to climate change adaptation to date (NRC, 2010a). Workshop participants noted that infrastructure can be constructed in adaptable modular units, with shorter expected longevities (or design revisit times) on the order of 10-20 years, which in practical terms reflects the non-stationarity concept. In general, insights drawn from historical and paleohydrologic records, plus practical assumptions, can provide a basis for establishing margins-of-error on designing infrastructure or repositioning existing assets (e.g., to higher ground). Such strategies have been used to avoid future urban flood damage in the Mississippi River basin (Interagency Floodplain Management Review Committee, 1994; NRC, 2009b; USACE, 2011).

Basic monitoring of key elements of the hydrologic cycle is essential to support analysis of hydrologic extremes with any confidence.

Addressing basic questions on the hydrology of extremes will require continuing commitments to monitoring networks and routine observations, through climate, weather, and hydrologic monitoring networks and their integration (see also, NRC, 2010b). Although the United States has an enviable record of hydrologic measurement, its hydrologic networks have become increasingly fragmented (NRC, 2009a). Absent firm commitments to retain observational networks by federal, state, and municipal agencies, estimation of the risk of hydrologic extremes will be compromised, as will the ability to prepare, adapt, and mitigate the impacts of these extremes as climate conditions change. As one example, the USGS stream gaging network has monitored flow in the nation’s rivers since 1889 (http://water.usgs.gov/nsip/history1.html), yet it and other USGS water monitoring programs are under continuing pressure to provide data with decreasing resources (NRC, 2009a).

Decisions about the design of hydrologic monitoring networks are increasingly confounded by hydrologic non-stationarity, not only because of a changing climate but also because of anthropogenic land cover change and water management effects. Procedures developed in the 1960s and 1970s to determine when stations can be discontinued are based on stationary statistical assumptions that are no longer defensible. These protocols therefore lead to management decisions regarding monitoring stations that are diametrically opposed to those that are applicable to a non-stationary world. The next generation of observational networks may include paleoclimatic and paleohydrologic data sets as well as newer technologies, such as precipitation radars, to augment basic data from stream gage networks.

Closing

The COHS-hosted workshop raised many questions and challenges in terms of characterizing hydrologic extremes, translating scientific knowledge to the policy and management communities, and identifying a productive future role for hydrologic sciences. Workshop participants confirmed the research findings that show that the water cycle is changing and indeed accelerating, but noted the many unknowns that remain with respect to the drivers of this change, the system response, and the implications for society. The issues discussed at the workshop are fundamental to the nation’s environmental security as it embarks on a major



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