A global perspective on changing sustainable urban water supplies
Introduction
Of the Earth's 1386 million cubic kilometers of water, only 2.5% of that quantity is freshwater and nearly one-third of this smaller amount is available for human use (Postel et al., 1996). Against this backdrop of a finite water supply, total water withdrawn for human uses has almost tripled in the last 50 yr from 1382 km3 yr− 1 in 1950 to 3973 km3 yr− 1; projections anticipate worldwide human water consumption will further increase to 5235 km3 yr− 1 by 2025 (Clarke and King, 2004): an amount greater than nine times the annual flow of the Mississippi River (Goudie, 2000). Over half the available freshwater supplies are already used for human activities and this proportion is increasing in response to growing agricultural, industrial, and residential demands (Postel et al., 1996, Vorosmarty and Sahagian, 2000). By 2025 an estimate 5 out of 8 people will be living in conditions of water stress or scarcity (Arnell, 1999). The consequences of insufficient water availability can result in degradation to human health, ecosystems, agricultural, and industrial output, while increasing the potential for conflict (Postel et al., 1996).
In conjunction with increasing demands on water supplies, has been an increase in the concentration of human habitation. Currently, more than 50% of humans currently live in cities (United Nations Centre for Human Settlements, 2001) and associated with the rapid increases in urbanization is the growth of urban water uses, combined industrial and domestic sectors (Falkenmark, 1998, Gleick, 2003, Jury and Vaux, 2005). In 2003, 74% of the population of developed countries resided in urban areas and this percentage is expected to increase to 82% by 2030; in developing countries 42% of the population lived in urban areas with an expected increase to 57% by 2030. The number of megacities, those cities with more than 5 million residents, is expected to increase globally from 46 to 61 between 2015 and 2030 with disproportionate increases in Asia and Africa (United Nations, 2004). Ensuring the sustainable supply of water for cities is of increasing concern (Lundqvist et al., 2005, United Nations Centre for Human Settlements, 2001).
Much of the recent research projecting future water availability has focused on the country as the unit of analysis; these analyses suggest that by 2050, nearly half the projected world's population will live in water scarce countries (Clarke and King, 2004). However, within countries expected to not experience water scarcity or stress in the next decade many include some densely habited regions that may have difficulty in providing sufficient water. For example, while Australia and the United States are not expected to experience water scarcity by 2015, cities in these countries such as Melbourne and Los Angeles may find it difficult to ensure the sustainable supply of water to meet the growing urban demand (Bjorklund and Kuylenstierna, 1998). While ensuring the sustainable supply of water for individual cities has been and continues to be a concern for many cities, (Niemczynowicz, 1996, Kupel, 2003, Fitzhugh and Richter, 2004), a global analysis is still needed to provide a context for the regionally specific problems.
The scarcity and sustainability of renewable water resources is affected by four comprehensive factors: (1) population, (2) per-capita water use, (3) climatic change, and (4) allocations for water conservation. These factors affect urban water use both individually and interactively. With increasing population, cities are increasing the water requirements into concentrated areas. With projected increases in urbanization, there will likely be similar increases in per-capita water consumption for some regions of the world, although the locations of these increases are sensitive to uncertainties in social and physical dynamics (Alcamo et al., 2003). Changes to global and regional climates are projected to influence both the total amount and variation of water throughout the globe (Easterling et al., 2000, Tudhope et al., 2001). Future climate scenarios suggest spatial variation in precipitation responses (Giorgi and Bi, 2005). Many regions are expected to have substantially altered water availability and because of increased variability in climatic conditions reduced water availability associated with short-term draughts will be more likely in all regions (Douville et al., 2002, Arnell, 2004, Thomson et al., 2005). Thus, while climate change will have a variable impact on regional water supplies, the potential for reduced water availability is a concern in most regions. Another anticipated factor that will reduce the amount of water available for withdrawal is the increasing awareness that a significant percentage of available water must be conserved for aquatic systems to maintain the production of instream ecosystem services (Richter et al., 2003, Smakhtinm et al., 2004). The amount of water necessary to maintain a given ecosystem function depends upon a variety of factors and will be specific to the ecosystem and process under consideration (Naiman and Turner, 2000, Baron et al., 2002). Our increasing knowledge about the importance of maintaining these ecological systems for their multitude of services suggest that future sustainable water management will require the dedication of significant quantities of freshwater for instream uses.
