Peak Water Limits to Freshwater Withdrawal and Use INAUGURAL ARTICLE
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Peak water limits to freshwater withdrawal and use INAUGURAL ARTICLE Peter H. Gleick1 and Meena Palaniappan Pacific Institute, 654 13th Street, Oakland, CA 94612 This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected in 2006. Contributed by Peter H. Gleick, April 8, 2010 (sent for review February 22, 2010) Freshwater resources are fundamental for maintaining human impact of human appropriations at various scales through the health, agricultural production, economic activity as well as critical use of rainfall, surface and groundwater stocks, and soil moisture. ecosystem functions. As populations and economies grow, new An early effort to evaluate these uses estimated that substantially constraints on water resources are appearing, raising questions more water in the form of rain and soil moisture—perhaps about limits to water availability. Such resource questions are 11;300 km3∕yr—is appropriated for human-dominated land uses not new. The specter of “peak oil”—a peaking and then decline such as cultivated land, landscaping, and to provide forage for in oil production—has long been predicted and debated. We pre- grazing animals. Overall, that assessment concluded that humans sent here a detailed assessment and definition of three concepts of already appropriate over 50% of all renewable and “accessible” “peak water”: peak renewable water, peak nonrenewable water, freshwater flows, including a fairly large fraction of water that is and peak ecological water. These concepts can help hydrologists, used in-stream for dilution of human and industrial wastes (3). It water managers, policy makers, and the public understand and is important to note, however, that these uses are of the “renew- manage different water systems more effectively and sustainably. able” flows of water, which we explain below. In theory, the use of Peak renewable water applies where flow constraints limit total renewable flows can continue indefinitely without any effect on water availability over time. Peak nonrenewable water is observa- future availability. Still, although many flows of water are renew- ble in groundwater systems where production rates substantially able, some uses of water will degrade the quality to a point that exceed natural recharge rates and where overpumping or conta- constrains the kinds of use possible. mination leads to a peak of production followed by a decline, In the past few years, various resource crises around water, SCIENCE similar to more traditional peak-oil curves. Peak “ecological” water energy, and food have led to new debates over definitions and SUSTAINABILITY is defined as the point beyond which the total costs of ecological concepts about sustainable resource management and use. Some disruptions and damages exceed the total value provided by energy experts have proposed that the world is approaching, or human use of that water. Despite uncertainties in quantifying has even passed, the point of maximum production of petroleum, many of these costs and benefits in consistent ways, more and or peak oil (4–7). More recently, there has been a growing dis- more watersheds appear to have already passed the point of peak cussion of whether we are also approaching a comparable point water. Applying these concepts can help shift the way freshwater for water resources, where natural limits will constrain growing resources are managed toward more productive, equitable, effi- populations and economic expansion. In this article, we define cient, and sustainable use. the concept of peak water and we evaluate the similarities and differences between water and oil, how relevant this idea is to surface water ∣ water use ∣ sustainable water management actual hydrologic and water-management challenges, and the implications of limits on freshwater availability for human and he Earth has substantial water resources, in numerous forms ecosystem wellbeing. Tand qualities, in various stocks and flows in the hydrologic Regional water scarcity is a significant and growing problem. cycle. Overall, the planet has a stock of approximately 1.4 billion Many possible indicators have been developed to measure water cubic kilometers of water, the vast majority of which (nearly 97%) scarcity, including both single-factor and weighted water mea- is salt water in the oceans. The world’s more limited freshwater sures (8). The United Nations has offered a definition of water stocks are estimated at around 35 million cubic kilometers. Most stress as regions where water consumption exceeds 10% of fresh water, however, is locked up in glaciers in Antarctica and renewable freshwater resources. Other definitions set per-capita Greenland, in permanent snow cover in mountains or high availability standards for defining scarcity (9–12). These kinds of