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Workshop Background and Objectives Flood damages have increased greatly over the last century. As noted by Pielke et al. (2002; Figure 1), a number of reasons for this have been suggested, the most common being development that has en- croached upon flood plains. More recently, climate change, as it might have and might in the future af- fect weather extremes, has been suggested as a possible cause, notwithstanding that the scientific evi- dence as to whether U.S. floods have increased is mixed. It is generally accepted that in a warming cli- mate, some acceleration of the hydrologic cycle can be expected, based on the fact that the water holding capacity of the atmosphere increases with temperature. Specifics, however, are much more difficult to ascertain, given strong regional variations in the manifestation of climate change, and the interaction of mechanisms that might (or might not) lead to increased flood frequency in a warming climate. A complication of making regionally specific predictions of the likelihood of climate change affecting flood frequency is the increasing awareness that methods of planning for hydrologic extremes have be- come outmoded in an era of global change. Milly et al. (2007) states that stationarity*, an assumption that underlies essentially all water resources planning, “is dead.” The question raised by the Committee on Hydrologic Science is, what is to replace it? This one-day workshop was intended to foster initial discussions among the science and applications communities of the following issues related to planning for hydrologic extremes (particularly flooding)†. • What should be the underpinnings and motivating science and applications questions in a new science of hydrologic extremes? • What can and should be the role of new observing methods, both in situ (including new sensor technologies) and remote sensing? How might approaches to the estimation of hydrologic extremes differ based on the richness of the historic observations? * A stationary time series is one whose statistical properties such as mean, variance, etc. are all constant over time. The assumption of stationarity underlies most traditional flood forecasting methods, including those codified by laws and regu- lations. Estimation of the “100-year flood,” for example, uses historical stream gaging data from rivers that are assumed to behave similarly over the period of record to precipitation events that are also assumed to be generated from the same random population of possible events. Changing climate or land use challenges this assumption. † See Stedinger and Griffis, 2008. 1
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2 Research and Applications Needs in Flood Hydrology Science FIGURE 1 U.S. flood damages, 1926-2000, in 1995 dollars. SOURCE: Replotted, with permission, from Pielke et al., 2002. • What should be the interface between the science of hydrologic extremes and applications issues, such as the need to replace standard methods, such as Bulletin 17B‡ and other methods that are based on stationary statistical methods? • How can advances in techniques for the accurate analysis of ancient flood events (e.g., House et al., 2002) aid estimation of future flood magnitudes and frequency, and understanding of the generative processes for extreme flood phenomena? It is emphasized that the workshop was intended to focus primarily on floods in a planning, rather than an operational forecasting, setting. The workshop opened with welcoming remarks by Eric Wood, chair of the Committee on Hydrologic Science (COHS), to the participants (Appendix A) on behalf of the Committee on Hydrologic Science (Appendix B) and the National Research Council’s Water Science and Technology Board. Workshop planning committee members Dennis Lettenmaier, Victor Baker, and David Ford then summarized the history, purpose, and agenda (Appendix C) of the workshop. Will Logan (NRC) explained that the pur- pose of the meeting was to highlight issues for future attention by participants and the National Research Council, but that no consensus findings or recommendations would result from the workshop. This would be left to any future follow-up consensus studies on topics highlighted by participants. Eric Wood then turned the floor over to Dennis Lettenmaier, who introduced the first speaker. ‡ The 1982 flood-frequency guidelines for federal projects in the U.S. (USGS, 1981).