), data assimilation (Kostas Andreadis), and Arctic hydrologic processes and change (Laura Bowling, Fengge Su, and Jenny Adam), among just a few. In more recent years, we’ve turned our attention from “what will happen if” questions to “what has and is happening” in the spirit of what Rich Vogel has termed hydromorphology. In that vein, I think in particular of Phil Mote’s declining mountain snowpack papers and current student Mu Xiao’s recent Water Resources Research paper on causes of declining Colorado River streamflows. All told, it’s been a great ride. In closing, I want to thank AGU, which has been my professional home for well over 40 years. I think especially of AGU’s motto “unselfish cooperation in research,” which typifies the many collaborators I’ve worked with over the years; it’s their shoulders on which I stand. Thank you to all. —Dennis P. Lettenmaier, University of California, Los Angeles

Dennis P. Lettenmaier was awarded the Hydrology Section Award at the Fall Meeting Hydrology Section Reception, which was held on December 16, 2000, in San Francisco, California. The medal recognizes outstanding contributions to the science of hydrology. Citation I think that it would be a great challenge to identify a single hydrologist who has never heard of Dennis Lettenmaier. Indeed, Dennis is a truly outstanding scientist, very well known to all of us for his numerous significant contributions to hydrology. In particular, he has been a key player in the integration of hydrological science—with the atmospheric science community on the one hand, and the water resources engineering community on the other. Dennis has combined an intimate knowledge of hydrological processes to address critical problems regarding the spatial and temporal behavior of streamflow and precipitation, land-atmosphere coupling, use of remote sensing with process-resolving hydrologic models, and hydrologic impacts and feedback of potential climate change. The popular “variable infiltration capacity” model (known mostly by its acronym as the 'VIC' model) is but one exemplary product of his research creativity. For 25 years, his research in these areas has been pioneering, innovative, and significant. There is no doubt that the quality of Dennis' work or its impact on our discipline is unquestionable. But when qualifications for the Hydrology Section Award are being evaluated, the service of the nominees to the hydrology community should not be underestimated, especially at the level of the few finalists. In fact, it appears that Dennis is not only a talented scientist who has made very significant scientific contributions to hydrology. It is now obvious that the vision and leadership that he provided when he unselfishly stepped forward a few years ago to direct for a year the NASAs Land Surface Hydrology Program (LSHP) already has, and will undoubtedly continue to have a tremendous impact on our discipline. While his primary responsibility was to reorganize and develop NASA's efforts in hydrological research, he championed the creation of a Committee on Hydrological Sciences at the National Research Council, something the community had lobbied for over many years. His leadership also fostered NASA's communication with other funding agencies such as NOAA and NSF to common research objectives and collaborative funding of research grants. Obviously, this has a tremendous impact on the overall funding given by NASA and other agencies in hydrological research. My colleagues for this nomination—Wilfried H. Brutsaert, Efi Foufoula-Georgiou, George M. Hornberger, Eric F. Wood and Is—wish to thank the Award Committee for trusting our nomination and for making the right choice in recognizing the outstanding contributions to our discipline of Dr. Dennis Lettenmaier. —Roni Avissar, Rutgers University-Cook College, New Brunswick, N.J., USA

First, I want to thank Roni for his kind words, as well as the Hydrology Section and its Awards Committee. I am deeply grateful for their actions.The citation for the award indicates that it is for contributions to macroscale hydrology, and for service. Let me reflect briefly on both.From my perspective, the macroscale hydrology work (which really represents efforts over at least a decade by numerous graduate students, post-docs, and others, in both my group and Eric Wood's at Princeton) began with a phone call in 1988. That phone call came from Pete Hawkins, who was then on leave form Utah State University at the EPA Environmental Research Laboratory in Corvallis. He had been asked to help put together a team to do supporting work for a report to Congress, which was to address the possible effects of climate change on the U.S. He had been asked specifically to find someone to do a study of climate change effects on hydrology and water resources in California. He asked me whether I thought I would be able to lead the hydrology part of such a study.After brief reflection, I said that we probably could do some kind of sensitivity analysis, assuming that an isolated single reservoir system in a catchment of moderate size was selected. Pete said that no, that was not what EPA had in minds—they wanted to do the entire state, or at least the Sacramento-San Joaquin basin. I told him that there were no models that were appropriate to that scale, and I had no idea how to do the problem. But, I was a bit hungry at that point, so I said I'd try to figure something out (as a side note, I've been supported essentially entirely by 'soft' money throughout my career, so there is a certain aspect of longevity that has to do with acquiring research funding). We did in fact figure something out, which was a bit of a kludge—modeling of relatively small catchments, linked together by a stochastic aggregation-disaggregation model. The reviewers (and I specifically remember several pages of comments by John Schaake, which we printed as an appendix to the resulting report) pointed out lots of problems with the approach. At that point, though, I think the reviewers recognized that there was not much around in the way of models that were better, and the work found its way into print.Shortly after this experience, Eric Wood and I began to talk about the form of a model that might be able to address macroscale problems—for instance, modeling water and energy fluxes of continental scale watersheds. Eric had recognized, much earlier than I, that there really was something to remote sensing, and he'd begun to think about how remote sensing data might be integrated into a surface water modeling framework. He'd also recognized that the then-emergent soil-vegetation-atmosphere transfer schemes (SVATS) which were designed to replace the earlier bucket models of hydrology in climate and weather prediction models wouldn't properly represent surface hydrology—even at large scales. He suggested that I might want to take a look at some work by Ezio Todini (the so-called Arno model), and its roots in work by the Chinese in Nanjing. Coincidentally, at about that time, Xu Liang