if total precipitation remains constant. Higher temperatures and runoff from intense rainfall can both negatively affect the physical and chemical characteristics of freshwater and thus water quality.
Despite considerable improvements in modeling, significant uncertainties remain in projections of precipitation—including its distribution, intensity, frequency, and other characteristics—as well as in related variables such as land use and land cover change. These uncertainties are compounded by uncertainties in our technical capacity to store, manage, and conserve water resources, as well as in socioeconomic, cultural, and behavioral issues that shape the use of water. Multisectoral planning and sophisticated decision-support tools can help water resource managers avoid the most undesirable consequences of climate change in their areas of responsibility (Bates and Kundzewicz, 2008; Gleick, 2000; Vorosmarty et al., 2000). Adaptive water management approaches at operational time scales will be particularly important (e.g., Georgakakos et al., 2005), and long-term strategic decisions need to be robust—that is, able to meet water management goals under a range of plausible future climate conditions (e.g., Dessai and Hulme, 2007; Lempert, 2002; Lempert and Collins, 2007; Lempert et al., 2003).
Observed changes in precipitation are broadly consistent with theoretical expectations and reasonably simulated by global climate models (Bates and Kundzewicz, 2008; Trenberth et al., 2007; Zhang et al., 2007a). While total precipitation in the United States has increased by about 5 percent over the past 50 years, there are significant regional differences, with generally wetter conditions in the Northeast and generally drier conditions in the Southeast and particularly the Southwest (Figure 8.1) (see also Field et al., 2007b). A wide range of climate models using different emissions scenarios predict that these regional trends will continue, with generally robust model results for the north and with high uncertainty for the south (Christensen et al., 2007; USGCRP, 2009a). Other factors in addition to temperature influence precipitation. Specifically, uncertainty remains in our understanding of the effects of aerosols on cloud formation and precipitation. For example, climate models underestimate the magnitude of the observed global land precipitation response to 20th-century volcanic forcing (Hegerl and Solomon, 2009) as well as human-induced aerosol changes (Gillett et al., 2004; Lambert et al., 2005).