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.

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