Dennis P. Lettenmaier

University of California, Los Angeles

2018 Robert E. Horton Medal Winner

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