weather events may be related to great recycling of water through the hydrologic cycle and may not be directly proportional to more extreme hydrologic events. Harry Lins of the U.S. Geological Survey posed the question of what the likely effects of an accelerated hydrological cycle might be on streamflow in general, and on floods in particular. Data and published literature indicate that the relative precipitation sensitivity (elasticity) of mean streamflow with respect to precipitation is much greater than that of peak streamflow, and that precipitation sensitivity decreases as flood return period increases. Hence, while flood peaks are quite likely to increase if precipitation increases, their fractional change relative to a given fractional change in the mean precipitation is less than the fractional increase in the mean flow.
Richard Vogel of Tufts University noted that multiple sources of uncertainty and non-stationarity are now inherent in nearly all water-resource planning problems, and it is important that the water resources engineering community shift away from the stationarity paradigm on which it has presumed for many decades. He described work that has analyzed several thousand flood peak records from across the United States. Although not yet definitive, his work suggests that, where non-stationarity is evident, it is more likely to be associated with changing land use (especially urbanization) than with climate change. Katie Hirschboeck of the University of Arizona argued for the necessity of moving beyond conventional methods for estimating the frequency of extreme hydrologic events. She described an approach based on parameterization of spatially and temporally varying hydroclimatic extremes (which she called synoptic hydroclimatology) as a starting place for making operationally useful decisions about the impacts of climate change on hydrologic extremes.
Gerald Galloway of the University of Maryland discussed the nature of guidance that can be given today to U.S. water management planners to deal with an uncertain hydrologic future. He acknowledged that floods are acts of nature, and flood consequences are a result of man, so while flood risk calculations are not precise, risk assessment provides insights and a basis for prioritization. U.S. water managers need to deal with present problems by using the Precautionary Principle in future planning with a newly developed national policy. Michael Hayes, of the University of Nebraska, Lincoln, spoke about the status of drought risk management in the United States. He highlighted several key issues that should be considered including the facts that drought is a local issue, monitoring is essential, mitigation and planning require innovative ideas, worse-case scenarios should be considered, and communication between scientists and the public is key.