WATER FOOTPRINT OF CITIES: A REVIEW AND PROSPECTS FOR FUTURE RESEARCH

ABSTRACT

Cities are hotspots of commodity consumption, with implications for non-local water resources. Water flows virtually into cities through this commodity exchange, meaning that local water issues have a global context. This form of water ‘teleconnection’ is being increasingly recognized as an important aspect of water decision making at the national scale. In cities and urban areas, virtual water flows are rarely acknowledged. The emphasis is on the hydrologic and engineered water balances. Through an extensive literature review of water footprint studies the potential and importance of evaluating water footprints and taking virtual flows into account in cities is evaluated. Specifically, the Water Footprint Assessment, life cycle assessment, and environmentally extended input-output methodologies are examined. As water is just one flux entering and leaving the urban boundary, the urban metabolism framework is investigated as a complimentary framework for analysis. Further, impacts of our trade decisions are discussed in terms of water scarcity and degradation and studies are reviewed for how they evaluate these impacts. Key themes and priorities for future research are also identified and discussed for urban water footprint analysis.

 

 

 

 

 

 

 

 

 

 

 

 

TABLE OF CONTENTS

 

 

List of Figures ………………………………………………………………………………………. v

List of Tables ………………………………………………………………………………………. vi

Acknowledgements ……………………………………………………………………………… vii

Index of Terms …………………………………………………………………………………… viii

 

Chapter 1 Introduction …………………………………………………………………………… 1

Chapter 2 Background: Water footprint studies at different spatial scales …….. 6

2.1 National scale ………………………………………………………………………………. 82.2 Subnational scale ……………………………………………………………………….. 10

2.3 Urban scale………………………………………………………………………………… 14

Chapter 3 Methodologies for water footprint analysis ……………………………… 19

3.1 Method 1: Water Footprint Assessment (WFA) ……………………………… 213.2 Method 2: Environmentally Extended Input-Output (EEIO) ……………. 24

3.3 Method 3: Life Cycle Assessment (LCA) ……………………………………… 26

Chapter 4 Complementary frameworks for water footprint analyses ………….. 30

4.1 Urban Metabolism (UM) …………………………………………………………….. 30

4.2 Water Scarcity ……………………………………………………………………………. 35

Chapter 5 Discussion …………………………………………………………………………… 39

5.1 The need for a water footprint of cities ………………………………………….. 395.2 A general approach for urban water footprint analysis …………………….. 405.3 Spatial scale or boundary for urban water footprint analysis ……………. 42

5.4 Required datasets for urban water footprint analysis ……………………….. 45 5.5 Applications of urban water footprint analysis: addressing sustainability

and resilience ………………………………………………………………………………….. 51Chapter 6 Summary …………………………………………………………………………….. 58

References ………………………………………………………………………………………….. 61

Appendix A Expanded Table 2 ……………………………………………………………… 77

Appendix B Mathematical representation of WF methods………………………… 82

Chapter 1 Introduction

 

 

Cities are the hub of global economic forces (Dobbs et al., 2012; Dobbs et al., 2011; van Vliet, 2002) with their links to distant and proximate locations through extensive exchange networks. Globally, cities are home to more than half of the world’s population and are expected to support nearly two-thirds by 2050 (United Nations, 2014). While this increased urbanization has come to signify greater socioeconomic opportunity and improved social welfare (Léautier, 2006; UNCHS, 2001), it is also creating additional stress on our water resources and the ecosystems they support (Averyt et al., 2013; McDonald et al., 2014; Moore et al., 2013; Savenije et al., 2014). This is further compounded by the environmental degradation that can result from aging and/or inadequate water infrastructure in cities (Charles, 2008; Hijdra et al., 2014; Rahm et al., 2013; Wu and Gao, 2010). Thus, it is becoming increasingly clear that cities hold the key to achieving sustainability targets because of their potential to address, and have an impact on global issues revolving around climate change (Georgescu et al., 2014; Marcotullio et al.,

2013; Zhao et al., 2014), biodiversity loss (Aronson et al., 2014; McKinney, 2006; Seto et al., 2012a), and water resources (Grimm et al., 2008; Groffman et al., 2014; Kennedy et al., 2012; Kennedy and Hoornweg, 2012; Satterthwaite, 2011; Shuster and Garmestani, 2014).

To track the trajectory and assess the resilience of pathways toward urban sustainability goals, there is a growing need to define and quantify flexible indicators and common standards for cities, both to account for their unique conditions and for comparable results (Aronson et al., 2014; Ferrão and Fernández, 2013; Folke et al., 1997; Kennedy and Hoornweg, 2012; Milman and Short, 2008; Satterthwaite, 2011; Tanguay et al., 2010; UN-Habitat, 2009). In addition, the use of city indicators to track and characterize flows and states could contribute to representing the connections (networks) among cities as well as with the global economy (Kennedy and Hoornweg, 2012). A variety of sustainability indicators have been proposed and implemented (see, e.g., (Singh et al., 2012) and references therein). The footprint family is a group of accessible and synthetic indicators that connect our consumer habits and production demands to the Earth’s resources (Ewing et al., 2012; Fang et al., 2014; Galli et al., 2012; Rushforth et al., 2013), and are creating ways to measure these impacts as a means to evaluate beyond normative economic benchmarks such as gross domestic product (GDP) (Ewing et al., 2012; Feng et al., 2011a; Kubiszewski et al., 2013).

