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Accounting for a scarce resource: virtual water and water footprint in the global water system
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ScienceDirect Accounting for a scarce resource: virtual water and water footprint in the global water system Hong Yang1,2, Stephan Pfister3 and Anik Bhaduri4 Effective water management and governance at all geographical levels can only be designed based on good quality information and thorough understanding of it. The accounting for water resources from virtual water (VW) and water footprint (WF) perspectives can generate information about water uses in production processes and flows of VW associated with the trade of commodities. There have been a large number of studies on VW and WF since the advent of the two concepts. The recent literature has seen an increase in explicit elaboration of local, regional and national water uses in the context of global economic and water systems. More sophisticated and systematic approaches, such as input– output (IO) models and Life Cycle Assessment (LCA) tools, have been employed to facilitate the analysis of complex interconnections of water uses across system boundaries and environmental impacts incurred. However, limitations and shortcomings remain in the current VW and WF studies with regard to policy relevance, data accuracy, methodological approaches and conceptual consistency. Further efforts are required from the scientific community to tackle these problems in order to enhance the usefulness of the concepts and the data generated for water resources management and governance at all geographical levels which are intrinsically connected through VW trade. Addresses 1 Swiss Federal Institute of Aquatic Science and Technology, Ueberlandstrasse 133, 8600 Duebendorf, Switzerland 2 Department of Environmental Science, University of Basel, Petersplatz 1, 4003 Basel, Switzerland 3 Ecological Systems Design, Institute of Environmental Engineering (IfU), ETH Zurich, 8093 Zu¨rich, Switzerland 4 Global Water System Project (GWSP), International Project Office, Walter-Flex-Strasse 3, 53113 Bonn, Germany Corresponding author: Yang, Hong ([email protected])
Current Opinion in Environmental Sustainability 2013, 5:599–606 This review comes from a themed issue on Aquatic and marine systems Edited by Charles J Vo¨ro¨smarty, Claudia Pahl-Wostl and Anik Bhaduri For a complete overview see the Issue and the Editorial Available online 26th October 2013 1877-3435/$ – see front matter, # 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cosust.2013.10.003
Introduction As the world’s available freshwater resources are limited and unevenly distributed, it is important to quantify how www.sciencedirect.com
and where available water volumes are appropriated: for producing certain commodities, for certain people [1]. With commodities being traded across economic and hydrological system boundaries, the use and consumption of water resources in one location can exert impacts on freshwater resources in other locations where trade occurs. This interconnection renders the local water resources and their management with a global dimension [2]. The concepts of virtual water (VW) and water footprint (WF) emerged in the early 1990s and early 2000s, respectively, amid the increasing water scarcity and interconnections of water uses worldwide. In a nut shell, VW is the water used for the production of commodities [3,4]; while WF is the volume of freshwater used during the production process, measured over the whole supply chain [5]. Given the uneven spatial distribution of global water resources and different environmental impacts of water use across geographical locations, some researchers have quantified WF weighted with water scarcity and/or pollution indicators to account for the environmental relevance of the water appropriation [6,7]. For convenience, we name the WF without weight as volumetric WF and with weight as weighted WF. When not specified, WF generally refers to either volumetric WF or weighted WF. There are many overlaps in the VW and WF concepts. Volumetric WF of a product is numerically equal to ‘VW content’. VW and WF can be categorized into green, blue and grey. ‘Green’ refers to the use of soil moisture, ‘blue’ refers to the use of withdrawn freshwater [8], and ‘grey’ concerns polluted water [5]. The term WF is usually used in the context where consumers or producers of products are concerned, whereas the term VW is mostly used in the context of international or interregional trade. There have been a large and still increasing number of studies on VW and WF issues since the advent of the two concepts. The technical approaches and the understanding of the relevant issues have evolved over time. This paper provides a critical review of the current knowledge and methodological development of the accounting for VW and WF. It highlights the up-to-date status of the studies with a focus on those of significance to the global water system. An outlook at the end provides prospects of the future VW and WF research in the context of increasing globalization of the world economy and consequently closer interconnections of water resources management across geographical boundaries. Current Opinion in Environmental Sustainability 2013, 5:599–606
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Figure 1
Methods
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VW and WF accounting methods and approaches.
