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Water sustainability: Science or science fiction? Perspective from one scientist
Water Sustainability: Science or Science Fiction? Perspective from One Scientist WARREN W. WOOD MS 430, U.S. Geological Survey, 12201 Sunrise Valley Drive, Reston, VA 20192, USA. E-mail: [email protected] and School of Geography and the Environment, University of Oxford, Mansfield Road, Oxford OX1 3TB, U.K.
ABSTRACT: A significant increase in the world's human population is predicted to occur in the next 100 years. This population will almost certainly require additional water resources for irrigated agriculture, domestic supply, and ecosystem support. Because most of the world's easily captured water is already identified and allocated, society must either change the present allocations or develop new sources to meet the expected demands. The philosophy of sustainability has been proposed as a management goal that will address water-resource-related issues. Sustainability is generally defined as the ability to meet the needs of the present generation without compromising the ability of future generations to meet their needs. "Meeting the needs" beyond basic survival requirements of a few liters of water per day requires many value judgments, and the assessment of these judgments lies outside the realm of science or engineering. Sustainability fails to address historical property fights or recognize that population increase is exponential although the resource is finite. Sustainability also implies stasis, yet uses and supply are temporally and spatially dynamic. Thus, the concept of sustainability is flawed not only as a goal, but also as a management objective and holistic sustainability (sustainability for all uses) is nearly impossible on a drainage-basin scale. Development of an alternative strategy based on water renewability that identifies and measures major sources, sinks, uses, and consumption in a system rather than "needs" can form the basis for informed policy decisions.
INTRODUCTION A high probability exists for doubling the world's human population in the next 100 years, although the estimates vary widely (Lutz et al., 2001). This increase will almost certainly require significant additional water. Over a thousand million people have an inadequate diet (Latham, 1984); however, that appears to be improving slightly (United Nations, 1992). Improving nutrition of the existing population, a laudable goal, will require additional irrigated agriculture. It is also generally recognized that in the future, a larger share of the water will be demanded for fisheries restoration, biodiversity, and general ecosystem maintenance. Adding to these environmental pressures for additional water is a strong positive correlation between standard of living and water use. World population increased 42% from 3.8 thousand million to 5.4 thousand million during a twenty year period between the 1970's and 1990's, while water used increased 300% (Postel, 1999). If national economies continue to grow, there is likely to be increased water demand, even with a static population. What will be the source of this water? The water-resource perspectives expressed in this manuscript are global in nature, but it is recognized because of the limited-trading market that most water-resource problems and solutions are local in nature.
It is generally believed that there is no significant volume of water entering or leaving the earth from space; thus, water on earth is finite, but renewable. Approximately 97% of the world's water resources are saline and located in the oceans and seas. Of the 2.5% of the world's water resources that are fresh water, approximately 69% is stored in glaciers and ice caps that are not readily available, 30% is stored in ground water, and less than 1% resides in rivers and lakes at any instant (Shiklomanov, 1993). The distribution of water on earth is not uniform, nor is it correlated well with population. For example, the Amazon River accounts for 15% of the global runoff and 0.5% of the population, while China has 21% of the population and 7% of the global runoff (World Resources Report, 1994; Population Reference Bureau, 1998). Approximately 110,000 km 3 of precipitation falls on the continents each year. Of this, approximately 50,000 km 3 are transpired from the native vegetation; 20,000 km 3 are transpired from irrigated crops and evaporated from surface reservoirs; 12,000 km 3 are utilized by man for industrial and domestic use; 20,000 km 3 are returned to the sea in unrecoverable floods; and 8,000 km 3 are unavailable for societal needs because of its remote locations (Postel et al., 1996). Of the water utilized by man, 69% is used for irrigation, 23% for industry, and 8% for domestic water supply (World Resources
Water ResourcesPerspectives:Evaluation, Management and Policy. Edited by A.S. Alsharhan and W.W. Wood. Published in 2003 by Elsevier Science, Amsterdam, The Netherlands, p. 45-51.
W.W. WOOD
Institute, 1992). This estimate for the percentage of irrigation use is approximately 10% larger than that given above by Postel et al. (1996). It is, however, within the uncertainty of the methods used in both approaches. Using the estimate from Postel et al. (1996) suggests that 32,000 km3/yr is the amount of renewable water resources that society can utilize without removing additional native vegetation, building new dams, or transporting water from remote locations. Implicit in this estimate is the temporal stability of this resource. Yet, we know that there have been large shifts in climate in the last 20,000 years, and from historical records, it is clear that measurable climate changes can occur decadally. The predicted increase in population will almost certainly require additional water for both domestic water supply and irrigated agriculture; however, the readily available water resources are already fully allocated. We are, in fact, borrowing from the future as unreplenished ground water from storage is used to produce approximately 8% of the total food crops (Postel, 2001). Worldwide, 12 to 15 million hectares of land, or about 1% of the total cropland, are lost each year to degradation due to salinization, erosion, desertification, and urbanization (Kendall and Pimentel et al., 1994). Additionally, nutrient depletion, over-cultivation, over-grazing, and acidification contribute to the reduction of yields. If other factors remain constant, the per capita food production will decline as the aquifers are dewatered and cropland is lost or its productivity is reduced. Irrigation of approximately 17% of the agricultural land produces 35 to 40% of the food crops (Postel, 1999). Thus, with increased food demands, there will almost certainly be a need for increased agricultural irrigation. Associated with many grain crops is the loss of arable land due to salinization. Between 1.5 and 2 million hectares of arable land are lost each year to salinization, or approximately 0.1% of the total cropland (Kendall and Pimentel, 1994). High concentrations of solutes are detrimental to plant growth and to the tilth of the soil. In advanced cases of salinization, salts form on the land surface and completely inhibit plant growth. Because of its relatively low value per unit mass and thus high transportation costs, water is not traded extensively like other commodities. Significant amounts of water, however, are used in the production of grains and meats, and this "virtual" or "embedded" water is traded worldwide (Allan, 1996). The lack of well-defined property rights to water is another limitation of trading. In the United States where some jurisdictions permit private
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ownership of water, an active market in trading water rights has recently developed (McCoy and Zachary, 1997), but these markets are local and constitute a minute portion of the total volume of annual water use. In some cases, water can be "wheeled" like electricity, thus reducing long transport for individual water molecules. That is, water moved to one area is used locally, and then new water from this area is passed on to the next user and so on until the transfer is complete. We are approaching, or in some areas have reached, a critical point in water resources where the total readily recoverable volume of water falling on a watershed has been allocated. The policy/management questions associated with these conditions are: 1) how does society reallocate the presently available water to accommodate changing needs, and/or 2) how does society acquire new water? Water Uses
