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projections are not based on downscaled models and, as a result, do not capture spatial variability at finer scales. Relatively few studies have dealt specifically with projected hydrologic trends in the southeast- ern United States and this remains an area in need of future research. Both spatial and seasonal vari- ability will greatly affect local systems, the latter of which will not be captured in projections of annual averages. An earlier study (Cruise et al. 1999) used downscaled data from the Hadley Center GCM and a simple regionalized stochastic stream flow model to examine the impacts of climate change on water quality in the southeastern United States. Although, their hydrologic model relied on a number of simpli- fied assumptions and is therefore of limited applica- tion, their results illustrate the potential variability across wet and dry season stream flow conditions that are not captured in projections of annual averages. ® , , m 4. oU.. V'11714 m w # "W11111 J1 41 151% hy o- l ,z *6 MI y i ru, ` 'V M , Oil . 45 ll Figure 4.10 Median changes in runoff interpolated to USGS water resources regions from Milly et al. (20 05) from 24 pairs of GCM simulations for 2041 -2060 relative to 1901 -1970. Percentages are fraction of 24 runs for which differences had some sign as the 24 -run median. Results replotted from Milly et al. (2005) by Dr. PC. D. Milly, USGS. Figure 2 -6. Mid - century projected changes in runoff for U.S. regions (Source: Lettenmaier et al. 2008).