impoundments on stream ecology, and (3) a better understanding of water quality problems caused by irrigated agriculture (NRC, 1989).

Regional development and migration have placed further stress on our water sources. Large populations have migrated to warmer climates in California, Nevada, Arizona, Texas, and Florida, causing growth rates of 85 percent to more than 400 percent between 1970 and 2009 in those states while the national population has increased by less than 50 percent (Figure 1-2). In some places, these changes have necessitated infrastructure to collect and move water on a grand scale (e.g., the infrastructure on the Colorado River, the California State Water Project, and the Central Arizona Project).

An even broader perspective on this migration is provided in the U.S. county-level population projections through 2030 prepared by the U.S. Global Change Research Program (Figure 1-3). Continued development of these population centers in the southwest and arid west and continued migration from population centers in the eastern and midwestern United States will require substantial transformation in the way water is procured and used by the people who live and work in these geographies.

The shift in population and associated water demand is further complicated by potential impacts of climate change on the water cycle. Increases in evapotranspiration due to higher temperatures will increase water use for irrigated agriculture and landscaping while changes in precipitation patterns (see Figure 1-4) may diminish the ability of existing water infrastructure to capture water. This is particularly important in the western United States where shifts in the timing and location of precipitation and decreases in snowfall are expected (NRC, 2007).

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FIGURE 1-2 Population growth in selected states between 1970 and 2009.
SOURCE: Data from U.S. Census (2010b).

Considerable uncertainty remains about the impacts of climate change on water supplies. Improvements in models and the collection of additional data are likely to reduce the uncertainties associated with these estimates in coming decades. However, the pressures placed on water supplies by the combination of population growth and the likely impacts of climate change necessitate a reexamination of the ways in which water is acquired and used, before all of the questions about climate change impacts on the hydrological cycle are resolved (NRC, 2011a).

NEW APPROACHES TO WATER MANAGEMENT

The increase in population coupled with the decreased rate of construction of reservoirs, dams, and other types of conventional water supply infrastructure is leading to a new era in water management in the United States. The pressures on water supplies are changing virtually every aspect of municipal, industrial, and agricultural water practice. These changes in water management strategies take two principal forms: reducing water consumption through water conservation and technological change and seeking new sources of water.

Reducing Water Consumption

Improvements in water efficiency and programs for water conservation have begun to change our national water use habits, reducing per capita water consumption. More changes of this kind are likely in the future across many sectors. In Table 1-1, selected data on water use collected by the U.S. Geological Survey (Kenny et al., 2009) are summarized, where changes in water use by both agriculture and industry are clearly evident.

While the U.S. population grew from roughly 150 million to 300 million persons during the 60-year period, industrial water use—an application that was once the third highest use of water in the United States—grew only modestly between 1950 and 1970 and has been on the decline for 45 years now. These decreases are due to increased efficiency, higher prices for water and energy, and a shift away from water-



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