Methodological development in VW and WF accounting Many methods have been developed for VW and WF accounting. Figure 1 provides a summary of the commonly used methods. In general, the approaches of the accounting can be categorized into two groups: bottom-up and top-down [9]. The complexity of the water system, the scope of the accounting, the nature of the issues tackled, and the data availability determine the selection of the approaches. Bottom-up approach
The bottom-up approach departs from the smallest unit feasible in assessing VW and WF and then aggregates each unit to desired scale and period. The amount of water required for the production of a unit of product, measured in physical or monetary term, is called ‘VW content’, expressed in m3/kg or m3/$. It can be viewed as the inversion of water productivity. VW content of a product is the ‘building block’ in accounting for VW flows and WF. The agricultural sector is the largest water user in the world and has a substantial impact on the global water system. A large number of VW and WF studies have focused on agricultural products, especially staple food crops due to their high water intensity in production and the importance for food security. Process-based crop growth models supported by GIS techniques have been commonly applied to estimate crop water consumptive use and VW content [10,11]. The crop modeling approach has provided a systematic tool to account for green and blue water consumptive uses in crop production and spatial variations in VW content, and enabled analyses of impacts of changes in various input factors, such as water availability, fertilizer application, as well as climate change, on crop yield and VW content. Current Opinion in Environmental Sustainability 2013, 5:599–606
While VW focuses primarily on water quantity, WF also emphasizes the environmental impact of water use [7,12]. One approach is the accounting for the grey WF [5], which refers to the volume of freshwater required to assimilate the load of pollutants based on natural background concentrations and existing ambient water quality standards. Another approach is to incorporate environmental impact assessment into the WF accounting, which typically weights the water uses with water scarcity/stress indicators [13,14]. Life Cycle Assessment (LCA) is an ISO standardized procedure to assess environmental impacts of a product or service over its whole life cycle. It is mainly a bottomup approach, although it may partially utilize input– output (IO)-derived values in the case of hybrid LCA. The LCA approach has been increasingly applied in the studies of VW and WF. In the LCA approach, the system boundary is defined in the first step (goal and scope definition) followed by data collection covering the information of processes and trade as well as specific emissions/resource uses of each process involved (life cycle inventory). The third step consists of assessing the overall resource consumption in respect to environmental damage (e.g. water appropriation). This step allows for comparing products from different regions which have resource uses under different environmental conditions (e.g. water scarcity). Top-down approach
The top-down approach departs from the highest level defined by the system boundary. The analysis then breaks down to lower levels according to the subsystem boundaries, for example, economic sectors, river basins, and countries. IO and multi-region IO (MRIO) models have been widely used in the current literature to account for WF and VW [15]. www.sciencedirect.com
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An IO table/model represents the monetary transactions of goods and services among different sectors of economic system. It provides a technique to specify how the substances flow among sectors through supplying inputs (including water) for the outputs (where VW is embedded) in the economic system. The IO approach accounts for VW and WF in economic sectors by distinguishing the direct water use and indirect water use. The direct water use coefficient (DWUC) refers to the amount of direct water intake to produce one monetary unit of production. DWUC is the conventional measure for the sectoral water use intensity. The total water use coefficient (TWUC) for one monetary unit of production reflects the water use throughout the whole supply chain, for example, from agriculture to textile to clothing. It provides a more complete picture of water use intensity. The MRIO analysis is a variant of IO analysis, operating on large databases combining the IO tables of many regions. While IO assumes that imported goods and services are being produced with the same technology as the regional/domestic technology in the same sector, MRIO endogenously combines regional/domestic technical coefficient matrices with import matrices from multiple regions or countries into one large coefficient matrix. Thus, MRIO captures trade supply chains between all trading partners as well as feedback effects [16].