Water resources use can be classified into different categories: domestic water supply and waste treatment, livestock watering, irrigation, transportation, industrial (resource development/ manufacturing), power generation, recreation, flood control, and ecosystem maintenance. These uses are not mutually exclusive as water is generally used in multiple ways as it moves through the basin, and any use typically impacts one or more of the other uses. One also must distinguish carefully between use and consumption. For example, in North America, power generation is the largest user of water, but its consumption is minuscule relative to irrigation. It appears that society's most critical, non-fungible water uses are domestic water supply, irrigation, livestock watering, and ecosystem support. Public recognition of the ecological importance of waterresource management has come to the forefront only in the last 40 years; thus, only small amounts of water have been formally allocated for ecosystem maintenance (Gleick, 2000). This late recognition, after most water already has been allocated, coupled with the difficulty of establishing the direct economic benefit of biodiversity and ecosystem maintenance in market-based economies, results in the smallest formal water allocation among the major uses. Improvement in freshwater fisheries and species diversity for ecosystem maintenance will almost certainly require significant additional water appropriation in the future. In countries with democratic governments and property rights with environmental laws, and policies of pricing (including scaled water charges,
Water Sustainability: Science or Science Fiction? Perspectivefrom One Scientist
subsidies, taxation) generally control water use. Because of the local nature of climate, topography, geology, historical development, infrastructure, and other factors, organizing water policy strategies around drainage basins might be a rational approach. Typically, however, there are many different and competing political jurisdictions in any given drainage basin, making decisive, coherent political action involving water-resources policy difficult. Additionally, thousands of private water and wastewater purveyors exist (National Research Council, 2001). This private ownership and a lack of universal property fights of water coupled with the fact that rivers are frequently used as boundaries between jurisdictions further inhibit the establishment of a coherent water policy. Water policy as generated by legislative bodies tends to be descriptive, immediate, asks the question "how," seeks human-based assessment, and is rigid. That is, it is designed as a "one size fits all" solution. In contrast, scientists and engineers recognize the need for long-term studies, ask the question "why," seek peer-review assessment, and demand variability with each case, depending on the circumstances. Thus, in political environments, it is not surprising that there are few opportunities for scientists and engineers to make direct contributions to water policy. Water scientists and engineers can provide important answers to "what if" policy questions related to physical, biological, and chemical stresses to the environment. It is because of their understanding of the full dimension of the water cycle that hydrologists can frame discussion and provide insight into what the potential tradeoffs might be. I suspect that the greatest immediate contributions by scientists and engineers to the solution of societal water problems lie in improving the efficiency and productivity of present water uses. Although long-term water-resource allocation is clearly a policy issue, policy cannot create "new" water. It can, however, foster the intellectual and economic environment where scientists and engineers excel.
Potential Areas for Increased Efficiency and/or Conserving Water Political or technological solutions to saving water must be distinguished from virtual and real forms of "saving" water. "Real" savings actually generate additional water supplies, but virtual savings are "paper" savings. Further, saving water is scale-dependent. On a global scale, the hydrologic cycle is closed: no water is lost; it only changes location and physical state. On the scale of river
basins, however, it is extremely important to know where and how the major sources and sinks of the water occur. Postel (1999) uses an interesting example of irrigation in the ancient Nile valley to illustrate efficiency as a function of scale. This system was not efficient when viewed from the perspective of individual farms because much of the water spread over each field drained off without benefiting the crop. The runoff from upstream farmers became the supply for downstream farmers, however, and the system's efficiency was much greater when examined from the perspective of the entire Nile valley. In general, there are five ways to "save" or make more productive use of the water available: 1) reduce evaporation and un-recycled seepage loss, such as those seepage losses to a closed basin or an ocean; 2) shift water from one use to another with higher food value and lower water demands; for example, growing wheat rather than cotton or raising chicken rather than beef; 3) develop more efficient crops, cultivation, and irrigation methods; 4) substitute capital for water, thereby increasing efficiency; and 5) capture of a greater percentage of the water in rivers before it is discharged to the oceans or closed basins. This last option is becoming less viable as the true costs of social disruption, inequitable benefits, loss of productive agricultural land, and loss of biodiversity and ecosystems are weighted against the societal benefits (McCully, 1996). In addition, the number of remaining topographically-suitable sites is limited. It also seems unlikely that a concerted world-wide effort will be mounted to harvest more water by removing native vegetation, although removal of invasive species is part of the Republic of South Africa's program of water conservation (Maitre et al., 1999). Deforestation continues to remove trees that use large quantities of water; thus, some gains in runoff are expected. In some cases, however, this deforestation also may lead to salinization (Cook et al., 1993), to increased sediment loading of streams, and to other harmful impacts. It appears that most of the increased productivity or savings must come from the other areas. The "green revolution" (in the last half of the 20th century) that increased agricultural productivity so significantly through modification of crops and increased irrigation is expected to continue. Evaporative water losses from irrigated fields are difficult to measure because the loss by transpiration and evaporation is commonly lumped together as a measured or calculated parameter in water-balance studies. That is, both processes return water to the atmosphere in the vapor phase. Transpiration by
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crops is necessary and not considered a loss; so, it is necessary to separate transpiration and evaporation to get a measure of the true loss. If one assumes that 10% of the water applied to irrigation is lost to evaporation, this would amount to 250 x 109 m3/yr worldwide. If this water could be saved and applied to grain production, it would generate approximately 250 million tons of grain, or 15% of the total annual grain harvest -- not an insignificant amount and about equal to the world's annual reserve amount. Note that approximately 1,000 tons of water are required to grow 1 ton of wheat (Ritschard and Tsao, 1978); 3,500 tons of water to grow one ton of chicken (Pimentel et al., 1997), 10,000 tons of water to grow one ton of beef (Falkenmark, 1994); and 17,000 tons of water to grow one ton of cotton (Kendall and Pimentel, 1994). Approaches to reduce evapotranspiration can be grouped into four general areas: agronomic, technical, managerial, and policy/institutional. As stated above, scientists and engineers typically have little direct influence in policy/institutional aspects such as establishing water-use organizations, setting conservation-oriented pricing, or establishing a legal framework for property fights. They can, however, make important contributions in the other areas of reducing evaporation. Clearly, great strides are anticipated in plant physiology (whether by genetic engineering or traditional selection methods) so that plants will require less water for the same yields, or yields will be increased for the same water application (Somerville and Briscoe, 2001). Development of plant species that can produce high protein fodder from saltwater irrigation or plant growth in salt marshes is the long-term goal of genetic modification. Development of species that require fewer pesticides or lower concentration of nutrients will limit or minimize degradation of runoff water, making it available for other uses. Development and implementation of inexpensive micro-irrigation systems that deliver the correct amount of water to the roots at the precise time required by a plant is the ultimate goal of irrigation. For poor countries with subsistence agriculture, where the improvements are needed most, this must be accomplished on small, hectare-size plots with hand labor and a minimum of technically sophisticated hardware. Examples of this approach are Treadle pumps and portable, inexpensive dripirrigation systems (Postel, 1999). Expansion of furrow irrigation using surge irrigation or low energy precision application (LEPA) on existing sprinkler systems result in significant increases in productivity and can quickly recover capital
