Water Management, Conservation, and Reuse in the Western United States
Henry Vaux, Jr.
In the United States the patterns of precipitation, water use, and water management problems are highly variable. In examining these patterns it is common to divide the country into relatively dry and wet regions at the 100th meridian. By coincidence, the 100th meridian is the line that separates the relatively moist eastern region of the country where rainfall exceeds 500 mm annually from the relatively arid western portion of the country where, with some exceptions, annual rainfall is less than 500 mm annually. As a general rule, agriculture cannot be carried out productively without supplemental irrigation unless precipitation exceeds 500 mm annually. The hydrologic circumstances and the problems of water management tend to be somewhat different in the relatively moist eastern portion of the country than they are in the arid and semiarid regions. In the moister regions, the focus tends to be on maintaining and enhancing the quality of water. Periodic droughts cause times of water scarcity, but as a general rule scarcity tends not to be as significant a problem in the eastern United States as it is in the West.
By contrast, in the semiarid and arid western portions of the country, water scarcity tends to be the predominant water problem and water quality has been managed more carefully for longer periods of time simply as one means of preventing scarcity from intensifying. Although the western United States and particularly the southern portion of the West is often characterized as the place of wide open spaces, for the last decade or so it has been the most urbanized portion of the country. The urbanization of the southwest continues as many of the cities there (for instance, Albuquerque, Denver, Las Vegas, Los Angeles,
Phoenix, and Salt Lake City) are among the fastest growing cities in the United States.
This paper is intended to provide an overview of the water situation in the United States. However, the focus will be on the semiarid western portions of the country since the hydrologic circumstances and problems there tend to be more closely akin to those encountered in Iran. The broad overview that follows is divided into five sections. The first of these explores the problems of water scarcity and describes some of the mechanisms that have been developed to manage scarcity. The second section focuses on water quality and describes the strategies that have been used to address water quality issues in the United States. The third section focuses on groundwater and groundwater management. Groundwater accounts for a significant portion of the water supply in most regions of the West. While the lessons of groundwater management are reasonably well known, efforts to apply those lessons have not always been successful. The final two sections focus on the problems of consumptive and nonconsumptive uses, respectively. Early efforts to develop and manage the water resources of the West were focused on acquiring sufficient water to serve consumptive uses. In the last two or three decades instream uses or nonconsumptive uses, particularly those related to the maintenance and enhancement of the environment, have become quite important, and this has led to efforts to rebalance the quantities of water allocated among consumptive and nonconsumptive uses.
Water recycling and reuse is one of the main focuses of interest here. Recycling and reuse is becoming increasingly important within each of the subject matter domains to be discussed in this paper. As a consequence, an effort has been made to emphasize the current and anticipated future role of reclaimed and reused water in managing scarcity, preserving and enhancing water quality, managing groundwater, and in the management of consumptive and nonconsumptive uses.
WATER QUANTITY: THE PROBLEM OF SCARCITY
Throughout the western United States water supplies are scarce. Simply stated, this means that there is not as much water as people wished there were. There is not enough water to fulfill simultaneously all of the wants for water for agricultural, domestic, industrial, and environmental uses. The bases of this scarcity are several and are more complicated than just a simple lack of water. The occurrence of water over the western landscape is variable both in time and location. Virtually all of the West has climates that include both wet seasons and dry seasons. Thus, for example, California has a Mediterranean climate with a dry summer and wet winter season. Much of Arizona and neighboring areas have two wet seasons, a rainy winter period and a short summer monsoon period. Here, precipitation, which is not great to begin with, can be concentrated in five or six rainfall events. Similarly, throughout much of the West, places where water occurs plentifully often are not the same as places where water is used. Much of
the water supply is generated in the form of snow pack in the higher elevations while the places of use tend to be in the coastal zones and broad lowland valleys.