Accounting for VW flows and WF in economic sectors and across regions (basins) Figure 2 depicts the spectrum of VW and WF accounting with respect to the complexity of the concerned systems, relevance to consumers, producers and policy makers, as well as significance in the global water system. From left to right, the complexity, policy relevance and significance in global water system tend to increase. From right to left,
the accuracy of data, consumer interests and producer relevance of the accounting tend to increase. Early studies of VW and WF were mostly concentrated on the left side of the spectrum, quantifying water uses of per unit of product and over production chain, such as agricultural commodities, a cup of coffee, a pair of shoes, a cotton shirt, and so on. These items are often directly related to daily consumptions of individual people and hence have high relevance to raising public awareness of water consumption. The focus of the recent studies has mostly on the right side of the spectrum, with attempts to increase policy relevance. Many studies have quantified VW and WF for economic sectors, catchments/basins, regions and nations and the globe. With the distinction of direct and indirect water uses, the IO and MRIO analysis enables quantification of water use intensities in individual economic sectors over their whole supply chains. Many studies have investigated sectoral water uses in the national and regional economic systems with the IO tables at country and regional levels [17,18–20]. Some have conducted key sector analysis considering both economic growth potential and water use intensities to determine the direction of sectoral transformations for sustainable growth in a country or region. Studies on China using the IO models on the relevant issues have been notably many in number. One focal region is the North China Plain where water scarcity is severe [2,21,22,23,24–26]. A common conclusion is that North China in spite of its poor water endowments virtually exports water to the other regions in the form of mainly agricultural products (as the region is the major wheat and corn producing area in China), while South China with abundant water resources imports VW from the rest of the regions. Given the computational and labor intensity of the MRIO approach, only few globally comprehensive and sectorally detailed models have been
Figure 2
Increase
Primary items
Production chain
Accuracy of values (data) Consumer interest Producer relevance Economic/ industrial sector
Catchment & river basin
Complexity of the accounting system Policy relevance Significance in global water system
Administrative region(s)
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Increase Current Opinion in Environmental Sustainability
Spectrum of VW and WF accounting.
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developed so far. Two most recently developed global MRIOs are the World Input–Output Database (WIOD) [27] and the Eora MRIO tables [28], both with the coverage for the period 1990s and 2000s. Water accounting using global MRIO tables has showed substantial water savings globally through VW trade [29].
Incorporating environmental impact in VW and WF accounting It has been widely recognized that water uses can have different impacts on the environment, depending on where water is used, what type of water is used, and how much water is polluted. There has been an increasing effort to incorporate environmental impacts, primarily water pollution and water scarcity, in VW and WF assessment. Water scarcity index has been used as a weight for converting total water use into scarce water use [14]. It offers a way for incorporating water scarcity into MRIO analyses of global water uses, and characterizing national WFs and trade balances in terms of scarce water. Together with water exploitation index (WEI) established based on national statistics, the water stress metric can be built in the MRIO framework to identify countries with relatively high abstraction in relation to water availability. More generic modeling of water uses and related environmental impacts per country has been conducted, including uncertainty assessment with continental and global coverage for crops and power production [11,29,30]. Combined with MRIO data, the global average values with uncertainty induced by underlying variability can be used for crops with unknown origin or that are traded on stock markets, such as cotton or coffee [11]. Given the nature of the LCA analysis, the method has often been used in assessing environmental impact of water uses. A number of studies which combined trade and water stress assessment for specific products and services have been published recently [6,29,31,32]. They generally quantified WF with weights representing degrees of environmental impact. For example, studies on meat production indicated the importance of looking at water sources and related impacts when analyzing meat consumption, since pasture and grain fed production systems differ widely [12], which is also true for milk production [33]. A case study on large water transfers from Southern to Northern China has indicated positive net effects from a blue water scarcity perspective [26]. The result is in contrast with the common perception of the irrationality of the water transfer project viewed from the volumetric WF perspective as mentioned earlier. So far, studies which specifically quantify grey WF have mostly focused on nitrogen and phosphorus input from agriculture into rivers and other water bodies [34]. Accounting for grey WF from industries is generally absent due to the lack of data on pollution loads in individual sectors. Current Opinion in Environmental Sustainability 2013, 5:599–606
Usefulness, limitations and gaps of VW and WF accounting What has been learnt from the current studies?
