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3

Water Use

In the twentieth century, future levels of water use were usually estimated by simply extrapolating historical per capita use rates and multiplying them by the projected growth in population. This approach is fundamentally flawed because it ignores the possibility that the social, behavioral, economic, and technological determinants of use can change over time. Although water use is clearly related to the amount of water that is available, it is also determined by a host of other factors such as price, cultural norms, landscaping preferences, and the availability and use of water-conserving technology. One reason for the failure to consider these variables in water resources planning is that the impacts of these variables on water use are not well understood. More effective water resources planning and management in the twenty-first century will require a clear understanding of the variables that determine water use. The ability to manage scarcity by economizing and “regulating” demand constitutes one important option available to the water manager.

DETERMINANTS OF CONSUMPTIVE WATER USE

In the water resources arena, the term “use” has different meanings. Consumptive uses involve changes, either in phase or quality, that render water unavailable for reuse. (Water that has been severely degraded can sometimes be treated for reuse, as discussed in Chapter 2.) For nonconsumptive uses, changes in the properties of water usually are not sufficient to bar its subsequent reuse. Diversionary uses, such as for domestic (household), commercial, public, industrial, and agricultural sectors, are frequently but not always consumptive. Instream uses (e.g., hydropower) are almost always nonconsumptive.

There is an urgent need for improved technologies that will permit water to be used more efficiently by all sectors. However, the development of such



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Page 24 3 Water Use In the twentieth century, future levels of water use were usually estimated by simply extrapolating historical per capita use rates and multiplying them by the projected growth in population. This approach is fundamentally flawed because it ignores the possibility that the social, behavioral, economic, and technological determinants of use can change over time. Although water use is clearly related to the amount of water that is available, it is also determined by a host of other factors such as price, cultural norms, landscaping preferences, and the availability and use of water-conserving technology. One reason for the failure to consider these variables in water resources planning is that the impacts of these variables on water use are not well understood. More effective water resources planning and management in the twenty-first century will require a clear understanding of the variables that determine water use. The ability to manage scarcity by economizing and “regulating” demand constitutes one important option available to the water manager. DETERMINANTS OF CONSUMPTIVE WATER USE In the water resources arena, the term “use” has different meanings. Consumptive uses involve changes, either in phase or quality, that render water unavailable for reuse. (Water that has been severely degraded can sometimes be treated for reuse, as discussed in Chapter 2.) For nonconsumptive uses, changes in the properties of water usually are not sufficient to bar its subsequent reuse. Diversionary uses, such as for domestic (household), commercial, public, industrial, and agricultural sectors, are frequently but not always consumptive. Instream uses (e.g., hydropower) are almost always nonconsumptive. There is an urgent need for improved technologies that will permit water to be used more efficiently by all sectors. However, the development of such

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Page 25 technologies is hindered by the surprising lack of information about the basic determinants of consumptive uses. Even in the agricultural arena where substantially more data are available, what is known about consumptive use is incomplete. Additional research is needed on the determinants of the levels of all these uses. Little is known about the determinants of water use in commercial establishments or for public purposes such as cemeteries, golf courses, and parkland irrigation. Although these uses are often small when compared with domestic, industrial, and agricultural uses, they will need to be managed as part of any comprehensive scheme to address scarcity in the coming decades. Studies to identify the parameters that affect the level of commercial and public uses and the variability of those uses are especially needed. The determinants of domestic uses have been identified for some large municipal areas and frequently form the basis of sophisticated schemes for managing water use. For other large municipalities, these determinants have not been identified with any degree of accuracy, and the determinants of domestic use have not been characterized at all for medium- and small-sized communities. It has been more than 30 years since the last comprehensive study of the determinants of domestic use (Howe and Linnaweaver, 1967), although a limited study of single-family residential indoor and outdoor water use was recently completed in 12 communities, mostly in western North America (AWWARF, 1999). Given that many medium-sized and smaller communities cannot afford to conduct their own studies of determinants, an updated comprehensive study would be of great value. It is well understood that the level of industrial demand for additional water is extremely sensitive to pollution control laws and regulations (NRC, 1994). With the advent of the Clean Water Act, industry became much more conscious of water use, and a high percentage of water used by industry today is recycled. Studies of how industrial use levels respond to changes in pollution control laws and regulations and other variables would be helpful. Completion of such research will permit water planners and managers to estimate accurately the effect of changes in water pollution regulations on the levels of industrial consumption and use. In general, agricultural irrigation water use is known to depend on climatic variables (e.g., temperature and humidity), crop type, and the uniformity with which water is applied. The relationship of these variables to levels of use has been characterized for some areas but not for others. Precise knowledge of these determinants and their relationship to crop yield is virtually a precondition for careful and effective agricultural water management. A comprehensive characterization of the determinants of agricul-