Historically, these imbalances between the times and places where water occurs and the times and places where it is used were redressed by developing facilities that would allow water to be captured and stored in wet times and places and transported to places of use in dry times. The result is that one of the prominent characteristics of today’s western waterscape is a vast maze of dams and canals. Parts of this maze are so imposing physically that they can be seen from outer space. The storage and conveyance facilities of the western United States have converted a portion of the highly variable supply into firmer supplies some of which can be “guaranteed in all but the most severe of drought conditions.” This, in turn, has allowed major cities such as Los Angeles and Phoenix to grow far beyond the limits imposed by local water supplies and have resulted in a vast and extensive irrigated agriculture that supplies significant portions of the nation’s food and fiber demands at all seasons. Available water supplies have also allowed the region to develop industrially and have supported historically such important industries as primary metal production and aircraft fabrication. Today, the fabrication of computer chips and the manufacture of equipment that is dependent upon semiconductors requires a reliable supply of high quality water.
Robust economic and population growth has continued almost without a pause in the western United States for more than 100 years. During all of this time water scarcity has continued to intensify as the demands for water to support larger populations, more and more extensive irrigated agriculture, more sophisticated and higher valued industry, and recreational purposes have grown far more than in proportion to the population. Scarcity is exacerbated by outmoded water laws and institutions, many of which were designed to address conditions that prevailed in the 19th and early 20th centuries. The need to modernize institutions has been underscored in recent decades as it has become clear that it is not possible to address water scarcity exclusively or even primarily by continuing to build impoundment and transport facilities. There are several reasons for this. First, the attractive damsites that are relatively close to centers of agriculture and population have already been developed, and what remains are damsites that are either difficult and costly to develop or so remote from places of use as to make water conveyance financially infeasible. Second, civil works such as dams and canals have become more expensive because of engineering difficulties and remoteness even while the competition for public funds to pay for such facilities has become far more acute than in the past. Increasingly, in the last five decades, government in the United States has been called upon to provide a vast array of new social services and regulatory support. The result is that public funds are not as easily available to support civil works as they were in the past. Third, it is now recognized that the environmental consequences of dams and canals are more serious and far reaching than was thought to be the case during the heyday of dam building. By altering flows, water temperatures, and other key environ-
mental parameters, dams and diversions are now understood to contribute to the loss of biodiversity, to impair fish habitat, and to constrain the capacity of the environment to provide waste assimilation and other services.
The consequence is that water managers in the West have turned to a broad array of techniques to assist in the management of scarcity. Many of these techniques are subsumed under the title of conservation and demand management. Their purpose is to regulate and limit the use of water thereby bringing demand more into balance with supplies. One way in which this is done is by adopting and employing technology that makes more efficient use of water. Flow constrictors and low flush toilets can reduce domestic uses substantially. Closed conduit irrigation systems that allow for precise application of water and reduce losses due to soil nonuniformity have resulted in substantial savings in the water needed to irrigate certain crops. Similarly, water efficient industrial technology permits the manufacture of all manner of commodities at significant water savings.
Programs of education have also been successful in reducing the demand for water, particularly in the domestic and agricultural sectors. Domestic users tend to be more careful and use less water when they know how much water they actually use. These users also respond to education programs that explain where water comes from and elaborates on the problems of managing it. Similarly, agricultural water users tend to practice more careful water management when they know with some accuracy how much water they use. Programs to educate agricultural users in the benefits and techniques of appropriate irrigation scheduling and various means of managing water on nonuniform soils have also been effective.
Finally, there are some rather straightforward institutional measures that can be employed to manage scarcity. These include pricing and markets. Appropriate pricing of water is also an effective way of managing demand and inducing conservation. Historically, water in the United States has been priced only to recover the costs of treating and transporting it. The water itself has been free and has sometimes been treated as if it were free. Policies that call for the price to more nearly reflect the scarcity value of water have been quite effective in reducing water use, particularly in times of drought. Pricing practices such as progressive rate structures that mimic traditional marginal cost pricing rules have led to widespread conservation both by encouraging adoption of water saving technology and by encouraging more careful and efficient use and management of water. This is true both in households and on the farm. Pricing policies tend to be quite controversial, but there is little doubt that they are effective.