The application of more systematic and sophisticated models in recent years has enabled comprehensive analysis of interconnections of water uses across economic sectors, administrative regions and hydrological systems (catchment and river basin) through the VW trade. Such accounting enhances the relevance of the information to water resources management which is primarily conducted in the context of economic sectors and regions, as well as river basins. For example, the study by Zhang et al. [23] explicitly quantified the changes in internal and external WF in Beijing between 2002 and 2007 with specification of economic sectors and factors contributing to the changes and identified the sources where the external WF originates. The results provide basis for the assessment of the effects of economic development and water policies in Beijing over the period studied. It also helps to formulate strategies for the future water resources management of the city. The effort to incorporating environmental impact into VW and WF accounting facilitated the assessment of damages or benefits (in much lesser circumstances) of human appropriation of water resources in specific locations and their repercussions to other locations. The LCA approach has demonstrated particular advantage by investigating the life cycle of water and pollutants in the systems concerned. The IO based approaches help identify high water intensity sectors by looking at not only the direct water use and wastewater discharge at the final stage of the process, but also the total water use and the environmental damage incurred in the entire supply chain of the concerned sector — with each stage of the production processes often located in different regions, basins and countries. Information/data gaps
Concerning data on VW and WF, major problems remain in quality and accuracy of the source data and modeled water uses in the production processes. While many agricultural processes are covered [11,35] and hydropower has global estimates too [29,36], there is a large lack in knowledge of water consumption and use in the industrial sectors. For the trade data, the aggregation of products into sectors often mixes products with very different water productivities. Also problematic is the inconsistency between sectors reported by different countries. Lacking up-to-date high quality data has often impeded the studies to provide timely information on VW and WF status. IO tables typically have a long time-lag due to heavy task in data collection and processing. For example, the complete set of the provincial level IO tables in China is only available up to 2007. Given the rapid economic development of the country, the usefulness of the results www.sciencedirect.com
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using the 2007 data is largely discounted. Furthermore, its processing step is more a ‘black-box’, not allowing for transparent reporting. The problem of lacking detailed data is especially true for global IO tables. For example, the existing MRIO tables based on GTAP distinguish 129 regions, each breaks down into typically 57 sectors [11]. Some of the regions in these tables are single countries; some are groups of countries. Many areas of critical water problems in developing countries are not distinguished. In addition, the global MRIO databases do not distinguish differences within large countries such as China or the USA.
starts. The reason is that it is not possible and feasible to put labels of WF concerning the green, blue, grey components and spatial and temporal dimensions of the water uses in a product. Even if this were done, it may do little good to the environment and ecosystem in the water scarce regions before we are sure that the alternative water uses in these regions are less harmful. Gaining complete information on the complicated interconnections of water uses in different locations remains a great challenge to the scientific community; even greater is to convey the information to the policy makers and general public for appropriate actions.
Mainly owing to the data constraint, most IO based studies have conducted static analysis which only provides ‘snapshot information’ using the data of a single year or an average over a period of time. Dynamic studies investigating changes in WF at different times are rare. Also, the existing studies have mostly only provided descriptions on ‘what the situation is’, but no explanation on ‘how the situation is shaped’. Studies investigating the driving forces of the changes remain very limited, with a notable exception of the study by Zhang et al. [23].
Another pitfall is related to the impact of the shift of a region’s WF to external sources on the destination regions, countries and river basins. As shown in the study by Zhang et al. [23], Beijing has shifted its WF to other regions, including water scarce regions, by increasing external WF. An intuitive conclusion would be that the shift would increase the water pressure on the destination regions (e.g. the two peer reviewers of Zhang’s paper both raised this concern). However, a further scrutiny would suggest that the actual effects of the shift depend on whether it leads to improvement of water use efficiency in the destination regions and whether the total water use will increase or decrease. Much effort is needed to investigate these issues on individual case basis. A general conclusion cannot be established.