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investment (Moseley, 1998); they are capitalintensive, however. Minimizing degradation of water resources is a fruitful area for saving water. That is, if the properties are not changed or contaminants are either not introduced or successfully removed from water, the water is then available to downstream users in its original condition. Although technology permits relatively easy identification of point sources of contamination and has provided mechanisms for treating domestic sewerage and industrial discharge, agricultural runoff containing sediments, fertilizers, pesticides, and herbicides has resisted large-scale remediation in most areas. Technical remediation strategies for agricultural runoff range from land management and control of applications of agricultural chemicals to removal of these chemicals and sediments by buffer strips of riparian vegetation. Preliminary studies suggest that these buffer strips between crop fields and streams are a passive, economical method to significantly reduce sediment transport and some overland runoff of fertilizer, pesticides, and herbicides to streams (U. S. Environmental Protection Agency, 1995). Furthermore, riparian buffer strips have other environmental benefits to wildlife. Although domestic water supply constitutes only about 8% of total water use, the cost per unit volume is the greatest of any water use. Thus, there are strong economic incentives for its conservation. Gleick (2001) uses a dramatic illustration of water loss by pointing out that a greater volume of water leaks from the Mexico City water distribution system than is used by the city of Rome! The use of treated sewerage for irrigation of ornamental plants (Gori and Lubello, 2000), Xeriscaping (Gregg and Curry, 1995), and treated wastewater for domestic supply (Haarhoff and Van der Merwe, 1996) are solutions that have proven effective in certain areas. The recent example of New York City saving 6 or 7 thousand million U. S. dollars by improving the environment of the catchment area rather than building filtration facilities (PCAST, 1998) illustrates the significant financial savings possible by alternative methods of water treatment. This approach not only improved the environment of the catchment area for many other uses, but also saved money that can be applied to repair leaking delivery pipes, install water meters, or supply low-flush toilets -- approaches that save water. Policies of subsidies, taxes, and environmental regulations are the most common approaches to control of industrial uses of water in the marketdriven economies of democratic societies. Because of the various uses in industry/manufacturing/
Water Sustainability: Science or Science Fiction? Perspective from One Scientist
mining, it is impossible to make generalizations for savings and water reclamation. A recent example of the impact of environmental regulations on water use in the United States, however, is the requirement that heated water be cooled to the ambient temperature of the receiving body prior to its discharge. This requirement has reduced the amount of water withdrawn for thermal cooling (Solley et al., 1998). Seemingly, industry has found it more economical to cool and reuse the bulk of the previously used water rather than withdraw new water.
Sustainability To address these water-resources problems, many popular press, radio, and television programs have proposed the concept of sustainability. Sustainability is on everyone's radar; it's the new "catchword." Sustainability is a relatively new term, less than 30 years old. According to the Oxford English Dictionary, it was first used in the early 1970's to explain an economy that was unchanging in time. In 1987, the Brundtland Commission of the United Nations popularized the word in the sense now used by managers of natural resources. This commission defined sustainability as "the ability to meet the needs of the present generation without compromising the ability of future generations to meet their needs." This philosophy appears to be a wealth distribution concept and does not address important water-management questions, such as sustainability at what level, for whom, in what time frame, using which economic model? Sustainability of any resource is considered a desirable goal by some analysts, although many resources are not renewable or recyclable on societally-relevant time scales, and thus, are not sustainable. Do we stop developing fossil fuels because they are not sustainable? The concept of sustainability does not provide the critical mechanisms for the distribution of the resource among competing factions, it is simply a philosophical goal without exact guidelines. Is this the correct way to cast a resource management problem? Because water has many different uses and functions in a drainage basin, sustainability requires a holistic approach; that is, what might be sustainable for one use is unsustainable for another, adding complexity to the concept of sustainability. Implicit in the concept of sustainability is the delay in use or consumption of a resource; yet the economic benefits from such a delay are not defined. A further concern is that sustainability implies stasis when, in fact, uses and consumption are dynamic both temporally and
spatially. Clearly, the source is dynamic both in mass and space viewed on almost any relevant time scale. This awareness is of particular importance when predicting effects of long-term climate changes on water resources. A more fundamental problem with the philosophy of sustainability is the flawed logic of comparing a finite resource with an exponential population growth - sustainability is ultimately impossible for critical, non-fungible uses. At some point, population demand will outstrip resource supply - it is the classic Malthusian argument. In the past, the availability of unallocated water and under-utilized, arable land as well as improvements in technology have saved humanity from the starvation predicted by Malthus (1798). We now have allocated most of the easily available water and arable land. Although we are adding cropland each year by deforestation, we are experiencing a net annual loss of arable land because of salinization, urbanization, desertification, and erosion (Barrow, 1991). Technical improvements continue to boost productivity but are ultimately constrained by the thermodynamics and kinetics of the systems, a concept not addressed by sustainability. A strategy of water-resource management based on annual water renewability that identifies major uses, consumption, sources, and sinks in a system rather than "needs" is an approach that can be incorporated in policy decisions on how water should be allocated. In general, greater clarity of water-resources management issues can be gained by separating the different roles of policy, science, and engineering. It must be emphasized that scientists and engineering cannot make the valuebased allocation decisions required for establishing policy, although they can help avoid unintended consequences by practicing a "what if" evaluation of suggested water policies that physically, chemically, or biologically stress a system. CONCLUSIONS It is my perception that there has been a failure by policy makers and the general public to recognize that utilization of water, land, and air isintimately related. What is needed in water-resources management is a more holistic conceptual framework that encompasses basin-scale hydrologic systems. It is clear that technological advances are to be the source of "new" water, as there remain opportunities for significant improvement in technology. The kinetic and thermodynamic limits of most processes are not presently constraining development. Specifically, it is desirable to identify
