Dr. Peter H. Gleick Ms. Meena Co-founder and President Palaniappan Pacific Institute for Studies in Director, International Water and Development, Environment Communities Initiative and Security Pacific Institute for Studies in Oakland, California Development, Environment [email protected] and Security Oakland, California [email protected] * THE CONCEPT OF PEAK WATER The new concept of ‘peak water’ is described here in the context of bated the timing of the point of maximum production of petroleum, global and local water challenges. Three different definitions are pro- or ‘peak oil’ (Bardi, 2009; Kerr, 2007; Duncan, 2003; Bentley, 2002). vided: ‘peak renewable,’ ‘peak non-renewable,’ and ‘peak ecological’ More recently, there has been a growing discussion of whether we are water, with specific examples of each and their role in characterizing also approaching a comparable point for water resources, where natu- water problems and solutions. Regions around the world are increas- ral limits will constrain growing populations and hinder economic ingly experiencing peak water constraints, as evidenced by a growing expansion (Gleick and Palaniappan, 2010). In this article, we present competition for water, increasing ecological degradation associated and review the concept of ‘peak water,’ evaluate the similarities and with human extraction of water from surface and ground water sys- differences between water and oil, and offer some thoughts about the tems and political controversies around water. Understanding the applicability of this concept to hydrologic and water-management links between human demands for water and peak water constraints challenges. Brief recommendations are made for avoiding these peak can help water managers and planners move towards more sustain- constraints. able water management and use by moving away from peak limits, by cutting non-renewable water use to more sustainable levels and Humanity faces serious water challenges. These include the failure by restoring aquatic ecosystems as a way to reduce ecological damage to meet basic human needs for safe water and sanitation, growing from exceeding ‘peak ecological water’. water contamination, the consequences of extreme events such as floods and droughts, disruptions in aquatic ecosystems, increasing Key words: peak water, peak ecological water, peak renew- concerns about water shortages and scarcity and the new risks to water able water, peak non-renewable water, fresh water resources, resources and systems from climatic changes. Considering the total climate change volume of water on Earth, however, the concept of ‘running out’ of water on a global scale is of little practical utility. There are huge vol- Purpose of the article umes of water – many thousands of times the volumes that humans appropriate for all purposes. The world’s fresh water stocks (< 3 per In the past few years, resource challenges around water, energy and cent of all water) are estimated at around 35 million km3. Much of this food have led to new debates over definitions and concepts about sus- fresh water is locked up in the icecaps of Antarctica and Greenland, tainable resource management and use. Energy experts have long de- permanent snow cover in mountains or high latitudes, or deep ground * This is a condensed and modified version of an article that appeared in the Proceedings of the National Academy of Sciences, Gleick, P.H. and M. Palaniappan. 2010. Peak Water: Conceptual and Practical Limits to Freshwater Withdrawal and Use. Proceedings of the National Academy of Sciences (PNAS), Vol. 107, No. 25, pp. 11155–11162 Washington, D.C. June 22, 2010. On the Water Front I Dr. Peter H. Gleick and Ms. Meena Palaniappan 41 water. Only small fractions are available to humans as ‘green’ or ‘blue’ duction and the growing effort to develop and substitute alternatives. water in river flows, accessible surface lakes and ground water, soil The phrase ‘peak oil’ refers to the point at which approximately half of moisture, or rainfall (Shiklomanov, 2000; CSD, 1997; Falkenmark et the existing stock of petroleum has been depleted and the rate of pro- al., 1989). Table 1 shows the distribution of the main components of duction peaks. In his classic paper, Hubbert (1956) correctly predicted the world’s water. In the early 2000s, total global withdrawals of water that oil production in the United States would peak between 1965 and were approximately 3700 km3 per year (excluding water used directly 1970. Indeed, in 1970, oil production in the US reached a maximum as rainfall or soil moisture), a tiny fraction of the estimated stocks of and has since declined (Fig. 1). fresh water. The concept of a roughly bell-shaped oil production curve has A more accurate way to evaluate human uses