An aspect that has been mostly neglected in VW and WF accounting is about climate change and uncertainty consideration of all involved steps. Uncertainties can stem from the input data which are often very coarse and subject to high level of aggregation, the analysis procedure which often involves many assumptions, and the rapid change in the system status and the boundary conditions. Climate change is expected to have significant impact on hydrological regimes on both temporal and spatial dimensions. This will impose impacts on economic activities, particularly agriculture, and consequently VW and WF associated. Interpretation pitfalls
The interpretation of the generated data on VW and WF is often contentious and subject to acute debate. Many studies have found that regional and international trade patterns of VW can be little explained by water scarcity patterns. By now, a consensus has been more or less established in the VW and WF community that the concepts and the relevant data alone are not sufficient to support the decision making on optimal water uses and VW trade [1,37]. Yet, many still consider the mismatch of water endowments and VW trade as inefficient use of water resources without any further analysis of other involved factors influencing water use decisions (economic, comparative advantages, political and cultural reasons). One consequence of this water centric conclusion is the suggestion to label WF in products to draw consumers’ consciousness of their direct and indirect water consumption. However, this attempt is doomed to fail before it www.sciencedirect.com
Incorporating green water and water pollution in VW and WF accounting
In most VW and WF accounting for entire economic systems, water uses concern only blue water. Green water or soil moisture is not considered. This is because except for the agricultural sector and the sectors to which agriculture, livestock or forestry provides raw materials, all other sectors exclusively use blue water. Including green water would greatly increase the share of agricultural water use and thus derive biased conclusions in assessing the value of water use across different sectors. On the other hand, green water is an extremely valuable resource with large potential in biomass production and should be included in the analysis. One option is to consider green WF with distinction from blue water uses. This way of accounting helps gain a complete picture of water appropriations in the economic systems. The information is important for water planning and management. Green water is the main water source of agricultural production, while irrigation is the single largest withdrawn water user in most of the countries. Enhancing the green water use efficiency is closely associated with improving land management. For a given region and country, this could reduce the demand for blue water for irrigation. However, it should to be noted that there is no clear boundary between green and blue water in the hydrological system, since blue water can become Current Opinion in Environmental Sustainability 2013, 5:599–606
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green water in case of river floods or capillary rise from groundwater. A complete separation of green and blue WF may be inappropriate. Finding an appropriate way to harmonize the blue and green WF accounting is both scientifically and practically important. The lack of information has generally impeded the inclusion of grey water in WF assessment involving multiple industrial sectors. The grey WF consists of many different pollutants that have adverse effects on the environment. This is the issue where LCA can make a greater contribution. The development of MRIO data including a more complete set of resource use and effluence for each sector and region is necessary. For impact assessment, more sophisticated approaches based on risk assessment may enhance the credibility of grey WF accounting compared to the currently applied critical load approach [12]. One more critical problem which has not drawn adequate attention is the conceptual basis of grey WF. In reality, water required for diluting polluted water to meet the certain standards is not available in many regions. In this case, the grey WF accounted does not exist. In the global context, there is a possibility that the world’s currently accessible water would not be enough to dilute the polluted water to meet the prevailing standards, leaving the global assessment of grey WF little meaningful. This situation calls for the development of a more pertinent indicator that can better reflect the environmental impacts of water uses of human society.