The footprint family is comprised of the ecological footprint (EF), carbon footprint (CF), and water footprint (WF), among other environmental footprints. The WF quantifies the total volume of freshwater used to produce the goods and services consumed by an individual, region, or nation (Hoekstra and Chapagain, 2007). It includes both the amount of direct and virtual, also referred to as indirect or embedded (Allan, 1998; Allan, 1993; Hoekstra and Chapagain, 2007; Hoekstra and Hung, 2002), water consumption. The virtual water component quantifies the physical amount of water needed to produce goods in one region that are then exported to the region of consumption. The WF is expressed in volumes of water, typically cubic meters (m³), and is an analogue of the EF (Hoekstra, 2009; Rees, 1992; Wackernagel and Rees, 1998). The EF assesses the amount of biodiverse land needed, in equivalent units of global hectares, to provide for a region’s demands and ability to assimilate waste flows. The CF focuses on evaluating greenhouse gas emissions in terms of carbon equivalents, evaluated in kilograms or tons.  These environmental footprint assessments (EFAs) (Hoekstra and Wiedmann, 2014) have been estimated at the global, national, and regional scales as well as for products and businesses (see e.g. (Fang et al., 2014;

Francke and Castro, 2013; Hoekstra and Mekonnen, 2012; Huijbregts et al.,

2008; Moore et al., 2013; Sovacool and Brown, 2010; Wackernagel et al., 1999; Wiedmann and Minx, 2008)). The EF is unique in that it accounts for the biosphere’s capacity, whereas the WF and CF do not explicitly have capacity measurements built into their methodology (Ewing et al., 2012). However, the concept of the ‘planetary boundary’ can be thought of as the limit of the global footprint of humanity (Gerten et al., 2013; Hoekstra and Wiedmann, 2014; Rockstrom et al., 2009b), thus serving as an estimate of global capacity. For carbon, the reference to global capacity of 350 ppm has gained popularity as a threshold number to be targeted which has already been surpassed (Hansen et al., 2013; Hansen et al., 2008; Rockstrom et al., 2009b). The capacity for the CF has been stated as 18-25 giga-tonnes of carbon equivalency which is in a range that would ensure that the global temperature does not rise more than 2 degrees (Hoekstra and Wiedmann, 2014; Pandey et al., 2011). The global capacity for freshwater use, estimated by Gerten et al. (2013), is 2,800 km³/year. The planetary boundary for freshwater may be useful for informing international policy and governance but water scarcity is a very regional and local issue, due to the spatiotemporal variability of the hydrological drivers and processes (Averyt et al., 2013; Mubako et al., 2013b; Poff et al., 2010; Steffen et al., 2015). Indicators of water sustainability ultimately will need to account for the spatial and temporal dependency of water scarcity as well as cross-scale interactions with the global system.

In urban areas, it is common to identify two different kinds of water balances in terms of direct flows: engineered and hydrologic. Hereafter the term urban area and city are used interchangeably to encompass different urban boundaries and spatial scales such as a central or satellite business district, suburb, or larger metropolitan area. The engineered water balance is controlled by the water demanded by and supplied to the city and the subsequent wastewater that is generated, while the hydrologic balance accounts for all of the natural inflows, outflows, and changes in storage in the urban basin. There are also important interactions between the engineered and hydrologic balances, e.g. combined sewer overflows, leakage from aging infrastructure, among others (Bhaskar and Welty, 2012; Padowski and Jawitz, 2012). Analogously, in the context of the WF of an urban area, a third water balance can be identified in relation to the virtual water flows entering and leaving the urban area through the products consumed and produced within.

The concentration of people and economic activity in urban areas leads to an imbalance between the virtual water inflows and outflows, with a bias towards certain commodities and economic sectors as compared with the average patterns of the complete national economy. For example, there is a concentrated demand for food and agricultural products in urban areas while limited land space is available for food production. Further, when evaluating the urban WF, the focus can be on the flows and networked exchanges coming across the boundary or it can be on the interflows within the urban boundary. In either case, the characteristics of these flows, including flows associated with energy use (Kenway et al., 2011) and food consumption, and their implications for urban water supply decision making are poorly known.

The goals of this review are to identify and clarify the need for WF analysis at the urban scale, assess the strengths and drawbacks of current WF methodologies in the context of urban WF analysis, and identify and suggest areas for future research in urban WF analysis. It is recognized that the original intent of the WF methodology was to examine global trends of consumption and water use (Hoekstra, 2009). The urban WF seeks to incorporate this global dimension while recognizing that, in cities, water impacts and decision making are highly localized. The paper is structured as follows. Chapter 2 provides background information from previous WF studies done at different spatial scales. National scale studies are included in this chapter because these have been thus far the primary focus of WF analysis and they serve to highlight the need for studies at subnational and urban scales. Chapter 3 reviews the main methodologies used for WF analysis. Chapter 4 discusses urban metabolism and water scarcity in the context of WF analysis. Chapter 5 is a discussion of key themes for the WF and recommendations are made for future research in urban WF analysis. The need for a water footprint is addressed, and discussion of setting appropriate boundaries with constrained data is examined for this analysis. Urban sustainability and resilience are also discussed in terms of WF analysis.

Finally, in chapter 6, main findings are summarized.

WATER FOOTPRINT OF CITIES: A REVIEW AND PROSPECTS FOR FUTURE RESEARCH

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