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methods and techniques that can economically save a significant portion of the water presently underutilized or lost from the system. If, however, energy prices decline significantly or the rate of technological innovation for desalinization increases, then desalinization of seawater could provide some new water. Similarly, if one could grow grains or fodder with saline water, some of the world's food concerns would be assuaged. At this time, it appears that the greatest savings almost certainly will come from agriculture and reclamation of degraded water now discharged to the sea because these are the processes that constitute over 90% of the total human-related consumption. The concept of sustainability has been proposed to resolve water-resource problems, but it is fundamentally flawed both as a goal and as an operational policy in several areas. It does not incorporate an economic model that justifies delayed use or non-use of a resource. It is based on the implied assumption that it is possible to satisfy all the demands on a finite resource in spite of an exponentially increasing population and increased water use. It does not address the important questions of distribution of the resource, and it implies a static use and supply as a function of time. The concept of water renewability and quantification of major water uses, sources, and sinks in a drainage basin can provide a viable base on which to evaluate water-resources policies for an increasingly thirsty and hungry world. This concept of additional water information for policy development is widely recognized by the scientific community (Food and Agriculture Organization of the United Nations, 2001). If technologies are developed to address watersupply problems, they must become affordable and available in areas of subsistence agriculture where much of the population growth is expected to occur. There is no viable international market in water, so water savings generated in one area on earth cannot be readily transferred to another except by embedded water. That is, competitive advantages and trade may dictate the "water-rich" areas specializing in production of some goods, while "water-poor" areas specialize in other goods. The goods are then traded, not the water. There is reluctance on the part of most consumers to pay for benefits that will not be delivered until sometime in the future. There is also significant inertia in customs (social behavior) and consumer choices. Furthermore, the long lifecycles of dams and other infrastructure investments are impediments to change in water use. Thus, overcoming economic,
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social, and political obstacles necessary to make use of technological changes is a significant challenge. REFERENCES Allan, J.A., 1996. Policy responses to the closure of water resources. In Water policy: allocation and management in practice; edited by P. Howsam and R. Cater. London: Chapman and Hall. Barrow, C.J., 1991. Land degradation - development and breakdown of terrestrial environments. Cambridge, U.K.: Cambridge University Press. Cook, P.G., A.L. Telfer, and G.R. Walker, 1993. Potential for salinization of groundwater beneath mallee areas of the Murray Basin. Centre for Groundwater Studies Report No. 42/Engineering and Water Supply Department (S.A.) Report No. 93/6. Adelaide, Australia. Falkenmark, M., 1994. Landscape as life support provider, water-related limitations. In Population: the complex reality, ed. P. Graham-Smith, 103-116. London: The Royal Society. Food and Agriculture organization of the United Nations, 2001, accessed January 2, 2001 at h ttp ://www.f ao. o rg/g to s/g to sp ub~pub 2 6. hml. Gleick, P.H., 2000. The changing water paradigm - a look at twenty-first century water resources development. Water International 25:127-138. Gleick, P.H., 2001. Our water. Scientific American 284(2): 41-45. Gregg, T., and J. Curry, 1995. Xeriscape: promises and pitfalls. Proceedings of Conservation 96, 165-168. Denver, CO: American Water Works Association. Gori, R., and C. Lubello, 2000. Pilot plant for reclaimed wastewater reuse in nurseries. Water Science and Technology 42( 1-2): 221-226. Haarhoff, J., and B. Van der Merwe, 1996. Twenty-five years of wastewater reclamation in Windhoek Namibia. Water Science and Technology 33(10-11): 25-35. Kendall, H.W., and D. Pimentel, 1994. Constraints on the expansion of the global food supply. Ambio 23(3): 198-205. Latham, M.C, 1984. International nutrition and problems and policies. In World food issues. Ithaca, N.Y: Cornell University, Center for Analyses of World Food Issues, International Agriculture. Lutz, W., W. Sanderson, and S. Scherbov, 2001. The end of world population growth. Nature 412: 543-545. Malthus, T. R., 1798. An essay on the principle of population, as it affects the future improvement of society with remarks on the speculations of Mr. Godwin M., Condorcet, and other writers, Printed for J. Johnston in St. Pauls Church-yard, London. Maitre, D.C.L., D.C. Scott, and C. Colvin, 1999. A review of information on interactions between vegetation and groundwater. Water, South Africa 25(2): 137-152.
Water Sustainability: Science or Science Fiction? Perspectivefrom One Scientist
McCoy, C., and G.P. Zachary, 1997. Bass play in water may presage big shift in its distribution. Wall Street Journal, New York, July 11. McCully, P., 1997. Silenced rivers. London: Zed Books. Moseley, L., 1998. Deaf Smith County producer pleased with drip irrigation. The Cross Section, 44, no. 3. Lubbock, TX: High Plains Underground Water Conservation District. National Research Council, 2001. Envisioning the agenda for water resources research in the twentyfirst century. Washington, D.C.: National Academy Press. PCAST, 1998. Teaming with life - investing in science to understand and use America's living capital. Washington, D.C.: The President's Committee of Advisors on Science and Technology. Pimentel, D., J. Houser, E. Preiss, O. White, H. Fang, L. Mesnick, T. Barsky, S. Tariche, J. Schreck, and S. Alpert, 1997. Water resources- agriculture, the environment, and society. Bioscience 47(2): 97106. Population Reference Bureau, 1998. World population data sheet wall chart. Washington, D.C.: Population Reference Bureau. Postel, S.L., 1999. Pillar of sand. New York: W.W. Norton. Postel, S.L., 2001. Growing more food with less water. Scientific American 284, no. 2:46-51. Postel, S.L., G.C. Daily, and P.R. Ehrlich, 1996. Human appropriation of renewable fresh water. Science 271: 785-788.
Ritschard, R. L., and K. Tsao, 1978. Energy and water use in irrigated agriculture during drought conditions. Lawrence Berkeley Laboratory Publication LBL-7866. University of Califomia at Berkeley. Shiklomanov, I.A., 1993. World fresh water resources. In Water in crisis, a guide to the world's fresh water resources, ed. P. H. Gleick. New York: Oxford University Press. Somerville, C., and J. Briscoe, 2001. Genetic engineering and water. Science 292: 2217. Solley, W.B., R.R. Pierce, and H.A. Perlman, 1998. Estimated use of water in the United States in 1995. U. S. Geological Survey Circular 1200. Reston, VA: U.S. Geological Survey. United Nations, 1992. World food supplies and prevalence of chronic undernutrition in developing regions as assessed in 1992. New York: Statistical Analysis Service, Statistical Division, Economic and Social Policy Department, United Nations Food and Agriculture Organization. U.S. Environmental Protection Agency, 1995. Water quality functions of riparian forest buffer systems in the Chesapeake Bay watershed. U.S. Environmental Protection Agency Report EPA-R-95-O04. Annapolis, MD: Chesapeake Bay Program. World Resources Institute, 1992. World resources report 1992-1993. New York: Oxford University Press. World Resources Institute, 1994. World resources report 1994-1995. New York: Oxford University Press.