Water markets have also been used successfully to ration large water wants among relatively fixed sources of supply. By relying on voluntary bargaining, markets ensure that both buyer and seller gain in the exchange of water and also help to ensure that water is allocated to its highest valued uses. Markets have been slow to develop because of concern about third party effects and worries that instream uses and common pool uses cannot compete on a comparable basis with consumptive uses. The controversy also extends to concerns that most water
to be transferred will come from agriculture and rural areas and that the financial returns from selling or leasing the water may enrich places other than areas of origin. In spite of these concerns, water marketing has proved an excellent means of coping with limited water supplies in specific drought situations and in some regions of the West where markets have been well-established for decades. It seems clear that the full potential of water markets remains to be realized.
Although augmentation of developed supplies still remains an option to meet growing demands for consumptive uses, issues related to financing and environmental impact will likely constrain the extent to which new supplies will be developed in the future. In many regions of the West, the only possible source of additional water supply is reclaimed and recycled water. The Orange County Water District in southern California has been a pioneer in developing recycled water supplies to repel salt water intrusion to local aquifers and to replenish those aquifers that are a source of domestic supply. The city of Phoenix, Arizona, has developed a wastewater recycling plan that will virtually eliminate the discharge of wastewater, treated or not, into an adjacent river. As will be noted later, strict water pollution control laws that embody stringent regulations on what may be discharged to public waterways have provided strong incentives for industrial and municipal wastewater recycling.
lntensifying water scarcity will remain a dominating feature of the western water scene for the foreseeable future. Only the development of low cost seawater desalting technology could provide any substantial relief. Western water managers have had some success in managing water scarcity by utilizing a wide variety of management techniques, including supply augmentation, pricing, education, water saving technology, markets, and water recycling. In the future these techniques together with new technology and innovative management systems will all have to be employed to stretching limited water supplies to serve the growing demands for additional water from almost every water using sector.
MANAGING WATER QUALITY
There is a tendency in the United States to forget that deteriorating water quality can reduce available water supplies just as surely as drought. This is due, in part, to the practice of separating considerations of water quantity from considerations of water quality and the tendency to treat these independently of each other. The fact is that the amount of water available for any purpose in any location is a function of the quality of available water supplies. Thus, it is important to recognize that considerations of water quantity and water quality are intimately related to each other and should be considered jointly with each other. In this paper, they are separated only to simplify and highlight the various issues related to each.
Historically, in the United States, the maintenance and enhancement of water quality was considered to be the prerogative of the states. Western states tended
to have reasonably effective water quality control laws and policies because the prevailing water scarcity made clear the importance of not allowing water quality to deteriorate. Despite this fact, there was a tendency nationwide for states to compete for the location of new business by promising freedom from stringent water quality regulations. This provided states with an incentive to weaken water quality policies and regulations so as not to be at a competitive disadvantage when promoting economic growth.
Beginning about 1970, the U.S. Congress passed a series of water pollution control and water quality laws that created strong national programs to restore and maintain water quality. These programs were aimed at reducing (and ultimately eliminating) most point source pollutants. (Point source pollutants are those that are discharged to the environment from a discrete point, for example, an outfall). The federal pollution control effort had two distinct designs, one aimed at controlling industrial discharges to the nation’s waterways and the other focused on the management of waste in public sanitary systems.