was looking for a dissertation topic, and I pointed her in the appropriate direction. Her 1994 JGR paper, and various others that followed, lay out the form of the so-called Variable Infiltration Capacity (VIC) macroscale model, around which a good deal of work at Washington and Princeton has revolved over the last ten years. There is no point (or space) for details here, but I think some of the distinguishing features of this entire line of work are worth mentioning. First, the model balances both energy and water at the land surface. This is not, seemingly, a big deal, except that the energetic aspect of the model, not addressed in most hydrologic models, addresses what atmospheric modelers see as the primary purpose of a land surface model—to partition energy at the land surface. Hydrologists, on the other hand, view their task as partitioning of precipitation into 'direct' runoff and infiltration. This dichotomy has resulted in a lot of talking past each other by both communities, a problem which is avoided when models are put to the test of balancing both water and energy. Second, the model is designed to represent explicitly those processes for which there are representations appropriate to the macroscale (which for convenience we typically take to be some number of tens of kilometers). Other processes are parameterized based on abstractions of the underlying process physics—especially the effects of variability in infiltration, but also spatial variability in precipitation, land cover, and so on. In so doing, there exists a pathway for improvement in the parameterizations, and in fact much of the work related to VIC over the years has dealt with improved parameterizations; for instance, of ground heat flux, snow redeposition and spatial variations in melt, and so on. We view the model not as an end product, but as a structure within which there is a continuing evolution of understanding. No doubt, at some point in the future, the time will come to discard the entire structure, and we will move on. In the meantime, though, the model has provided a framework, and a pathway for model improvements. At least in the realm of surface water hydrology, this has been a difficulty in the past; the model structures most widely used in practice do not provide for an obvious pathway for improvements, and the cost of discarding the entire structure is viewed as being too high. The end result has been stagnation.I do want to comment briefly on the service citation, which may be somewhat unusual for this award. I believe the reference is probably mostly to the year I spent on Intergovernmental Personnel Act (IPA) loan to NASA Headquarters in 1997B1998, although I should point out that I also spent a year on IPA loan to USGS' headquarters in 1985B1986. In both cases, the work I did was more related to the management of science than to its execution (at NASA headquarters, I was the program manager for Land Surface Hydrology). In various conversations with others in the research community, the tone of questions regarding both IPAs has often been to the effect that it must have been somehow distasteful to actually be part of the science bureaucracy, if only for a brief time. Nothing could be farther from the truth. In both cases, far from the stereotype of being overconstrained, difficult to get things done, and so on, I was given wide latitude in implementing new ideas. In the case of the USGS, that meant heading a review of a national water quality monitoring network. At NASA, it meant deciding the direction of a program, including writing research solicitations, and designing the review process. I had wide latitude in both cases, including the opportunity to make mistakes. Certainly, while others within those agencies had ideas about what needed to be done, I did not have anyone telling me what I couldn't do. My experiences in both cases were entirely positive, and I'd highly recommend the experience to others.In fact, this brings me to my closing point. One aspect of the 'D.C. experience' is having a better appreciation for how the research community interactss—with funding agencies, of course, but also with the research and policy review process, and with the international science structure. The latter includes, for instance, the oversight process of the NAS/ NRC, interaction of the federal agencies with Congress, and the nation at large. One good example is the U.S. Global Change Research Program, the responsibilities of which go well beyond research. The hydrology community has unfortunately been somewhat limited in its visibility, in part because there are only a few members of the community who have the experience in dealing at the interface between the various elements I've mentioneds—the funding agencies, the international science structure, the Academy, and the Congress. In my mind, we need more members of our community who are interested in going beyond their own research to help represent the broader community at the above interfaces. Opportunities do exist. I would also like to urge those within the 'science establishment' — agency program managers, heads of international programs, and others, to reach out more broadly to the science community for support, and to help find 'new blood' so that we are able to broaden our base.Finally, I would be entirely remiss were I not to recognize the contributions of many students of mine over the years. We truly do stand on the shoulders of our students, and this award is really for their work. I have had the opportunity to work with a number of outstanding students, and I was very pleased to see many of them in the audience when the award was presented in San Francisco. While in no way meaning to diminish the contributions of my Ph.D. students over the years, of whom I am extremely proud, I do want to make a short comment about the Masters students, whose efforts may be less recognized. In a state institution like the University of Washington, many of the students entering the graduate program do so with the intention of earning a master's as the terminal degree. The master's students who have studied with me have done so recognizing (I hope!) that what is expected of them is much more than the minimum that would meet their degree requirements. In particular, they have participated in an academic research program, and the end result of their work has, in most cases, been a journal article or similar publication. I am well aware of many cases in which these students have spent countless hours 'going the extra mile' to produce publication-quality results. In many, if not most, cases, the resulting publications have probably had much less value to their careers in engineering consulting firms, government agencies, and elsewhere, than they have to me. They have made the effort nonetheless, and I believe they deserve special recognition, along with the Ph.D. students and staff who have helped me over the years. Thanks to all of you. —Dennis P. Lettenmaier, University of Washington, Seattle, Wash., USA