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Page 26tural water use for all regions where irrigated agriculture is practiced is therefore essential. Results from this research should permit the development of improved water demand models for each sector that uses water consumptively. Efforts should be made to generalize these models so that they can be used widely and can be applied by water managers in different regions of the country. Although improved data should lead to better models, improvements in the specification and estimation of such models should also be accorded research priority. If possible, models should be designed to account for demand management that may become necessary during short-term recurrent drought and projected long-term supply problems. Conservation-induced reductions in individual and systemwide water demand can be used to alleviate temporary water shortages, avoid increased water supply infrastructure and consumer costs, and extend the ability of existing supplies to meet current and growing demands. Research to develop more intelligent demand management methods, which are valuable for domestic, industrial, and agricultural water use, is an important dimension of the solution. Research on the determinants of water consumption and the development of models should also encompass drought responses. It is well known that some techniques that work well in drought management, such as public appeals, are not particularly effective in helping to manage scarcity over the long run. Studies focused on the special circumstances of drought management should improve the nation's capacity to manage droughts of the future. The water resources research agenda for the twenty-first century should give priority to: comprehensively characterizing the determinants of water use in the domestic, commercial, public, and industrial sectors and determining with greater precision the relationship of agricultural water use to such variables as climate, crop type, and water application rates for all regions where irrigated agriculture is practiced. AGRICULTURAL WATER USE Agriculture is of special significance because it uses most of the developed water supply in many regions of the United States and because the patterns of agricultural water use are likely to change in the future. Irrigated agriculture accounts for almost two-thirds of total developed water supply withdrawals ( Figure 3-1) and over 84 percent of consumptive use ( Table

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Page 27 ~ enlarge ~ FIGURE 3-1 Trends in water withdrawal in the U.S. by sector. Total water withdrawals include irrigation, public water supplies, rural withdrawals, and industrial uses other than for thermoelectric and hydroelectric power generation. Note that irrigation use is nearly all consumed by evaporation in contrast to the other withdrawals, which are largely recycled. SOURCE: Solley et al. (1998). 3-1). In many of the semi-arid western states, agriculture uses between 75 percent and 90 percent of the developed water supply. A significant quantity of irrigation water has been made available to growers in the western United States at heavily subsidized prices. As the competition for water has intensified and reallocations have become inevitable, irrigation withdrawals have decreased about 10 percent in the last 20 years ( Figure 3-1). However, population growth will likely fuel the demand for water to support irrigation (CAST, 1997; NRC, 1996a). Agricultural water use will probably continue to dominate total consumptive use even though its magnitude may be somewhat less than it was in the latter twentieth century. There has been no significant increase in the availability of developed supplies of groundwater or surface water since the mid-1970s (Solley et al., 1998). Despite this and despite the decrease in withdrawals noted above, irrigated acreage has actually increased because of conservation, efficient timing, and improved water delivery, particularly east of the Mississippi River where humid conditions dominate (Solley et al., 1998). In the long run, however, it will be difficult to sustain irrigated agriculture now that the fresh water supply has been nearly completely developed. New knowledge

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Page 28 TABLE 3-1 Consumptive Use of Water in the United States by Sector, 1995 Sector % of Total Consumption Domestic–Commercial 8.0 Industrial–Mining 4.1 Thermoelectric 3.3 Irrigation–Livestock 84.6 NOTE: Estimates are based on information submitted by states, tribes, and other jurisdictions that do not use identical survey methods. SOURCE: Solley et al. (1998). will be needed if irrigation water is to be managed in order to avoid contamination from chemicals and salts and to prevent erosion. Salinization of both ground and surface waters continues to be a problem on many of the irrigated lands throughout the West, and many of the techniques used for the management of drainage waters ultimately fail to maintain salt balances. Research should continue on the development of more efficient water management techniques. For example, improvements in the precision of water application to crops at the appropriate time will likely increase the efficiency with which water is used. Such improvements may also result in a decrease in pollutant loading from irrigated agriculture and thus improve overall water quality. In some regions, it may be necessary to convert irrigated land to dryland farming. Some lands overlying the Ogallala aquifer, within which the groundwater has been in decline for decades, are candidates for such conversion. The Ogallala aquifer underlies a vast portion of the High Plains stretching from southern South Dakota to northwest Texas and accounts for 30 percent of all the irrigation water used in the United States. The most severe depletion has been in Texas, which had lost about 25 percent of its portion of the aquifer by 1990. Research is needed to characterize the conditions under which dryland farming can be profitable. There may also be possibilities for developing crop varieties that are specially adapted to dryland conditions and produce higher yields than would otherwise be expected. One component of a general research program should focus on the development of sustainable agricultural practices that include improved crop varieties for dryland cultivation. Similarly, the potential of genetic alteration to improve crop water use is increasing and should be comprehensively explored. Crops using different pathways of photosynthesis have clear water use differences in the field