For point source industrial discharges, a permit system was developed and enforced that requires all firms that discharge to waterways to acquire a permit to do so. In addition, the law required the use of the best available technology (BAT) to treat wastewater prior to discharge. Over the course of the years BAT came to be defined in a way that included an economic test. This test was intended to avoid bankrupting large numbers of firms through regulation. Firms that were not eligible for permits because of failure to employ the best available technology were barred from discharging. The permit discharge system was initially backed by an enforcement system that severely limited its effectiveness. Subsequently, the application of a provision in some older legislation that gave citizens the right to identify and file enforcement actions led to workable regimes of enforcement and resulted in an effective wastewater management strategy. One result of this strategy has been to encourage firms to recycle and reuse both process and cooling water. In the arid and semiarid southwestern United States manufacturing firms typically recycle extensively with the result that the demand for water for industrial purposes has grown at rates far lower than the rates of growth in demand for other purposes.
The companion strategy and program that was applied to the control of wastewater from public sanitary systems was an immense civil works program in which the federal government subsidized 75 percent of the costs of constructing wastewater treatment systems. Initially, such systems were required by law to include at least secondary treatment. Today, most such systems include some form of Advanced Wastewater Treatment (AWT), which in the past was sometimes called tertiary treatment. The total costs to the federal government of the wastewater treatment subsidization over the nearly 30 years of its existence totals approximately $100 billion. The primary criticism of this strategy is that the large subsidy led to treatment technologies that were unnecessarily capital intensive and delayed the development and adoption of innovative technologies,
such as artificial wetlands, beyond the time when they could have first been employed effectively as part of a comprehensive wastewater management strategy. Despite the shortcomings of the permit and technology strategy and the capital intensive public wastewater treatment strategy, the twin strategies have been very effective—even if unnecessarily costly—in managing point source contaminants.
In spite of the success of laws and policies governing the management of discharges to surface waterways, water quality in the United States continues to be subject to deterioration. One explanation lies with the fact that early policies failed to address in any adequate way the problems of pollution from nonpoint sources. For example, agriculture was exempted from early pollution control policies. Moreover, policies aimed at point source pollutants rely for their effectiveness on the fact that points of discharge can be identified and subjected to controls. This is not the case with nonpoint source discharges. Uncontrolled nonpoint source discharges are now the most significant cause of water contamination in the United States.
Nonpoint source discharges affect the quality of both ground and surface water. Groundwater pollution can be especially serious for several reasons. First, as a general rule the self cleansing and diluting properties of groundwater are not nearly as robust or effective as those of surface water. Second, groundwater is particularly susceptible to contamination from discharges made many decades ago. Fertilizer residues, toxic chemicals, and other materials discharged onto and into the soil can pose a serious hazard to groundwater quality for many years. Often such threats cannot be identified until the groundwater is already contaminated. Even in instances where spills and other potential sources of contamination that are already in the soil profile can be identified, the costs of clean-up may be astronomical. In the last 10 to 15 years effective techniques for cleaning up groundwater contamination in situ have been developed, and these hold considerable promise for keeping the costs of clean-up at reasonably manageable levels. Yet, protection of groundwater quality remains one of the major water resource management challenges for the future in the United States.
The struggles of the last several decades to fashion techniques and policies for managing nonpoint source contaminants in the United States now appear to have resulted in the targeting of best management practices (BMPs) and comprehensive watershed management as the two preferred strategies. Best management practices are aimed at specific land use activities while watershed management entails the holistic management of watershed lands in ways that restrict activities to areas that are least likely to contribute to water pollution. Thus, best management practices require the adoption and implementation of a prescribed set of practices for conducting activities such as lumbering and agriculture. The prescribed practices are intended to reduce or eliminate altogether the entrainment of contaminants in surface water runoff or deep percolation through the soil profile. One example of a BMP is the use of closed conduit irrigation tech-
nologies that allow precision application of water to avoid overirrigation that contributes to run-off and deep percolation.
Comprehensive watershed management strategies are plans and practices for managing watersheds as a hydrologic unit and regulating activities. The resulting regulations aim to direct activities that can generate nonpoint source contamination to the least susceptible areas of the watershed and provide strong protections for susceptible areas such as riparian zones. Riparian zones offer defenses from contamination originating elsewhere but can themselves be the source of nonpoint source pollution if not carefully managed.