Outlook to future water accounting in the context of global water system Improve data quality and accessibility at all levels
Given the data problems specified earlier, there is a great need for improving the completeness and reliability of data for water availability, water uses, water pollution and trade of commodities on all geographical scales. New developments should better utilize the hydrological, agronomic, economic and trade models to improved data quality with higher spatial and temporal resolutions and refine analysis on the relevant dimensions. In addition, models in environmental sciences and technologies should also be utilized to help identification and monitoring of water pollution and quantification of environmental impacts. Given the anticipated impact of climate change on water availability, water uses and economic development, incorporating climate change projections in VW and WF studies is important to facilitate the development of future water policies to support sustainable economic development and improvement of human and nature’s well-beings. Current Opinion in Environmental Sustainability 2013, 5:599–606
Integrated assessment to enhance policy relevance for national and global water governance
Although the VW and WF data alone are not sufficient to support the decision making, it is important to incorporate them in the integrated analysis to tackle complex problems relating to water resources management today and in the future. Several aspects can be highlighted in this regard. Inter-regional VW trade in dealing with regional water scarcity. Many countries of water scarcity have significant regional variations in natural (including water endowments) and socio-economic conditions. VW trade provides one option to alleviate regional water scarcity [38]. Studies of different scenarios concerning economic structural (particularly crop patterns) adjustments and land uses may be conducted to assess their impacts on regional economy and water uses. The results allow policy makers to foresee the outcomes of each of the options concerning the adjustment of economic structures and promoting VW trade. However, the feasibility of each of the structure adjustment options must be further assessed by considering the socio-economic conditions, opportunity costs, environmental impacts and other trade-offs. Inter-basin water transfer versus VW transfer. Inter-basin water transfers have been conducted in many areas in the world, especially in high population density areas. The trend is expected to continue with the economic development and population growth. The rationale of VW and real water transfers is an issue of debate in the political arena and the scientific community. A comprehensive assessment of trade-offs taking into consideration the natural and socio-economic conditions is necessary to reach a conclusion. The information on VW embodied in the trade is needed for such an assessment. Implications of the local/regional water policies for the trade partners. Given the interconnections of local water uses with other regions in the global water system, VW and WF assessments need to pay attention to the repercussions of the local/regional water policies to other regions/ countries and river basins. For example, the EU Water Framework Directive (WFD) of 2000 was a response to the growing pressure on European water resources by pollution, overexploitation and endangered wetlands [39]. The implementation of WFD is conducive to the European waters and related ecosystems. However, the impact may not always be positive when viewed in a broader geographical context. The implication for the trade between the EU and the Middle Eastern and North Africa (MENA) countries is a case in point. The problems associated with agricultural water use in the EU could be exported, at least partially, to the water scarce MENA countries due to the possible increase in import of the EU countries from this region. www.sciencedirect.com
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Overall, with increasing domestic demand for food and water, as well as tightening environmental and resource protection policies, water-rich food export countries (mostly developed countries) are likely to reduce the amount of VW export [40]. This could leave water poor import-dependent countries without enough water to sustain their populations. For these countries, improving water use efficiency through enhancing productivities should be taken as a long-term strategy to reduce the vulnerability to water scarcity.
Conclusion (1) The current studies of VW and WF have made substantial progress in the elaboration of the interconnections of water uses in production and consumption of final products in the local, regional, national and international economic systems. (2) The methodological progresses have allowed more sophisticated but also complex quantification of VW and WF and assessment of their environmental impacts. (3) There remains a lack of policy relevance of VW and WF accounting to support water resources management and governance at all geographical levels. (4) Issues on climate change impact and uncertainties have generally been absent in VW and WF accounting and should be incorporated in the future. (5) Conceptual frameworks for incorporating green and grey WFs into VW and WF accounting are weak. There is a big challenge to harmonize the conceptual bases of blue, green and grey WFs and environmental impacts in water accounting. (6) The repercussions of local water management to the global water and economic systems through trade of goods and services require broader as well as in depth studies with joint efforts from different disciplines.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1. Hoekstra AY, Mekonnen MM: From water footprint assessment to policy. Proc Natl Acad Sci U S A 2012, 109:E1425. One of the major papers addressing the relevance of WF to water policies. 2. 3.
4.
