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ity of hydrologic models for use with climate change projections (Box 2 -1) and focused on water balance models as particularly useful for identify hydrologic consequences of changes in temperature, precipi- tation, and other climate variable. In addition, a number of other types of models have been applied to modeling hydrologic effects of climate change (Examples are shown in Table 2 -4). One limitation of many of these models is that they do not incor- porate physiological changes in plants or changes in plant communities resulting from increased temper- ature and atmospheric CO 2' Box 2 -1. Criteria for evaluating hydrologic models for use in climate change impact assessment (Gleick 1986). Accuracy of the hydrologic model Degree to which model accuracy depends on the climatic conditions used to develop and calibrate the model Availability of input data, including historical data Accuracy of the input data Model flexibility and ease of use Compatibility with existing general circula- tion models Compared to surface waters, far fewer studies have assessed the potential impacts of climate change on groundwater. Indeed far less is known about groundwater recharge and levels even under current conditions. Groundwater systems will generally respond more slowly to climate change than surface water systems and, as compared to surface water, climate effects on groundwater may be more heavily influenced by changes in precipitation than tempera- ture (Kundzewicz et al. 2007). However, in warm periods, temperature effects are likely to be more pronounced (Kundzewicz et al. 2007). Recharge rates are determined by precipitation minus the combined effects of evapotranspiration and surface runoff. Warmer temperatures and longer growing seasons are expected to increase evaporative demand (Allan et al. 2005). As with surface water, ground- water recharge will be affected by changes in the frequency and magnitude of intense precipitation events as well as total precipitation amounts. Several studies document generally increasing stream flow in the eastern and southeastern regions of the United States over the last century (tins and Slack 2005, Mauget 2003) consistent with trends in precipitation. This overall pattern was observed in the South - Atlantic Gulf, but the region showed more variability than other regions of the U.S. For exam- ple, a number of stations documented low stream flow, particularly in Georgia (tins and Slack 2005). While precipitation is a major driver of runoff, increases and decreases in precipitation do not neces- sarily correspond to equal increases and decreases in runoff (Karl et al. 2009). Rose (2009) found a high degree of elasticity in the rainfall -runoff rela- tionship in the southeastern U.S. in that small devia- tions in rainfall amounts resulted in proportionally greater deviations in runoff. These differences were largely driven by differences in elevation and water- shed relief. For example, the runoff /rainfall ratio for the Blue Ridge region was more than twice that of the Coastal Plains or Piedmont regions in North Carolina, indicating that stream flow in areas with high topographic relief might be more susceptible to changes in precipitation regimes. Furthermore, the relationship between rainfall and runoff was more tightly correlated in the Blue Ridge than the Coastal Plains or Piedmont (Rose 2009). Milly et al. (2005) looked at global runoff projec- tions (2041 -2060) using a set of models from the IPCC Fourth Assessment Report (2007). The results for the United States are replotted in Lettenmaier et al. (2008) and show general agreement among a majority of model runs for slight increases (2 -5 %) in runoff in the Southeast (Figure 2 -6). However, these