Dr. Timothy A. Cohn, U.S. Geological Survey (USGS) statistical hydrologist and expert on flood risks, water quality, and hydrologic trends, died at his home in Reston, Va., surrounded by his loved ones on 20 February 2017, a few days short of his 60th birthday.
During his 30 years with the USGS, Tim developed innovative tools to help scientists, engineers, and policy makers better understand water quality and streamflow (especially floods) in a manner that informs and protects Americans. His tools and concepts are used by federal agencies and are incorporated in software used worldwide.
Tim was a hydrologist with a great gift for mathematics and statistics, and he never lost sight of the importance of hydrologic science to sound public policy. He shared his ideas and insights freely with many colleagues in the USGS and other agencies of government and with his university colleagues. For his outstanding contributions to the USGS, Tim was awarded the Department of the Interior’s Distinguished Service Award.
Tim’s influence extended beyond those tools. To be with Tim was ennobling. His friend and colleague Bill Hooke, in his 20 February blog post, said that Tim was “skeptical of shortcuts and premature judgments.” Hooke continued, “To dialog with him…was always an education. To leave was to leave with a stronger determination to be more thorough, to do higher-quality work, be more self-critical.”
Tim always exhibited curiosity and followed through by learning about a diverse range of topics, including the medical science involved in his treatments for mantle cell lymphoma, the disease that ultimately took his life. Hooke went on to say of Tim, “He had a great sense of humor but never employed it at anyone else’s expense. To be around Tim was to experience dignity and respect.”
Tim was born in Boston, Mass., on 26 February 1957. He grew up in Cambridge, dropping out of high school to go to Colombia and work on Ingetec’s Chivor Hydroelectric Project, an experience that kindled a lifelong love of science and water. He then earned a B.A. in mathematics from Swarthmore College (Swarthmore, Pa.) and an M.S. and Ph.D. in water resource systems engineering from Cornell University (Ithaca, N.Y.), where he developed a lifelong collaboration with his major professor, Jery Stedinger.
Lasting Contributions to Hydrology
Tim made lasting contributions to three major topics in hydrology. First, he developed an unbiased and accurate method for estimating how much pollution flows down a river in a given year on the basis of very limited data and highly nonlinear relationships. The rivers flowing into Chesapeake Bay were his test bed, and his methods have been incorporated into the most commonly used statistical software designed to address this issue. He made the method properly handle “censored data” (data values reported as less than some limit of detection), and he developed the means to assess the accuracy of the estimates that the method produced.
The second topic was his examination of the role of long-term persistence (the tendency of data values to stay above or below their mean value for long periods) in hydrologic records that results in the appearance of trend-like behavior. With his USGS colleague Harry Lins, Tim published “Nature’s Style: Naturally Trendy,” which illustrates how readily long-term persistence in hydrologic records at timescales of decades to centuries can be mistaken for trends. It is an important cautionary tale for all scientists who aim to characterize the signature of human actions on natural phenomena such as streamflow.
Probably his most notable contributions have been in the area of flood hazard estimation. Working with Jery Stedinger and others, he developed a set of mathematical tools for analysis of long-term flood records that are partly based on instrumental records but also contain historical, geological, or botanical evidence of large floods in the distant past. He also developed a much-improved approach for eliminating the influence of “low outliers” (years with no real flood events) from the analysis of flood frequency. As was typical for Tim’s work, he stressed the importance of quantifying the uncertainty, and he reminded practitioners of just how little we actually know. The analysis methods he developed are at the core of the new interagency manual for flood frequency analysis (now in the final stages of review), the first update of official flood frequency methods in more than 40 years.
A Keen Interest in Science for Public Good
Alongside this record of technical accomplishment, Tim was keenly interested in the public policy aspects of his science. He served as an American Geophysical Union (AGU) Congressional Science Fellow in the office of Senator Bill Bradley (D-N.J.), and he was president of the AGU Societal Impacts and Policy Sciences focus group. While he served as the USGS science advisor for hazards, Tim took a leadership role in a series of workshops promoting collaboration between the government and the private sector to reduce disaster loss (sponsored by the National Science and Technology Council’s interagency Subcommittee on Natural Disaster Reduction). This activity resulted in the volume Living with Earth’s Extremes, which Tim coedited with Kathleen Gohn and Bill Hooke.
Tim also served on several National Academy of Sciences studies related to floods and risk reduction. He was a tireless contributor to the science community through his service on the Board of Governors and Executive Committee of the American Institute of Physics and his role as an associate editor of AGU’s journal Water Resources Research.
Enthusiasm for Life
Tim’s enthusiasms extended well beyond his work. He was an avid distance runner. He once said, “Ultramarathons are more fun than marathons. You don’t just run. You talk. You stop for meals. You’re in community.” He served as president of Reston Runners and also was a board member of Reston Association, working to improve the Virginia community he had lived in for more than 30 years.
Tim is survived by his wife, Sarah S. Humphrey; children, Alexander Cohn and Hannah Cohn; mother, Barbara P. Norfleet; two brothers; three nephews; and a niece. He is also survived by many grateful hydrologists who knew the joy of interacting with him and many more who have benefited from his research.
—Robert M. Hirsch (email: [email protected]), U.S. Geological Survey, Reston, Va.