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Page 29 ~ enlarge ~ Not only does agriculture consume the majority of freshwater supplies in the United States, it can also change the timing and magnitude of stream flow via drainage tiles and irrigation ditches. (Kramer and Boyer, 1995), and it may be possible to convert crops to more efficient pathways in the future. However, it is unclear whether enough genetic alteration can be achieved to accomplish such fundamental changes at this time. More fruitful approaches may be to use genetic engineering to promote deep rooting and increase the fraction of the plant devoted to grain and fruit in dry conditions. In addition, genetic control that improves crop quality may lead to a reduction in the need for weed- and pest-control chemicals, thereby resulting in greater economic return for the water used (NRC, 1996a) and improved water quality. Research should be pursued in all these areas as should research aimed at identifying potentially undesirable side effects from crop gene alteration. Priority should be given to research on agricultural water use that focuses on: developing improved crop varieties for use in dryland agriculture

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Page 30 (like those that sustain grain and fruit development in dryland conditions or are disease-resistant and drought-tolerant); improving the sustainability of irrigated agriculture, including more efficient management of salt balances and management of agricultural drain waters; and developing crops for more efficient use of water, and optimizing the economic return for the water used to irrigate them. ENVIRONMENTAL WATER USE Much of what is known about aquatic ecosystems and the role of water in supporting environmental services is fragmented and piecemeal. There is a critical need to understand aquatic ecosystems in a broad systems context because intensive water development has dramatically altered these systems, as manifested by extinct and endangered species, loss of wetlands and riparian areas, and loss of biological productivity (Abell et al., 2000; Dahl, 1990; Moyle and Leidy, 1992). Research is needed to help determine the water requirements of aquatic and riparian ecosystems necessary to maintain certain environmental functions, such as provision of wildlife habitat, flood control, and assimilation of contaminants. In addition, there is now much interest in the removal of dams and the restoration of original flow regimes in riverine corridors in many parts of the country. Yet, there is limited information about how the timing of certain hydrologic events controls ecosystem structure and functioning. A systematic understanding of the relationships among biological, hydrologic, and geologic factors will ultimately be needed if efforts to alter hydrologic regimes are to be successful. Systems approaches to modeling and to understanding aquatic ecology have been ongoing; however, much additional research is needed if the promise of these approaches is to be realized. Similarly, understanding the relationship between land and water resources is essential to managing water on a watershed basis (NRC, 1999b). There has been substantial research on the relationship between land use and its effect on water quality and quantity, but much remains to be learned. The success of efforts to manage water on a watershed basis has been hindered somewhat by the tendency to study terrestrial and aquatic ecosystems independently of each other and even more so by the tendency to base studies on geographic rather than watershed-based boundaries. The scientific foundations upon which advances in watershed management will be based will need to include extensive knowledge and understanding of the

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Page 31 ~ enlarge ~ Certain flow regimes, like annual floods, are vital for maintaining the habitat of many migratory birds, such as these Sandhill cranes along the Platte River in Nebraska. These hydrologic regimes are often disrupted by dam construction. Photograph courtesy of Elizabeth Rogers. relationships between terrestrial and aquatic ecosystems. Additional research is also needed on issues related to the protection of species diversity in aquatic habitats. It is common knowledge that the decline and extinction of aquatic species is attributable to changes in flow regimes and water quality (NRC, 1992b). Thus, a comprehensive assessment of potential limiting factors and the means of managing those limiting factors in optimal ways is needed. Research aimed at developing strategies for managing aquatic habitats for the purpose of preserving biodiversity and maintaining ecosystem health is needed as well. Priority should be given to research aimed at developing a broad and comprehensive understanding of aquatic habitats. Such research should: elucidate the behavior of aquatic ecosystems in a broad, systematic context; describe the interrelationship between aquatic and terrestrial eco-

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Page 32 systems to support the development of improved regimes of watershed management; support the enhancement and restoration of species diversity in aquatic ecosystems; and describe how water-quality parameters can be manipulated to maintain and enhance aquatic habitats.