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<br />the Colorado River. These include several combinations of hypothetical changes in temperature and <br />precipitation and scenarios derived from three state-of-the-art GCMs were used to develop inputs for use <br />in modeling the Colorado River Basin. The use of more than one GCM has two advantages: first, reliance <br />on one GCM may give a false impression of accuracy; and second, the use of more than one GCM <br />highlights model differences and similarities and permits a broader analysis of outcomes and sensitivities. <br />The data on temperature and precipitation changes due to a doubling of carbon dioxide come from the <br />Goddard Institute for Space Studies (GISS) model, the Geophysical Fluid Dynamics Laboratory (GFDL) Q- <br />flux model, and the United Kingdom Meteorological Office (UKMO) model (Hansen, et al., 1983, 1988; <br />Manabe and Stouffer, 1980; Manabe and Wetherald, 1987; Wilson and Mitchell, 1987). Each of these models <br />is an equilibrium run, i.e. carbon dioxide is doubled all at once in the models and a new equilibrium climate <br />is established. <br /> <br />In addition, data from a GISS transient run were incorporated into our analysis. In the transient <br /> <br /> <br />run, the GISS model was started with same amount of greenhouse gases in the atmosphere as measured <br /> <br /> <br />in 1958, and the concentration of gases was gradually increased. We developed a climate scenario that <br /> <br /> <br />reflected the decadal average of temperature and precipitation changes that occur in the years 2030 to 2039. <br /> <br />These data were presumed to provide an indication of how much change will occur over the next 40 years, <br /> <br /> <br />given the assumptions in the GISS model concerning the future rate of greenhouse-gas emissions (Hansen, <br /> <br /> <br />et al., 1988). The changes in temperature and precipitation predicted for the Colorado River Basin by each <br /> <br /> <br />of these GCM runs is given in Table 2. <br /> <br />METHODS OF ANALYSIS I: HYDROLOGIC MODELING <br /> <br /> <br />Backaround <br /> <br /> <br />Once scenarios of climate change are developed, hydrologic models can be used to estimate <br /> <br /> <br />impacts on water resources. If accurate estimates of future water availability are to be calculated, regional <br /> <br />9 <br />