Hoff H: Global water resources and their management. Curr Opin Environ Sustain 2009, 1:141-147. Allan JA: Policy responses to the closure of water resources: regional and global issue. In Water Policy: Allocation and Management in Practice. Edited by Howsam P, Carter RC. London, UK: Chapman and Hall; 1996:3-12. Yang H, Zehnder AJB: Virtual water—an unfolding concept in integrated water resources management. Water Resour Res 2007, 43 http://dx.doi.org/10.1029/2007WR006048.
5.
Hoekstra AY, Chapagain AK, Aldaya MM, Mekonnen MM: The WF Assessment Manual: Setting the Global Standard. London, UK: Earthscan; 2011, . This manual sets the benchmark for the assessment of WF and is widely followed in the WF studies.
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Ridoutt BG, Pfister S: A revised approach to water footprinting to make transparent the impacts of consumption and production on global freshwater scarcity. Global Environ Change 2010, 20:113-120.
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23. Zhang ZY, Shi MJ, Yang H: Understanding Beijing’s water challenge: a decomposition analysis of changes in water footprint between 1997 and 2007. Environ Sci Technol 2012, 46:12373-12380. This paper is the first attempt for a decomposition analysis of the factors contributing to the changes in Beijing’s water footprint between 1997 and 2007. It demonstrated empirically the role of VW strategies in Beijing’s water resources management and economic development. 24. Zhang ZY, Shi MJ, Yang H, Chapagain A: An input–output analysis of trends in VW trade and the impact on water resource and uses in China. Econ Syst Res 2011, 23:431-446. 25. Feng KS, Siu YL, Guan DB, Hubacek K: Assessing regional virtual water flows and water footprints in the Yellow River Basin, China: a consumption based approach. Appl Geogr 2012, 32:691-701. 26. Lin C, Suh S, Pfister S: Does south-to-north water transfer reduce the environmental impact of water consumption in china? J Indust Ecol 2012, 16:647-654. 27. Timmer M (Ed): The World Input–Output Database (WIOD): Contents, Sources and Methods. 2012. www.wiod.org (Downloaded on 30 August 2013). 28. Lenzen M, Kanemoto K, Moran D, Geschke A: Mapping the structure of the world economy. Environ Sci Technol 2012, 46:8374-8381. 29. Pfister S, Saner D, Koehler AA: The environmental relevance of freshwater consumption in global power production. Int J Life Cycle Assess 2011, 16:580-591. 30. Steen-Olsen K, Weinzettel J, Cranston G, Ercin AE, Hertwich EG: Carbon, land, and water footprint accounts for the European Union: consumption, production, and displacements through international trade. Environ Sci Technol 2012, 46:10883-10891. 31. Berger M, Warsen J, Krinke S, Bach V, Finkbeiner M: Water footprint of European cars: potential impacts of water consumption along automobile life cycles. Environ Sci Technol 2012, 46:4091-4099. 32. De Boer IJM, Hoving IE, Vellinga TV, Van de Ven GWJ, Leffelaar PA, Gerber PJ: Assessing environmental impacts associated with freshwater consumption along the life cycle
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of animal products: the case of Dutch milk production in Noord-Brabant. Int J Life Cycle Assess 2013, 18:193-203. 33. Zonderland-Thomassen MA, Ledgard SF: Water footprinting – a comparison of methods using New Zealand dairy farming as a case study. Agric Syst 2012, 110:30-40. 34. Liu C, Kroeze C, Hoekstra AY, Gerbens-Leenes W: Past and future trends in grey water footprints of anthropogenic nitrogen and phosphorus inputs to major world rivers. Ecol Indicat 2012, 18:42-49. This paper is one of the few attempts in the hitherto literature that conducted a large scale assessment on nitrogen and phosphorus input to major rivers in the world. 35. Mekonnen MM, Hoekstra AY: The green, blue and grey WF of crops and derived crop products. Hydrol Earth Syst Sci 2011, 15:1577-1600. 36. Hoekstra AY, Mekonnen MM: Global Water Scarcity: Monthly Blue Water Footprint Compared to Blue Water Availability for the World’s Major River Basins. UNESCO-IHE (Value of Water Research Report Series No. 53); 2011:. www.waterfootprint.org. 37. Wiedmann T, Barrett J: Policy-relevant applications of environmentally extended mrio databases—experiences from the UK. Econ Syst Res 2013, 25:143-156. This study illustrate whether and to what extent there is policy uptake of results from environmentally extended multi-region IO (EE-MRIO) models and how it may be improved. 38. Faramarzi M, Yang H, Mousavi J, Schulin R, Binder C, Abbaspour K: Analysis of intra-country virtual water trade strategy to alleviate water scarcity in Iran. Hydrol Earth Syst Sci 2010, 14:1417-1433. This paper is so far the only systematic study which demonstrated how the virtual water concept can be useful in supporting the national and regional water and food policies within a country taking into consideration water, land and climate constraints. 39. Aldaya MM, Allan JA, Hoekstra AY: Strategic importance of green water in international crop trade. Ecol Econ 2010, 69:887-894. 40. Suweis S, Rinaldo A, Maritan A, D’Odorico P: Water-controlled wealth of nations. Proc Natl Acad Sci 2013, 110:4230-4233.
