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<br />basin are predicated on streamflow data gathered within the last 80 years. In fact, the Colorado River <br /> <br /> <br />Compact of 1922 was based upon less than 20 years of data, and as a result allocated more water than is <br /> <br />likely to be available in an average year. The ability of this system to function under altered climatic <br /> <br /> <br />conditions has not been seriously considered. <br /> <br />Scenarios of Climate Chance for Imoact Assessment <br /> <br /> <br />To assess the implications of global warming for water resources, regional-scale details of future <br /> <br /> <br />changes are needed for temperature, precipitation, evaporation, wind speed, and other hydroclimatological <br /> <br /> <br />variables. Because our ability to predict these details is limited, climate-impact analysis must rely upon the <br /> <br />development of scenarios. Such scenarios can be either hypothetical or derived from General Circulation <br /> <br /> <br />Models (GCMs), paleoclimatic reconstructions, or recent historical climate analogues (WMO, 1987; USEPA, <br /> <br />1989). <br /> <br />Hypothetical scenarios are simple combinations of changes in variables (usually temperature, <br /> <br /> <br />precipitation, and potential evapotranspiration) that are consistent with global changes expected as a result <br /> <br /> <br />of the greenhouse warming. While such scenarios are limited by the fact that they may not be internally <br /> <br /> <br />consistent, they provide a very useful means of testing hydrologic vulnerabilities. If constructed <br /> <br /> <br />systematically, hypothetical scenarios can be used to develop sensitivity studies that delineate the relative <br /> <br /> <br />importance of changes in temperature and precipitation to changes in runoff. Subsequently, as estimates <br /> <br /> <br />of future temperature and precipitation improve, the impacts on water resources can be easily estimated. <br /> <br />Table 1 lists the range of hypothetical scenarios used in a variety of studies. The values chosen <br /> <br /> <br />typically reflect best estimates of changes in important climatic variables, although extreme values are <br /> <br /> <br />occasionally chosen to explore where a system might fail to perform as expected or designed. Thus, the <br /> <br /> <br />practice of using hypothetical temperature increases of 1, 2. 3. or tf Celsius reflects the consensus that <br /> <br /> <br />greenhouse warming will produce, temperature rises in this range, given an equivalent doubling of <br /> <br />5 <br />