years (Trenberth, 1999; Groisman et al., 2005; Kharin et al., 2007; NRC, 2010) and by increases in precipitation levels in the United States over the 20th century (Groisman et al., 2004). Yet a clear picture of how precipitation translates into the hydrologic extremes is frustrated by observations and studies made by the U.S. hydrologic science community. Recent analyses of U.S. Geological Survey (USGS) long-term streamflow records show few statistically significant trends in floods from annual maximum streamflows as a result of intense precipitation within the United States (USGS, 2005). Evidence for changes in droughts in the United States, determined by the balance between precipitation and runoff, is mixed. Trends of increasing precipitation across much of the eastern and central United States appear to have reduced drought severity and length, while a general warming in parts of the West appears to have increased atmospheric evaporative demand more rapidly than precipitation, resulting in longer and more frequent and severe droughts (Groisman et al., 2004; Andreadis and Lettenmaier, 2006).

Floods and droughts are also complicated by the presence of other factors and are not simply climate-driven phenomena. Anthropogenic land-cover change such as deforestation and reforestation, urban expansion, and the pervasive impact of water engineering—impoundment, irrigation, and water diversions, as well as other social factors—confound these signals of change (Vörösmarty et al., 2005; Trenberth, 2011). Yet floods and droughts remain a primary concern for water managers. In the context of these factors, there is a pressing need for decision-makers to better understand the complexity of these interactions and to recognize the limits and opportunities of the current knowledge base upon which their decisions will rest. The implications for water management, agriculture, and other sectors of the U.S. economy, especially in light of widely publicized predictions of increased frequency and severity of hydrologic extremes as the climate warms, have yet to be fully articulated.

The workshop, Global Change and Extreme Hydrologic Events: Testing Conventional Wisdom, was convened by the NRC Committee on Hydrologic Science in January 2010 to probe the conventional wisdom surrounding the acceleration of the hydrologic cycle and its implications. The workshop, sponsored by the U.S. Nuclear Regulatory Commission, the National Aeronautics and Space Administration, and the National Oceanic and Atmospheric Administration, provided a forum for the science and engineering applications communities to identify differing perspectives and to seek common ground on the issue of climate-change-induced floods and droughts. In addition, the workshop provided an opportunity to recognize and potentially begin to transcend the array of contrasting definitions, scientific agendas, methodologies, and observations that separate the climate science, hydrologic, and engineering applications communities as they address the hydrologic extremes question. The statement of task, organized as a series of questions was as follows:

1. Is the global hydrologic cycle accelerating and what does this acceleration look like? Is precipitation becoming more intense? Is drought frequency and severity becoming more prominent?

2. Are hydrologic fluxes associated with floods and droughts changing at the regional scale?



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