To identify the relative importance of these different factors affecting urban consumption and regional availability of water we used a modified ecological footprint (EF) approach. EFs combine the variation in consumption and production patterns of renewable resources to assess the sustainability of a societal practice (Wackernagel and Rees, 1996, Folke et al., 1997). EFs quantify human–ecosystem relationships by estimating the area required to sustainably provision ecosystem services, including food and raw material production, atmospheric gas regulation, and water purification (Wackernagel and Rees, 1996, Folke et al., 1997, Luck et al., 2001, Wackernagel et al., 2002, York et al., 2003). Part of the usefulness of the footprint resides in the potential for comparison amongst ecosystem services using a consistent metric, area. EFs sum the amount of six land-use classes required to satisfy a particular human lifestyle: cropland, grazing land, forest, fishing ground, built-up land, and land required to absorb emitted CO2. The EF calculation applies a ‘world average productivity’ for each of the six productive areas and the final per capita EF value can be aggregated to different population categories such as city, region, or nation. The available per capita worldwide area for the six required areas is 2.1 ha, while the current average per-capita demand is 2.8 ha (York et al., 2003). Thus, sustainably supplying the global demand for ecosystem services requires 13 billion hectares, approximately 21% more area than available (Loh and Wackernagel, 2004).
Criticisms have been raised surrounding the implementation of ecological footprints. These criticisms include: (1) accounting for multiple services produced by the same ecosystem, (2) trade of services between regions, and (3) spatial heterogeneity in both the consumption and supply of services (van den Bergh and Verbruggen, 1999, Opschoor, 2000, Templet, 2000, Andersson and Lindroth, 2001). Solving these problems is an area of active research (Wackernagel et al., 2004). Several alternative methods for calculating footprints have recently been proposed to address these problems (Sanderson et al., 2002, Nijkamp et al., 2004, Zhao et al., 2005). One response to these problems has been the development of the spatially heterogeneous footprint (H-EF) (Luck et al., 2001), which computes footprints for individual ecosystem services accounting for both spatial heterogeneity in the demand and supply of a single ecosystem service. As such it addresses the important challenge of combing maps of ecosystem service production and consumption (Sanderson et al., 2002). The H-EF approach is especially useful for ecosystem services that are not generally traded between regions; the sustainable supply of water in particular seems to be amenable to this approach (Postel et al., 1996, Luck et al., 2001). While large financial investments allow for public works that transport water great distances, this is an expected outcome of disproportionately large water footprints. Thus, caution should still be exercised in the interpretation of H-EFs. However, this method has provided information to describe urban water consumption differences between China and the United States (Jenerette et al., in press-b) and compare urban water pricing, demand, and regional availability within the United States (Jenerette et al., in press-a).
The objective of our research was to evaluate the role of hypothesized constraints to urban water withdrawal and project alternative futures in urban water demand. We investigated the pattern of water availability, urban water demand, and sensitivity of these patterns to different scenarios for the 524 cities with populations larger than 750,000 residents in 2000. Combining geospatial patterns of water demand and availability are becoming useful approaches for describing spatial variation in water scarcity (Vorosmarty et al., 2000, Vorosmarty et al., 2005). Scenario testing was used to ask: how do population increases, changes in per-capita demand, climatic reductions in water availability due associated with potential climatic change, or maintaining water for instream uses, influence urban water consumption patterns? We also asked, which cities' water consumption patterns are the most sensitive to changes in these factors? Comparisons of alternate scenarios of future water demand and availability can provide information on likely future trajectories (Arnell, 1999). By answering our research questions we generated a spatially explicit and global analysis of the interaction between renewable water availability and consumption patterns for the 524 largest cities and their response to four general factors that modify the extent of urban water demand.
Section snippets
Methods
We overlaid two sources of global data, the location and population for all larger than 750,000 residents and the spatial distribution of renewable annual water run-off. We linked these data by computing the footprint area required to supply the water demand for each city for 1950, 2000, and 2015 populations. We also computed footprints for each city using different scenarios and evaluated the potential response of water footprint area to the four general factors for each city and overall.
Data
Results
The distribution of cities with more than 750,000 residents and the corresponding distribution of urban water footprints showed large variation throughout the Earth (Fig. 1). Some of the largest concentrations of large cities and cities with large footprints are within Southeast Asia. The Middle East region also has several cities with large water footprints. Within both North and South America, there were comparatively fewer cities with large footprints. The estimates of total urban water
Discussion
Our study investigated urban water withdrawal and identified the locations of likely urban water scarcity and stress in the next 15 yr throughout the globe. In the analysis of all cities with more than 750,000 inhabitants, there were an estimated 1.2 billion residents in 2000 and projected 1.5 billion residents in 2015. Assuming urban density varies between a 2000 and 40,000 residents km− 2 we estimate the surface area coverage of these cities ranges between 30,000 and 598,000 km2 for 2000 and
Acknowledgments
This research was supported by the National Science Foundation via the Urban Ecology IGERT (DGE-9987612).
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