The primary challenge with the implementation of these strategies in the United States is finding an appropriate balance between the personal freedom to choose how to undertake activities such as lumbering and agriculture, and regulations that are effective in controlling nonpoint source pollution. Experience with these strategies is limited, and it is not yet clear whether they will work. Much more experience with them will be required. In the meantime the search for new and innovative ways of managing nonpoint source contaminants will continue.
The semiarid western United States and particularly the more arid southwestern region is subject to soil salinization, which results from irrigation practices. Without proper management, soils can become salinated either from salts introduced with irrigation water or salts mobilized in the soil profile by the presence of irrigation water. In the absence of management, the salinization of soils will lead to a reduction in agricultural productivity and ultimately to sterilization of the soil itself. Proper management requires the application of sufficient quantities of water to leach salts below the root zone and appropriate drainage facilities to carry away leaching waters and prevent the build-up of groundwater tables and the water-logging of soils.
There are only a few places in the western United States where salinity is managed on a sustainable basis. Leaching widely occurs where salt is a problem, and it is sustainable in circumstances where drainage waters can be disposed of or recycled. In many areas, the disposal of drain water constitutes a major problem, and irrigators are unable to find workable options for the management and disposal of drainage waters. In these areas, salinity will remain a persistent problem and unless ways are found to deal with drain water it is likely that irrigated agriculture will ultimately become untenable.
Although the United States has had some successes in managing water quality, the successes are largely found in the management of point source discharges to surface waters. The strategy of regulating industrial discharges with permits and requirements for the use of best available technology, together with the strategy of subsidizing the construction of waste treatment systems to clean sanitary wastewaters, has worked reasonably well. These strategies emphasized the use of regulations and capital intensive treatment regimes, with the result that the use of market-like incentives and innovative treatment techniques were under-
utilized. This meant that the success in treating point source discharges was probably more costly than it needed to be.
The effective management of nonpoint source contaminants remains the major water quality challenge for the future. A number of techniques, including the implementation of best management practices and comprehensive watershed management, are available but have been implemented in a limited number of situations. More experience is needed with these techniques. In addition, it seems likely that the nonpoint source pollution problem will be difficult to solve without major new innovative techniques and methods for managing diffuse pollution.
THE MANAGEMENT OF GROUND WATER RESOURCES
Groundwater is a significant source of total water supply in the United States and is the source of approximately 25 percent of the drinking water. Major cities such as San Antonio, Texas, and Tucson, Arizona, are completely dependent on groundwater for drinking water supplies. Throughout the United States, but especially in the West, water managers have been slow to learn how to manage groundwater on a sustainable basis. Aquifers have frequently been excessively overdrafted with the result that pumping depths have become deeper than optimal, pumping costs have risen above affordable levels for some users, and water scarcity has intensified. Historically, the principal strategy for addressing this problem entailed the development of additional surface supplies to offset the decline in economically affordable groundwater. The viability of this strategy has been significantly reduced because of the financial costs of building the dams and canals necessary to provide additional surface supplies and because of adverse environmental impacts associated with these facilities.
Apart from the problems of managing groundwater quality that were discussed in the previous section, the primary problem confronting groundwater managers is that of regulating extractions to ensure economic sustainability. Overdraft is said to exist when the quantities extracted exceed the quantities recharged. Overdraft may be economically efficient in certain situations and thus it is not always bad. However, prolonged or permanent overdraft is always self-terminating as groundwater depths are drawn down to levels from which it is no longer economical to pump. As a general rule, in the absence of regulation groundwater will always be extracted inefficiently when extraction is organized in an individually competitive fashion. In these circumstances, extractors have an incentive to ignore the increments added to future pumping costs by extracting now rather than later. Monopolistic pumpers, to the contrary, have every incentive to account for these costs and thus tend to extract groundwater in an economically efficient fashion.