of water, however, would been proven for a well, an oil field, a region, and is thought to look at specific, often localised, stocks and flows of water and the im- hold true worldwide, although there is still a significant debate pact of human appropriations of rainfall, surface and ground water about when the world as a whole will reach the point of peak oil. stocks and soil moisture. An early effort to evaluate these uses esti- Forecasts range from the coming decade to substantially after 2025. mated that substantially more water in the form of rain and soil mois- One of many recent estimates suggests that oil production may peak ture –perhaps 11,300 km3/y – is appropriated for human-dominated as early as 2012 at 100 million barrels of oil per day (Gold and Da- land uses, such as cultivated land, landscaping and to provide forage vis, 2007). The actual peak of production will only be identified in for grazing animals. Overall, that assessment concluded that humans hindsight, and its timing depends on the demand and cost of oil, the already use, in one form or another, more than 50 per cent of all re- economics of technologies for extracting oil, the rate of discovery of newable and ‘accessible’ fresh water flows, including a fairly large frac- new reserves compared to the rate of extraction, the cost of alternative tion of water that is used in-stream for the dilution of human and energy sources and political factors. But a peak in the production and industrial wastes (Postel et al., 1996). It is important to note, however, consumption of non-renewable resources is inevitable. that these uses are of the ‘renewable’ flows of water (described in more detail below). In theory, the use of renewable flows can continue in- Analysis: comparison of peak production in definitely without any effect on future availability. In practice, how- oil and water ever, while many flows of water are renewable, some uses of water will degrade the quality or reduce quantities to a point that constrains the Does production or use of water follow a similar bell-shaped curve? kinds of use possible. In this context, the three concepts of ‘peak wa- In the growing concern about global and local water shortages and ter’ presented here may be especially useful. scarcity, is the concept of ‘peak water’ valid and useful to hydrolo- gists, water planners, managers and users? In the following sections, Peak resource production we consider the differences and similarities between oil and water. The focus is on the characteristics of renewable and non-renewable re- The theory of peak resource production originated in the 1950s with sources, consumptive versus non-consumptive uses, transportability the work of geologist M. King Hubbert and colleagues who suggested and substitutability (Table 2 summarises these characteristics for oil that the rate of oil production would likely be characterised by several and water), and then we define three forms of ‘peak water’. phases that follow a bell-shaped curve (Hubbert, 1956). The first phase is the discovery and rapid increase in growth in the rate of exploitation Key characteristics of renewable and non-renewable of oil as demand rises, production becomes more efficient, and costs resources fall. Second, as stocks of oil are consumed and become increasingly depleted, costs rise and production levels off and ultimately peaks. Fi- There are important differences between renewable and non-renewa- nally, increasing scarcity and costs lead to a decline in the rate of pro- ble resources. As traditionally defined, renewable resources are flow- or rate-limited while non-renewable resources are stock limited (Ehrlich et al., 1977). Stock-limited resources, especially fossil fuels, can be depleted without being replenished on a time-scale of practical interest. Stocks of oil, for example, accumulated over millions of years and are effectively independent of any natural rates of replenishment because such rates are so slow. Conversely, renewable resources, such as solar energy, are virtually inexhaustible over time, because their use does not diminish the production of the next unit. Such resources are instead limited by the flow rate, i.e. the amount available per unit time. Water demonstrates characteristics of both renewable and non-renew- able resources. Renewable water systems experience rapid flows from one stock and form to another, and the human use of water, with a few Figure 1. Total annual U.S. production of crude oil, 1900–2007. US pro- exceptions, has no effect on natural recharge rates. But there are also duction peaked in 1970. Source: USEIA (2008, 2009) stocks of local water resources that are effectively non-renewable cept On the Water Front I Dr. Peter H. Gleick and Ms. Meena Palaniappan 42 Table