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  Summary The National Research Council (NRC) Committee on Hydrologic Sciences (COHS) convened a workshop, titled Global Change and Extreme Hydrologic Events: Testing Conventional Wisdom, to promote dialogue across the science and water resource management communities with respect to climate change and its links to extreme hydrologic events, specifically floods and droughts. The workshop’s purpose was to probe the conventional wisdom that as the climate warms there will be an “acceleration” of the hydrologic cycle that will translate into potentially more frequent and severe floods and droughts. The issue is fundamental not only to the science of climate change but also to the capacity of the nation and, indeed, the world to adapt to changes in the Earth system in the 21st century. The workshop reviewed evidence supporting the conventional wisdom, assessed the degree to which the phenomenon— or at least its perception—is consistent across the atmospheric and hydrologic science realms, and assessed the effectiveness by which the scientific knowledge base is currently being translated into water policy and management. The workshop and deliberations of the host committee yielded several valuable findings as summarized here. Climate theory dictates that core elements of the climate system, including precipitation, evapotranspiration, and reservoirs of atmospheric and soil moisture, should change as the climate warms, both in their means and extremes. The issue rests theoretically on the Clausius-Clapeyron relation, which describes how a warmer atmosphere can hold more water vapor, which in turn will support more vigorous precipitation and surface wetting, and more intense evaporation and evapotranspiration. Although the current generation of climate models effectively simulates this phenomenon’s atmospheric components, there is mixed observational evidence on the hydrologic response to these postulated changes, namely, floods and droughts. This disconnect between climate model simulations and observational evidence is due in part to the pathways that these atmospheric changes take once they encounter the complexity of land-surface systems. Well- mixed and rapid atmospheric processes interact with heterogeneous substrates and storage and release processes that are regulated by vastly different time constants. In addition, traditional assumptions on the statistical distribution of hydrologic events used to analyze hydrologic extremes are predicated on stationarity, yet the recent record shows that this assumption is not accurate. Furthermore, the nature of hydrologic extremes is convolved with land cover change, 1   

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2   Global Change and Extreme Hydrology: Testing Conventional Wisdom    urbanization, and the operation of water management facilities such as dams, irrigation works, wells, and diversions. As a result, a coherent picture of the nature of likely future changes in hydrologic extremes has yet to evolve. A “grand challenge” thus faces the climate and hydrologic sciences communities—to understand the nature of ongoing changes in climate and hydrology and the apparent anomalies that exist in reconciling their extreme manifestations. The climate science, water science, and engineering applications communities have yet to establish sufficient interaction to appreciate the value of information products generated by each community. For example, critical terms are used freely with different meanings and research agendas have not been unified even around the arguably well-defined question of climate extremes. From a hydrologic perspective this lack of interaction has not only limited fundamental research on climate extremes but also impeded the translation of new and potentially useful outputs from scientists into the planning and management realm. Risk to the nation’s infrastructure from water-related extremes is a function of not only the climate-change- induced hydrologic hazards but also the exposure of assets (and their value) to these extremes, as humans continue to settle and build in hydrologically dangerous settings such as floodplains and river deltas. Without substantially greater interchange of research findings and ideas across these three communities as well as further understanding of the various dimensions of the risk, the design of effective climate change adaptation strategies will remain unrealized. Hydrologists stand in a useful position between climate change scientists and practitioners to tackle research that expressly links the character of climate variability and change to essential hydrologic process studies and metrics over many scales. With hydrologic processes as the intermediary, hydrologists could lay the groundwork for a more effective translation of climate research findings into applications. Although a full understanding of the hydroclimatology is yet to be secured, practical designs to cope with the possibility of elevated climate and hydrologic extremes based on historical time series and ad hoc margins of error are available for use and these techniques do rely on sufficient observational data. Basic monitoring of key elements of the hydrologic cycle provides an irreplaceable information resource that is particularly critical in a non-stationary environment. Addressing basic questions about the hydrology of extremes requires long and unbroken time series. Although the United States has an enviable record of hydrologic measurement, its ability to maintain this effort is jeopardized by an increasingly fragmented network of water quantity and quality monitoring. Furthermore, reliance on observations-based, a posteriori analysis—although practical in the short-term—may obscure the inherent value of research aimed at causality and improved forecasting.