The problem of efficient groundwater management, then, can be summarized as the problem of managing groups of independent extractors to account for future costs of extraction (the marginal user cost) and behave in the same
fashion as a monopolistic extractor would behave. This cannot be accomplished without metering and regulating the extractions of each individual. Quotas and pump taxes are the usual tools recommended for regulation, and quotas are the most often used technique.
Another technique that can be used with or without regulation is to supplement natural rates of recharge through artificial recharge. Artificial recharge can be accomplished either indirectly by percolating water through the soil profile underlying percolation basins or by injecting water directly into the aquifer. By increasing the total quantities of water recharged through artificial recharge, managers can increase the quantities of water that can be extracted sustainably. The possibilities for artificial groundwater recharge also open the prospect of groundwater storage or groundwater banking.
The concept of groundwater banking is very similar to the concept of surface water storage. Surplus flows in wet times can be stored in a groundwater basin and then extracted for use in dry times. Groundwater storage or groundwater banking has several advantages over its surface water counterpart. It avoids the adverse environmental effects of surface water storage systems and the recharge facilities and wells needed to make groundwater storage systems work are generally far less expensive than surface water storage facilities. The possibilities of effecting groundwater storage or banking with reclaimed wastewater are also beginning to be exploited. Artificial groundwater recharge and storage are being employed on a large scale in the water management schemes of Phoenix, Arizona, and Orange County, California, as well as about two dozen other sites around the country.
The lack of effective controls on groundwater extractions can constrain the use of groundwater storage and banking. Thus, it also constrains, often severely, the extent to which ground and surface waters can be managed conjunctively. In the absence of workable extraction controls, the party who pays for the recharge operation and recharges the aquifer has no guarantee that he or she can capture all of the benefits from recharge. Other extractors, who did not participate in the recharge operation, may be able to extract some of the recharge water at no cost other than the cost of extraction. Thus, the lack of effective pumping controls tends to create a barrier to the successful implementation of artificial groundwater recharge and groundwater banking programs. The full promise of groundwater recharge and banking in California as well as the full promise offered by the conjunctive use of ground and surface waters are unlikely to be realized until such time as state or local agencies are able to adopt some effective form of regulating groundwater extractions.
Finally, it is important to point out that the reluctance to implement groundwater management practices that include effective monitoring and control of extractions may be due to the fact that the benefits of such regulation are reasonably small. There are several studies in California that show that the benefits of groundwater regulation in most agricultural areas are relatively modest. By con-
trast, in two areas in which stringent groundwater management regimes have been developed in the last two decades, the benefits were apparently quite large. In the state of Arizona, the development of a comprehensive groundwater management plan became a quid pro quo for a large federal investment in a surface water storage and conveyance facility called the Central Arizona Project. The thinking here was that in the absence of groundwater controls, the need for additional supplies of surface water would continue almost indefinitely as the state’s groundwaters were drawn down to uneconomical pumping depths.
In a similar vein, nearly ten years ago the city of San Antonio, Texas, found its groundwater supplies endangered as a consequence of unrestricted extractions from neighboring users. San Antonio, which is one of the two or three largest cities in the United States that relies exclusively on groundwater, was able to motivate regulatory authorities in the state of Texas to take necessary actions to control extractions, thereby preserving the city’s water supply. Here again, the benefits from management and regulation of a groundwater resource were quite high. These experiences suggest that in the United States the existence of positive benefits from groundwater regulation is a necessary but not sufficient condition to induce programs of regulation. Experience shows that the benefits must be very large and quite visible before it is possible to move forward to comprehensive management programs.
The management of groundwater quality and quantity remains a major challenge in the United States. The techniques of managing groundwater quantities are better known and generally less expensive to apply than the techniques for managing groundwater quality. For the most part the management of groundwater remains within the prerogative of the states. One of the significant debates confronting the groundwater management community centers on the question of whether the federal government will have to assume substantial authority if the nation’s groundwaters are to be adequately protected.
