Historically, the United States has relied heavily on surface water, and to a lesser extent groundwater, to meet its freshwater needs. It would be easy to assume that precipitation is the most critical factor in determining surface water availability, and thus future water supplies will be controlled almost entirely by changes in average annual precipitation. In reality, however, the relationship between climate change and water supplies is more complex. For example, climate change directly affects temperatures, and hence evaporation from soil and water surfaces, plant transpiration, and mountain snowmelt. The average intensity, seasonality, mode (i.e., rain or snow), and geographic distribution of precipitation are also important for water management decisions. All of these characteristics are closely connected to storm patterns, which are modulated by regional and global patterns of variability on a range of time scales, and both storm patterns and patterns of variability may shift as climate change progresses (e.g., Kundzewicz et al., 2007; Lemke et al., 2007; Trenberth et al., 2007). Moreover, water cycling through soils, land cover, and geologic formations, as well as rainfall intensity and amount, all affect the volume of surface runoff as well as infiltration rates and groundwater recharge, making the response of water resource systems to climate change complex. Changes in land cover and land use will complicate projections of water resource availability as well as the detection and attribution of climate-driven trends; for example, land degradation with accompanying vegetation changes can be a dominant driver of changes in stream flow (Wilcox et al., 2008). In many coastal regions, sea level rise (see Chapter 7) will affect surface and groundwater resources.
The complex processes involved in the water cycle, combined with uncertainties in model projections of future precipitation changes, prevent any easy conclusions about how climate change will affect regional water supplies. Even if model projections do not show any significant changes in total precipitation, for example, shifts in seasonal precipitation patterns or average storm intensity may be critical for water-dependent sectors like agriculture. As discussed in Chapter 6 and the next section below, a higher fraction of rainfall is expected to fall in the form of heavy precipitation events as temperatures increase, and in many locations such a shift has already been observed (see also CCSP, 2008f; Bates and Kundzewicz, 2008). Higher temperatures are also projected to increase soil and surface water evaporation, producing overall drier conditions even 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., Georgaka-kos 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).
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