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ABSTRACT: A significant increase in the world's human population is predicted to occur in the next 100 years. This population will almost certainly require additional water resources for irrigated agriculture, domestic supply, and ecosystem support. Because most of the world's easily captured water is already identified and allocated, society must either change the present allocations or develop new sources to meet the expected demands. The philosophy of sustainability has been proposed as a management goal that will address water-resource-related issues. Sustainability is generally defined as the ability to meet the needs of the present generation without compromising the ability of future generations to meet their needs. "Meeting the needs" beyond basic survival requirements of a few liters of water per day requires many value judgments, and the assessment of these judgments lies outside the realm of science or engineering. Sustainability fails to address historical property fights or recognize that population increase is exponential although the resource is finite. Sustainability also implies stasis, yet uses and supply are temporally and spatially dynamic. Thus, the concept of sustainability is flawed not only as a goal, but also as a management objective and holistic sustainability (sustainability for all uses) is nearly impossible on a drainage-basin scale. Development of an alternative strategy based on water renewability that identifies and measures major sources, sinks, uses, and consumption in a system rather than "needs" can form the basis for informed policy decisions.
INTRODUCTION A high probability exists for doubling the world's human population in the next 100 years, although the estimates vary widely (Lutz et al., 2001). This increase will almost certainly require significant additional water. Over a thousand million people have an inadequate diet (Latham, 1984); however, that appears to be improving slightly (United Nations, 1992). Improving nutrition of the existing population, a laudable goal, will require additional irrigated agriculture. It is also generally recognized that in the future, a larger share of the water will be demanded for fisheries restoration, biodiversity, and general ecosystem maintenance. Adding to these environmental pressures for additional water is a strong positive correlation between standard of living and water use. World population increased 42% from 3.8 thousand million to 5.4 thousand million during a twenty year period between the 1970's and 1990's, while water used increased 300% (Postel, 1999). If national economies continue to grow, there is likely to be increased water demand, even with a static population. What will be the source of this water? The water-resource perspectives expressed in this manuscript are global in nature, but it is recognized because of the limited-trading market that most water-resource problems and solutions are local in nature.
It is generally believed that there is no significant volume of water entering or leaving the earth from space; thus, water on earth is finite, but renewable. Approximately 97% of the world's water resources are saline and located in the oceans and seas. Of the 2.5% of the world's water resources that are fresh water, approximately 69% is stored in glaciers and ice caps that are not readily available, 30% is stored in ground water, and less than 1% resides in rivers and lakes at any instant (Shiklomanov, 1993). The distribution of water on earth is not uniform, nor is it correlated well with population. For example, the Amazon River accounts for 15% of the global runoff and 0.5% of the population, while China has 21% of the population and 7% of the global runoff (World Resources Report, 1994; Population Reference Bureau, 1998). Approximately 110,000 km 3 of precipitation falls on the continents each year. Of this, approximately 50,000 km 3 are transpired from the native vegetation; 20,000 km 3 are transpired from irrigated crops and evaporated from surface reservoirs; 12,000 km 3 are utilized by man for industrial and domestic use; 20,000 km 3 are returned to the sea in unrecoverable floods; and 8,000 km 3 are unavailable for societal needs because of its remote locations (Postel et al., 1996). Of the water utilized by man, 69% is used for irrigation, 23% for industry, and 8% for domestic water supply (World Resources
Water ResourcesPerspectives:Evaluation, Management and Policy. Edited by A.S. Alsharhan and W.W. Wood. Published in 2003 by Elsevier Science, Amsterdam, The Netherlands, p. 45-51.
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Institute, 1992). This estimate for the percentage of irrigation use is approximately 10% larger than that given above by Postel et al. (1996). It is, however, within the uncertainty of the methods used in both approaches. Using the estimate from Postel et al. (1996) suggests that 32,000 km3/yr is the amount of renewable water resources that society can utilize without removing additional native vegetation, building new dams, or transporting water from remote locations. Implicit in this estimate is the temporal stability of this resource. Yet, we know that there have been large shifts in climate in the last 20,000 years, and from historical records, it is clear that measurable climate changes can occur decadally. The predicted increase in population will almost certainly require additional water for both domestic water supply and irrigated agriculture; however, the readily available water resources are already fully allocated. We are, in fact, borrowing from the future as unreplenished ground water from storage is used to produce approximately 8% of the total food crops (Postel, 2001). Worldwide, 12 to 15 million hectares of land, or about 1% of the total cropland, are lost each year to degradation due to salinization, erosion, desertification, and urbanization (Kendall and Pimentel et al., 1994). Additionally, nutrient depletion, over-cultivation, over-grazing, and acidification contribute to the reduction of yields. If other factors remain constant, the per capita food production will decline as the aquifers are dewatered and cropland is lost or its productivity is reduced. Irrigation of approximately 17% of the agricultural land produces 35 to 40% of the food crops (Postel, 1999). Thus, with increased food demands, there will almost certainly be a need for increased agricultural irrigation. Associated with many grain crops is the loss of arable land due to salinization. Between 1.5 and 2 million hectares of arable land are lost each year to salinization, or approximately 0.1% of the total cropland (Kendall and Pimentel, 1994). High concentrations of solutes are detrimental to plant growth and to the tilth of the soil. In advanced cases of salinization, salts form on the land surface and completely inhibit plant growth. Because of its relatively low value per unit mass and thus high transportation costs, water is not traded extensively like other commodities. Significant amounts of water, however, are used in the production of grains and meats, and this "virtual" or "embedded" water is traded worldwide (Allan, 1996). The lack of well-defined property rights to water is another limitation of trading. In the United States where some jurisdictions permit private
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ownership of water, an active market in trading water rights has recently developed (McCoy and Zachary, 1997), but these markets are local and constitute a minute portion of the total volume of annual water use. In some cases, water can be "wheeled" like electricity, thus reducing long transport for individual water molecules. That is, water moved to one area is used locally, and then new water from this area is passed on to the next user and so on until the transfer is complete. We are approaching, or in some areas have reached, a critical point in water resources where the total readily recoverable volume of water falling on a watershed has been allocated. The policy/management questions associated with these conditions are: 1) how does society reallocate the presently available water to accommodate changing needs, and/or 2) how does society acquire new water? Water Uses