CONSUMPTIVE USES OF WATER
For the first two hundred years of development in the United States the focus was on the development of sufficient water supplies to serve traditional consumptive uses. Consumptive uses are those that transform the water—either qualitatively or in phase terms—in ways that make it unfit for further utilization. Typical consumptive uses include irrigation, domestic household use, and industrial processes water use. (Industrial thermal uses are partly consumptive but most thermal waters are recooled and made available for other uses.) The narrow historic focus on consumptive uses of water tended to ignore and obscure important instream uses, particularly environmental uses, that were increasingly sacrificed to support the growth in consumptive uses. Today, it is widely recognized that water development and management activities must give balanced consideration to both consumptive and instream uses.
There is a need for new technologies that will allow water to be used more efficiently in all consuming sectors. Yet, there is a surprising lack of knowledge about the fundamental determinants of consumptive use, knowledge that would be needed to develop appropriate technology. Little or nothing is known about the determinants of public and commercial uses. Although these uses are small when compared with industrial, domestic, and agricultural uses, an understanding of the determinants of these uses will be needed if a comprehensive scheme to manage all uses is to be developed in the coming decades. It is well understood that industrial demands for additional water are highly sensitive to pollution control regulations. Nevertheless, the precise nature of the relationship between the quantities of industrial water demanded and pollution control laws has never been documented and specified.
The determinants of domestic use have been identified for a number of large metropolitan areas and are frequently used in devising sophisticated schemes for managing domestic water use. The determinants of domestic water use have not been investigated or specified for most medium and small sized communities. Additionally, it has been more than 30 years since the last comprehensive study of the determinants of domestic use. As water scarcity continues to intensify it will be important to have a comprehensive understanding of the variables that determine domestic consumptive uses so that sophisticated schemes of domestic water supply management can be applied to all communities and not just those who are able to afford the elaborate studies needed to identify the determinants of use.
Generally, water use for irrigation purposes is understood to be dependent upon climatic variables such as temperature and humidity, crop type, and the uniformity with which water is applied. The effect of these variables on levels of agricultural water use has been characterized for some areas but not for others. A complete knowledge of the determinants of agricultural water use is virtually a precondition for the development of efficient and effective schemes of agricultural water management. A comprehensive assessment of determinants of agricultural water use for all regions where irrigated agriculture is practiced will be essential in the future as the supplies of water available to grow food come under intensifying competitive pressure.
Agricultural water uses are especially important both because they account for more than 80 percent of the consumptive use in the United States (more in other parts of the world) and because the need for water to grow food for a growing global population is likely to increase in the coming decades. There will be compelling reasons for learning how to manage water efficiently, to minimize the impacts of fertilizer and nutrient residues on receiving water, and to manage salt balances in precise and realistic ways. The knowledge generated could result in agricultural management practices that make efficient use of scarce water supplies while having minimal impacts on water quality.
There are areas in the United States and elsewhere in the world where irrigated agriculture cannot be extended indefinitely because they depend upon non-
replenishable groundwater supplies or because lands are not suited to agricultural production over the long run. As irrigated lands go out of production it may be possible to carry on profitable dry-land farming. More knowledge is needed about the circumstances under which dry-land farming can be carried out profitably. One possibility is the development of crop varieties that are specially adapted to dry-land conditions and can produce higher yields than might otherwise be expected.
The potential for genetic alternation to improve crop water use deserves further and comprehensive exploration. Crops with different photosynthetic pathways have clear water use differences, and it may be possible to take advantage of such differences in the future. However, it is unclear whether genetic manipulation can be used to achieve changes in the fundamental properties of crop water use. It may turn out to be more productive to focus on genetic manipulation to increase crop rooting depth, improve crop quality, and develop crop characteristics that reduce the need for fertilizers and chemical control of pests.
