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<br />,. <br />I <br /> <br />changed precipitation patterns) that may obscure a long-term trend towards decreases in runoff for some <br /> <br />sub-basins. <br /> <br />Runoff results taken directly from GCMs show poor correspondence with results generated by <br /> <br />the NWSRFS model using GCM temperature and precipitation scenarios. In general, runoff and soil moisture <br /> <br />outputs from GCMs suggest less drying than the NWSRFS model, despite increased air temperatures and <br /> <br />PET. Rind, et 81. (1990) have concluded that soil moisture deficits and vegetation dessication are <br /> <br />understated in the GCM simulations because of their lack of realistic land surface models. Thus, even <br /> <br />though GCM estimates of PET may be quite high (reflecting higher temperatures), actual evapotranspiration <br /> <br />remains quite low in the models due to inadequate assumptions about evapotranspiration efficiency. <br /> <br />Overall, GCM predictions of runoff should be considered less reliable on a regional basis than those results <br /> <br />obtained by hydrologic modeling (WMO, 1987). <br /> <br />The statistical significance of these results cannot be assessed in a definitive manner. On the <br /> <br />one hand, because data generated by the sensitivity runs are highly correlated with data generated by the <br /> <br />base runs, sensitivity estimates of changes in the mean and standard deviation would be expected to be <br /> <br />reasonably accurate and statistically significant with respect to one another. At the same time, however, <br /> <br />the streamflows generated by the scenarios may not be significantly different from values compatible with <br /> <br />the historic streamflow series. Using the method put forth by K1emes (1985, App. B), our analysis suggests <br /> <br />that precipitation changes of more than 10% would be necessary before changes in runoff would be <br /> <br />significantly different from the historic streamflow series, even if the streamflow distribution were to remain <br /> <br />stationary. Moreover, temperature changes of .f C would not produce a statistically observable impact on <br /> <br />runoff, unless accompanied by precipitation decreases. This is consistent with the finding of K1emes (1985) <br /> <br />that precipitation changes of 15 to 20% would be required to generate statistically significant changes in <br /> <br />runoff in the Pease River (Texas) and the Leaf River (Missouri). This conclusion does not imply that the <br /> <br />impacts of climatic change are insignificant but does suggest the difficulty inherent in detectina the impacts <br /> <br />37 <br />