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The hydrologic cycle describes the continuous circu- lation and conservation of water on, above, or below the surface of the Earth and is thus uniquely tied to changes in temperature and precipitation. Hydrolog- ic patterns are driven by complex processes associated with precipitation, evaporation, and transpiration, which are typically incorporated into predictive models of future hydrologic system behavior. Because climate change may impact hydrological systems in a number of distinct but interrelated ways, this synthesis is treated separately from temperature and precipitation, with an understanding that projected changes in temperature and precipitation will be interrelated with impacts on hydrology. Changes in climate will have direct and indirect effects on the hydrological cycle and freshwater systems. A warm- er climate increases the capacity of the atmosphere to hold moisture while also increasing evaporation from land and water surfaces, resulting in a more vigorous water cycle (Huntington 2006). Global increases in continental runoff from major rivers, increases in evapotranspiration (ET) inferred from hydrologic budgets and increased ocean salinity, and evidence for increase in water -vapor at the surface over most northern latitudes, all point to ongoing and future intensification of the hydrologic cycle (reviewed in Huntington 2006). Altered precipi- tation regimes will directly affect stream flows and groundwater recharge, but the net effect on water levels will depend on how increasing temperatures and CO2 affect ET processes. Praskievicz and Chang (2009) review hydrological modeling of basin -scale climate change impacts, as well as impacts of urban development and interac- tions with climate change. They note that a number of factors influence basin hydrological response to climate change. Latitude and whether a basin is located in a relatively humid or arid region will affect potential for flood risk and water stress. Humid mid - latitude regions may generally experience increased runoff, whereas and regions may be more likely to experience a decrease in annual runoff. Eleva- tion will influence hydrologic response to warming according to whether the basin is dominated by rainfall or snowmelt. Basins dominated by snow - melt are likely to exhibit increased winter runoff and earlier spring peaks. The geology of the basin will also be an important factor. Groundwater domi- nated systems, particularly those with deep aquifers maybe less sensitive to changes in temperature in the short term whereas systems with shallow aquifers or those dominated by surface flows will respond more quickly. In addition to changes in mean hydrology, climate change will likely affect hydrological vari- ability. Even in areas that see only slight changes in annual runoff, the frequency of very low or very high flows may change significantly. There are several approaches to developing climate change scenarios for incorporation into hydro- logic models (Praskievicz and Chang 2009). One approach is to modify the historical average tempera- ture and precipitation by some fixed amount. A disadvantage of this approach is that these projec- tions may or may not provide realistic reflections of current atmospheric changes. An alternative and increasingly common approach is to use projections based on IPCC emissions scenarios that have been coupled with global circulation models downscaled to appropriate scales. These projections are then used as inputs in a hydrologic model to examine projected changes in runoff and other variables. Uncertainty associated with the choice of GCM, downscaling method, and choice of hydrological model can all impact projected changes to hydrology (Praskievicz and Chang 2009). Of these, the greatest source of uncertainty in the modeling chain appears to be the choice of GCM (Graham et al. 2007). However, fewer studies have addressed the range of outcomes obtained using different hydrologic models with a given climate scenario. Hydrologic models differ in their parameters and assumptions and their usefulness to various applications. Gleick (1986) identified six criteria for evaluating the applicabil-