Although much energy has been devoted to understanding consumptive uses of water much remains to be learned. A thorough and comprehensive understanding of the determinants of various consumptive uses will be needed if sophisticated water management schemes are to be developed. Agricultural uses are both large and important. Yet, understanding of agricultural water use is far from comprehensive. Agriculture water is frequently seen as the supplier of last resort because it accounts for such a large share of total consumptive use. It seems likely that food production will need to be increased sharply worldwide if a growing global population is to be fed. This suggests that it will be critically important to learn how to manage agricultural water supplies efficiently so that they yield optimum levels of production of food and fiber.
NONCONSUMPTIVE USES OF WATER
There are a number of water uses that by their nature do not render the water unfit for subsequent use. Among these are navigation, hydroelectric power generation, thermal cooling (within limits), and environmental uses. Of these, environmental uses have assumed particular importance because it is now understood that water development and use practices of the past have severely constrained and reduced the water available for environmental uses. Environmental uses of water provide both amenity values and service values. Amenity values include water-based recreational values, scenic values, and option values that accrue to individuals who appreciate the presence of pleasing water-based environments even if they never expect to see or use them. Service values include the benefits of environmental services provided by aquatic ecosystems such as air and water purification and the inherent environmental stability associated with diverse ecosystems. In instances in which the capacity of water-based environments to provide ecosystem services is impaired, it is very expensive to provide those
services artificially in lieu of the environmental services, and the resulting in lieu services are never as effective and efficient as they are when provided by the environment.
There is very little understanding of the role of aquatic ecosystems and the role of water in supporting environmental services, with the result that there is a compelling need to understand such ecosystems in a broad systems context. Additional research will be required to understand the determinants of water requirements for the maintenance of aquatic and riparian ecosystems in order to preserve their capacity to provide wildlife habitat, flood control and assimilation, and dilution of contaminants. There is much interest in restoring the flows of some of America’s major rivers such as the Colorado and Missouri to conditions that mimic their undeveloped states. If this is to occur it will be necessary to develop a systematic understanding of the relations between biological, hydrological, and geological factors.
It will also be necessary to develop a better understanding of the relationship between land and water resources if water is to be effectively managed on a watershed basis. Efforts to develop the knowledge needed for innovations in watershed management have been hindered by the tendency to study terrestrial and aquatic ecosystems independently of each other. Additional research will be needed on a whole range of issues that bear on the protection of species diversity in aquatic habitats. Only by preserving species diversity will it be possible to maintain environmental stability and ecosystem health in aquatic ecosystems.
The discussion in both this and the preceding sections provide a sample of the kinds of knowledge that must be acquired if the United States is to address successfully the water management challenges of the next several decades. Some of these issues may be usefully addressed by groups of scientists in a collaborative way that would yield richer results than if they were addressed independently. There are, of course, hundreds of such issues and those identified here are simply meant to be suggestive.
This brief overview of the water resources of the United States and the various problems and challenges that attend to the management of those resources is intended to paint a broad picture. The situation in the semiarid portion of the country was emphasized because the physical circumstances and problems of management there are more akin to those encountered in Iran. Several general conclusions emerge from the overview as follows:
Water scarcity will persist and intensify as population and the economy grow.
Learning how to manage water scarcity with reasonable efficiency will be a continuing challenge.
The management of water quality will continue to be challenging. Control of nonpoint source pollution will be particularly important as will the management of new chemicals and technologies to avoid further contamination. The contamination of groundwater will be a growing problem and in situ techniques will offer the best and least costly methods of clean-up.
The need to manage groundwater sustainably will continue to be important in many regions of the country.
The management of salinity will pose a continuing challenge in many areas where irrigated agriculture is practiced.
Additional research and technical innovation will be needed if consumptive and nonconsumptive uses are to be fully understood and managed effectively.