Water resources use can be classified into different categories: domestic water supply and waste treatment, livestock watering, irrigation, transportation, industrial (resource development/ manufacturing), power generation, recreation, flood control, and ecosystem maintenance. These uses are not mutually exclusive as water is generally used in multiple ways as it moves through the basin, and any use typically impacts one or more of the other uses. One also must distinguish carefully between use and consumption. For example, in North America, power generation is the largest user of water, but its consumption is minuscule relative to irrigation. It appears that society's most critical, non-fungible water uses are domestic water supply, irrigation, livestock watering, and ecosystem support. Public recognition of the ecological importance of waterresource management has come to the forefront only in the last 40 years; thus, only small amounts of water have been formally allocated for ecosystem maintenance (Gleick, 2000). This late recognition, after most water already has been allocated, coupled with the difficulty of establishing the direct economic benefit of biodiversity and ecosystem maintenance in market-based economies, results in the smallest formal water allocation among the major uses. Improvement in freshwater fisheries and species diversity for ecosystem maintenance will almost certainly require significant additional water appropriation in the future. In countries with democratic governments and property rights with environmental laws, and policies of pricing (including scaled water charges,
Water Sustainability: Science or Science Fiction? Perspectivefrom One Scientist
subsidies, taxation) generally control water use. Because of the local nature of climate, topography, geology, historical development, infrastructure, and other factors, organizing water policy strategies around drainage basins might be a rational approach. Typically, however, there are many different and competing political jurisdictions in any given drainage basin, making decisive, coherent political action involving water-resources policy difficult. Additionally, thousands of private water and wastewater purveyors exist (National Research Council, 2001). This private ownership and a lack of universal property fights of water coupled with the fact that rivers are frequently used as boundaries between jurisdictions further inhibit the establishment of a coherent water policy. Water policy as generated by legislative bodies tends to be descriptive, immediate, asks the question "how," seeks human-based assessment, and is rigid. That is, it is designed as a "one size fits all" solution. In contrast, scientists and engineers recognize the need for long-term studies, ask the question "why," seek peer-review assessment, and demand variability with each case, depending on the circumstances. Thus, in political environments, it is not surprising that there are few opportunities for scientists and engineers to make direct contributions to water policy. Water scientists and engineers can provide important answers to "what if" policy questions related to physical, biological, and chemical stresses to the environment. It is because of their understanding of the full dimension of the water cycle that hydrologists can frame discussion and provide insight into what the potential tradeoffs might be. I suspect that the greatest immediate contributions by scientists and engineers to the solution of societal water problems lie in improving the efficiency and productivity of present water uses. Although long-term water-resource allocation is clearly a policy issue, policy cannot create "new" water. It can, however, foster the intellectual and economic environment where scientists and engineers excel.
Potential Areas for Increased Efficiency and/or Conserving Water Political or technological solutions to saving water must be distinguished from virtual and real forms of "saving" water. "Real" savings actually generate additional water supplies, but virtual savings are "paper" savings. Further, saving water is scale-dependent. On a global scale, the hydrologic cycle is closed: no water is lost; it only changes location and physical state. On the scale of river
basins, however, it is extremely important to know where and how the major sources and sinks of the water occur. Postel (1999) uses an interesting example of irrigation in the ancient Nile valley to illustrate efficiency as a function of scale. This system was not efficient when viewed from the perspective of individual farms because much of the water spread over each field drained off without benefiting the crop. The runoff from upstream farmers became the supply for downstream farmers, however, and the system's efficiency was much greater when examined from the perspective of the entire Nile valley. In general, there are five ways to "save" or make more productive use of the water available: 1) reduce evaporation and un-recycled seepage loss, such as those seepage losses to a closed basin or an ocean; 2) shift water from one use to another with higher food value and lower water demands; for example, growing wheat rather than cotton or raising chicken rather than beef; 3) develop more efficient crops, cultivation, and irrigation methods; 4) substitute capital for water, thereby increasing efficiency; and 5) capture of a greater percentage of the water in rivers before it is discharged to the oceans or closed basins. This last option is becoming less viable as the true costs of social disruption, inequitable benefits, loss of productive agricultural land, and loss of biodiversity and ecosystems are weighted against the societal benefits (McCully, 1996). In addition, the number of remaining topographically-suitable sites is limited. It also seems unlikely that a concerted world-wide effort will be mounted to harvest more water by removing native vegetation, although removal of invasive species is part of the Republic of South Africa's program of water conservation (Maitre et al., 1999). Deforestation continues to remove trees that use large quantities of water; thus, some gains in runoff are expected. In some cases, however, this deforestation also may lead to salinization (Cook et al., 1993), to increased sediment loading of streams, and to other harmful impacts. It appears that most of the increased productivity or savings must come from the other areas. The "green revolution" (in the last half of the 20th century) that increased agricultural productivity so significantly through modification of crops and increased irrigation is expected to continue. Evaporative water losses from irrigated fields are difficult to measure because the loss by transpiration and evaporation is commonly lumped together as a measured or calculated parameter in water-balance studies. That is, both processes return water to the atmosphere in the vapor phase. Transpiration by
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crops is necessary and not considered a loss; so, it is necessary to separate transpiration and evaporation to get a measure of the true loss. If one assumes that 10% of the water applied to irrigation is lost to evaporation, this would amount to 250 x 109 m3/yr worldwide. If this water could be saved and applied to grain production, it would generate approximately 250 million tons of grain, or 15% of the total annual grain harvest -- not an insignificant amount and about equal to the world's annual reserve amount. Note that approximately 1,000 tons of water are required to grow 1 ton of wheat (Ritschard and Tsao, 1978); 3,500 tons of water to grow one ton of chicken (Pimentel et al., 1997), 10,000 tons of water to grow one ton of beef (Falkenmark, 1994); and 17,000 tons of water to grow one ton of cotton (Kendall and Pimentel, 1994). Approaches to reduce evapotranspiration can be grouped into four general areas: agronomic, technical, managerial, and policy/institutional. As stated above, scientists and engineers typically have little direct influence in policy/institutional aspects such as establishing water-use organizations, setting conservation-oriented pricing, or establishing a legal framework for property fights. They can, however, make important contributions in the other areas of reducing evaporation. Clearly, great strides are anticipated in plant physiology (whether by genetic engineering or traditional selection methods) so that plants will require less water for the same yields, or yields will be increased for the same water application (Somerville and Briscoe, 2001). Development of plant species that can produce high protein fodder from saltwater irrigation or plant growth in salt marshes is the long-term goal of genetic modification. Development of species that require fewer pesticides or lower concentration of nutrients will limit or minimize degradation of runoff water, making it available for other uses. Development and implementation of inexpensive micro-irrigation systems that deliver the correct amount of water to the roots at the precise time required by a plant is the ultimate goal of irrigation. For poor countries with subsistence agriculture, where the improvements are needed most, this must be accomplished on small, hectare-size plots with hand labor and a minimum of technically sophisticated hardware. Examples of this approach are Treadle pumps and portable, inexpensive dripirrigation systems (Postel, 1999). Expansion of furrow irrigation using surge irrigation or low energy precision application (LEPA) on existing sprinkler systems result in significant increases in productivity and can quickly recover capital