latitudes, or in deep groundwater inaccessible to humans for indicators inform decision making and offer insights into progress practical reasons. Only small fractions are readily available to on addressing water problems, but no single measure can com- humans in river flows, accessible surface lakes and groundwater, pletely describe the characteristics of water scarcity. Despite soil moisture, or rainfall (1). Table 1 shows the distribution of the the lack of clear and specific measures of scarcity, it is increas- main components of the world’s water. ingly apparent that some regions are experiencing limits to Serious water challenges face humanity, including the failure growth in water use due to natural, ecological, political, or to meet basic human needs for safe water and sanitation for economic constraints. billions, growing contamination of water with human and indus- trial wastes, the consequences of extreme events such as floods Concept of Peak Resource Production and droughts, ecological disruption in aquatic ecosystems, in- The theory of peak resource production originated in the 1950s creasing concerns about water shortages and scarcity, and the with the work of geologist M. King Hubbert and colleagues who growing risks from climatic changes that will affect regional suggested that the rate of oil production would likely be hydrology and water management. Considering the total volume characterized by several phases that follow a bell-shaped curve of water on Earth, however, the concept of “running out” of (13). The first phase is the discovery and rapid increase in growth water at the global scale is of little practical utility. There are huge in the rate of exploitation of oil as demand rises, production volumes of water—many thousands of times the volumes that hu- mans appropriate for all purposes. In the early 2000s, total global Author contributions: P.H.G. designed research; P.H.G. and M.P. performed research; 3 withdrawals of water were approximately 3;700 km per year, a P.H.G. and M.P. contributed new reagents/analytic tools; P.H.G. and M.P. analyzed data; tiny fraction of the estimated stocks of fresh water (2). and P.H.G. and M.P. wrote the paper. A more accurate way to evaluate human uses of water, how- The authors declare no conflict of interest. ever, would look at regional stocks and flows of water and the 1To whom correspondence should be addressed. E-mail: [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1004812107 PNAS ∣ June 22, 2010 ∣ vol. 107 ∣ no. 25 ∣ 11155–11162 Table 1. Major stocks of water on Earth (34) Distribution area, Volume, Percent of total Percent of 103 km2 103 km3 water, % fresh water, % Total water 510,000 1.386 million 100 — Total freshwater 149,000 35,000 2.53 100 World oceans 361,300 1.340 million 96.5 — Saline groundwater — 13,000 1 — Fresh groundwater — 10,500 0.76 30 Antarctic glaciers 13,980 21,600 1.56 61.7 Greenland glaciers 1,800 2,340 0.17 6.7 Arctic islands 226 84 0.006 0.24 Mountain glaciers 224 40.6 0.003 0.12 Ground ice/permafrost 21,000 300 0.022 0.86 Saline lakes 822 85.4 0.006 — Freshwater lakes 1,240 91 0.007 0.26 Wetlands 2,680 11.5 0.0008 0.03 Rivers (as flows on average) — 2.12 0.0002 0.006 In biological matter — 1.12 0.0001 0.0003 In the atmosphere (on average) — 12.9 0.0001 0.04 becomes more efficient, and costs fall. Second, as stocks of oil are peak of production will only be identified in hindsight, and its consumed and the resource becomes increasingly depleted, costs timing depends on the demand and cost of oil, the economics rise and production levels off and peaks at a point now known as of technologies for extracting oil, the rate of discovery of new re- peak oil. Finally, increasing scarcity and costs lead to a decline in serves compared to the rate of extraction, the cost of alternative the rate of production more quickly than new supplies can be energy sources, and political factors. found or produced. This last phase would also be typically accom- panied by the substitution of alternatives. The phrase peak oil Comparison of Peak Production in Oil and Water Does production or use of water follow a similar bell-shaped refers to the point at which approximately half of the existing curve? In the growing concern about global and local water stock of petroleum has been depleted and the rate of production shortages and scarcity, is the concept of peak water valid and use- peaks (see Fig. 1). In a now-classic paper, Hubbert (1956) ful to hydrologists, water planners, managers, and users? In the predicted that oil production in the United States would peak following sections, we consider the differences and similarities between 1965 and 1970 (13). In 1970, oil production in the between oil and water to evaluate whether a peak in the produc- United States reached a maximum and began to decline (Fig. 2). tion of water is possible, and in what contexts it may be relevant.