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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,
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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.