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investment (Moseley, 1998); they are capitalintensive, however. Minimizing degradation of water resources is a fruitful area for saving water. That is, if the properties are not changed or contaminants are either not introduced or successfully removed from water, the water is then available to downstream users in its original condition. Although technology permits relatively easy identification of point sources of contamination and has provided mechanisms for treating domestic sewerage and industrial discharge, agricultural runoff containing sediments, fertilizers, pesticides, and herbicides has resisted large-scale remediation in most areas. Technical remediation strategies for agricultural runoff range from land management and control of applications of agricultural chemicals to removal of these chemicals and sediments by buffer strips of riparian vegetation. Preliminary studies suggest that these buffer strips between crop fields and streams are a passive, economical method to significantly reduce sediment transport and some overland runoff of fertilizer, pesticides, and herbicides to streams (U. S. Environmental Protection Agency, 1995). Furthermore, riparian buffer strips have other environmental benefits to wildlife. Although domestic water supply constitutes only about 8% of total water use, the cost per unit volume is the greatest of any water use. Thus, there are strong economic incentives for its conservation. Gleick (2001) uses a dramatic illustration of water loss by pointing out that a greater volume of water leaks from the Mexico City water distribution system than is used by the city of Rome! The use of treated sewerage for irrigation of ornamental plants (Gori and Lubello, 2000), Xeriscaping (Gregg and Curry, 1995), and treated wastewater for domestic supply (Haarhoff and Van der Merwe, 1996) are solutions that have proven effective in certain areas. The recent example of New York City saving 6 or 7 thousand million U. S. dollars by improving the environment of the catchment area rather than building filtration facilities (PCAST, 1998) illustrates the significant financial savings possible by alternative methods of water treatment. This approach not only improved the environment of the catchment area for many other uses, but also saved money that can be applied to repair leaking delivery pipes, install water meters, or supply low-flush toilets -- approaches that save water. Policies of subsidies, taxes, and environmental regulations are the most common approaches to control of industrial uses of water in the marketdriven economies of democratic societies. Because of the various uses in industry/manufacturing/
Water Sustainability: Science or Science Fiction? Perspective from One Scientist
mining, it is impossible to make generalizations for savings and water reclamation. A recent example of the impact of environmental regulations on water use in the United States, however, is the requirement that heated water be cooled to the ambient temperature of the receiving body prior to its discharge. This requirement has reduced the amount of water withdrawn for thermal cooling (Solley et al., 1998). Seemingly, industry has found it more economical to cool and reuse the bulk of the previously used water rather than withdraw new water.
Sustainability To address these water-resources problems, many popular press, radio, and television programs have proposed the concept of sustainability. Sustainability is on everyone's radar; it's the new "catchword." Sustainability is a relatively new term, less than 30 years old. According to the Oxford English Dictionary, it was first used in the early 1970's to explain an economy that was unchanging in time. In 1987, the Brundtland Commission of the United Nations popularized the word in the sense now used by managers of natural resources. This commission defined sustainability as "the ability to meet the needs of the present generation without compromising the ability of future generations to meet their needs." This philosophy appears to be a wealth distribution concept and does not address important water-management questions, such as sustainability at what level, for whom, in what time frame, using which economic model? Sustainability of any resource is considered a desirable goal by some analysts, although many resources are not renewable or recyclable on societally-relevant time scales, and thus, are not sustainable. Do we stop developing fossil fuels because they are not sustainable? The concept of sustainability does not provide the critical mechanisms for the distribution of the resource among competing factions, it is simply a philosophical goal without exact guidelines. Is this the correct way to cast a resource management problem? Because water has many different uses and functions in a drainage basin, sustainability requires a holistic approach; that is, what might be sustainable for one use is unsustainable for another, adding complexity to the concept of sustainability. Implicit in the concept of sustainability is the delay in use or consumption of a resource; yet the economic benefits from such a delay are not defined. A further concern is that sustainability implies stasis when, in fact, uses and consumption are dynamic both temporally and
spatially. Clearly, the source is dynamic both in mass and space viewed on almost any relevant time scale. This awareness is of particular importance when predicting effects of long-term climate changes on water resources. A more fundamental problem with the philosophy of sustainability is the flawed logic of comparing a finite resource with an exponential population growth - sustainability is ultimately impossible for critical, non-fungible uses. At some point, population demand will outstrip resource supply - it is the classic Malthusian argument. In the past, the availability of unallocated water and under-utilized, arable land as well as improvements in technology have saved humanity from the starvation predicted by Malthus (1798). We now have allocated most of the easily available water and arable land. Although we are adding cropland each year by deforestation, we are experiencing a net annual loss of arable land because of salinization, urbanization, desertification, and erosion (Barrow, 1991). Technical improvements continue to boost productivity but are ultimately constrained by the thermodynamics and kinetics of the systems, a concept not addressed by sustainability. A strategy of water-resource management based on annual water renewability that identifies major uses, consumption, sources, and sinks in a system rather than "needs" is an approach that can be incorporated in policy decisions on how water should be allocated. In general, greater clarity of water-resources management issues can be gained by separating the different roles of policy, science, and engineering. It must be emphasized that scientists and engineering cannot make the valuebased allocation decisions required for establishing policy, although they can help avoid unintended consequences by practicing a "what if" evaluation of suggested water policies that physically, chemically, or biologically stress a system. CONCLUSIONS It is my perception that there has been a failure by policy makers and the general public to recognize that utilization of water, land, and air isintimately related. What is needed in water-resources management is a more holistic conceptual framework that encompasses basin-scale hydrologic systems. It is clear that technological advances are to be the source of "new" water, as there remain opportunities for significant improvement in technology. The kinetic and thermodynamic limits of most processes are not presently constraining development. Specifically, it is desirable to identify
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methods and techniques that can economically save a significant portion of the water presently underutilized or lost from the system. If, however, energy prices decline significantly or the rate of technological innovation for desalinization increases, then desalinization of seawater could provide some new water. Similarly, if one could grow grains or fodder with saline water, some of the world's food concerns would be assuaged. At this time, it appears that the greatest savings almost certainly will come from agriculture and reclamation of degraded water now discharged to the sea because these are the processes that constitute over 90% of the total human-related consumption. The concept of sustainability has been proposed to resolve water-resource problems, but it is fundamentally flawed both as a goal and as an operational policy in several areas. It does not incorporate an economic model that justifies delayed use or non-use of a resource. It is based on the implied assumption that it is possible to satisfy all the demands on a finite resource in spite of an exponentially increasing population and increased water use. It does not address the important questions of distribution of the resource, and it implies a static use and supply as a function of time. The concept of water renewability and quantification of major water uses, sources, and sinks in a drainage basin can provide a viable base on which to evaluate water-resources policies for an increasingly thirsty and hungry world. This concept of additional water information for policy development is widely recognized by the scientific community (Food and Agriculture Organization of the United Nations, 2001). If technologies are developed to address watersupply problems, they must become affordable and available in areas of subsistence agriculture where much of the population growth is expected to occur. There is no viable international market in water, so water savings generated in one area on earth cannot be readily transferred to another except by embedded water. That is, competitive advantages and trade may dictate the "water-rich" areas specializing in production of some goods, while "water-poor" areas specialize in other goods. The goods are then traded, not the water. There is reluctance on the part of most consumers to pay for benefits that will not be delivered until sometime in the future. There is also significant inertia in customs (social behavior) and consumer choices. Furthermore, the long lifecycles of dams and other infrastructure investments are impediments to change in water use. Thus, overcoming economic,
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social, and political obstacles necessary to make use of technological changes is a significant challenge. REFERENCES Allan, J.A., 1996. Policy responses to the closure of water resources. In Water policy: allocation and management in practice; edited by P. Howsam and R. Cater. London: Chapman and Hall. Barrow, C.J., 1991. Land degradation - development and breakdown of terrestrial environments. Cambridge, U.K.: Cambridge University Press. Cook, P.G., A.L. Telfer, and G.R. Walker, 1993. Potential for salinization of groundwater beneath mallee areas of the Murray Basin. Centre for Groundwater Studies Report No. 42/Engineering and Water Supply Department (S.A.) Report No. 93/6. Adelaide, Australia. Falkenmark, M., 1994. Landscape as life support provider, water-related limitations. In Population: the complex reality, ed. P. Graham-Smith, 103-116. London: The Royal Society. Food and Agriculture organization of the United Nations, 2001, accessed January 2, 2001 at h ttp ://www.f ao. o rg/g to s/g to sp ub~pub 2 6. hml. Gleick, P.H., 2000. The changing water paradigm - a look at twenty-first century water resources development. Water International 25:127-138. Gleick, P.H., 2001. Our water. Scientific American 284(2): 41-45. Gregg, T., and J. Curry, 1995. Xeriscape: promises and pitfalls. Proceedings of Conservation 96, 165-168. Denver, CO: American Water Works Association. Gori, R., and C. Lubello, 2000. Pilot plant for reclaimed wastewater reuse in nurseries. Water Science and Technology 42( 1-2): 221-226. Haarhoff, J., and B. Van der Merwe, 1996. Twenty-five years of wastewater reclamation in Windhoek Namibia. Water Science and Technology 33(10-11): 25-35. Kendall, H.W., and D. Pimentel, 1994. Constraints on the expansion of the global food supply. Ambio 23(3): 198-205. Latham, M.C, 1984. International nutrition and problems and policies. In World food issues. Ithaca, N.Y: Cornell University, Center for Analyses of World Food Issues, International Agriculture. Lutz, W., W. Sanderson, and S. Scherbov, 2001. The end of world population growth. Nature 412: 543-545. Malthus, T. R., 1798. An essay on the principle of population, as it affects the future improvement of society with remarks on the speculations of Mr. Godwin M., Condorcet, and other writers, Printed for J. Johnston in St. Pauls Church-yard, London. Maitre, D.C.L., D.C. Scott, and C. Colvin, 1999. A review of information on interactions between vegetation and groundwater. Water, South Africa 25(2): 137-152.
Water Sustainability: Science or Science Fiction? Perspectivefrom One Scientist
McCoy, C., and G.P. Zachary, 1997. Bass play in water may presage big shift in its distribution. Wall Street Journal, New York, July 11. McCully, P., 1997. Silenced rivers. London: Zed Books. Moseley, L., 1998. Deaf Smith County producer pleased with drip irrigation. The Cross Section, 44, no. 3. Lubbock, TX: High Plains Underground Water Conservation District. National Research Council, 2001. Envisioning the agenda for water resources research in the twentyfirst century. Washington, D.C.: National Academy Press. PCAST, 1998. Teaming with life - investing in science to understand and use America's living capital. Washington, D.C.: The President's Committee of Advisors on Science and Technology. Pimentel, D., J. Houser, E. Preiss, O. White, H. Fang, L. Mesnick, T. Barsky, S. Tariche, J. Schreck, and S. Alpert, 1997. Water resources- agriculture, the environment, and society. Bioscience 47(2): 97106. Population Reference Bureau, 1998. World population data sheet wall chart. Washington, D.C.: Population Reference Bureau. Postel, S.L., 1999. Pillar of sand. New York: W.W. Norton. Postel, S.L., 2001. Growing more food with less water. Scientific American 284, no. 2:46-51. Postel, S.L., G.C. Daily, and P.R. Ehrlich, 1996. Human appropriation of renewable fresh water. Science 271: 785-788.
Ritschard, R. L., and K. Tsao, 1978. Energy and water use in irrigated agriculture during drought conditions. Lawrence Berkeley Laboratory Publication LBL-7866. University of Califomia at Berkeley. Shiklomanov, I.A., 1993. World fresh water resources. In Water in crisis, a guide to the world's fresh water resources, ed. P. H. Gleick. New York: Oxford University Press. Somerville, C., and J. Briscoe, 2001. Genetic engineering and water. Science 292: 2217. Solley, W.B., R.R. Pierce, and H.A. Perlman, 1998. Estimated use of water in the United States in 1995. U. S. Geological Survey Circular 1200. Reston, VA: U.S. Geological Survey. United Nations, 1992. World food supplies and prevalence of chronic undernutrition in developing regions as assessed in 1992. New York: Statistical Analysis Service, Statistical Division, Economic and Social Policy Department, United Nations Food and Agriculture Organization. U.S. Environmental Protection Agency, 1995. Water quality functions of riparian forest buffer systems in the Chesapeake Bay watershed. U.S. Environmental Protection Agency Report EPA-R-95-O04. Annapolis, MD: Chesapeake Bay Program. World Resources Institute, 1992. World resources report 1992-1993. New York: Oxford University Press. World Resources Institute, 1994. World resources report 1994-1995. New York: Oxford University Press.
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