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ScienceDirect Accounting for a scarce resource: virtual water and water footprint in the global water system Hong Yang1,2, Stephan Pfister3 and Anik Bhaduri4 Effective water management and governance at all geographical levels can only be designed based on good quality information and thorough understanding of it. The accounting for water resources from virtual water (VW) and water footprint (WF) perspectives can generate information about water uses in production processes and flows of VW associated with the trade of commodities. There have been a large number of studies on VW and WF since the advent of the two concepts. The recent literature has seen an increase in explicit elaboration of local, regional and national water uses in the context of global economic and water systems. More sophisticated and systematic approaches, such as input– output (IO) models and Life Cycle Assessment (LCA) tools, have been employed to facilitate the analysis of complex interconnections of water uses across system boundaries and environmental impacts incurred. However, limitations and shortcomings remain in the current VW and WF studies with regard to policy relevance, data accuracy, methodological approaches and conceptual consistency. Further efforts are required from the scientific community to tackle these problems in order to enhance the usefulness of the concepts and the data generated for water resources management and governance at all geographical levels which are intrinsically connected through VW trade. Addresses 1 Swiss Federal Institute of Aquatic Science and Technology, Ueberlandstrasse 133, 8600 Duebendorf, Switzerland 2 Department of Environmental Science, University of Basel, Petersplatz 1, 4003 Basel, Switzerland 3 Ecological Systems Design, Institute of Environmental Engineering (IfU), ETH Zurich, 8093 Zu¨rich, Switzerland 4 Global Water System Project (GWSP), International Project Office, Walter-Flex-Strasse 3, 53113 Bonn, Germany Corresponding author: Yang, Hong ([email protected])
Current Opinion in Environmental Sustainability 2013, 5:599–606 This review comes from a themed issue on Aquatic and marine systems Edited by Charles J Vo¨ro¨smarty, Claudia Pahl-Wostl and Anik Bhaduri For a complete overview see the Issue and the Editorial Available online 26th October 2013 1877-3435/$ – see front matter, # 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cosust.2013.10.003
Introduction As the world’s available freshwater resources are limited and unevenly distributed, it is important to quantify how www.sciencedirect.com
and where available water volumes are appropriated: for producing certain commodities, for certain people [1]. With commodities being traded across economic and hydrological system boundaries, the use and consumption of water resources in one location can exert impacts on freshwater resources in other locations where trade occurs. This interconnection renders the local water resources and their management with a global dimension [2]. The concepts of virtual water (VW) and water footprint (WF) emerged in the early 1990s and early 2000s, respectively, amid the increasing water scarcity and interconnections of water uses worldwide. In a nut shell, VW is the water used for the production of commodities [3,4]; while WF is the volume of freshwater used during the production process, measured over the whole supply chain [5]. Given the uneven spatial distribution of global water resources and different environmental impacts of water use across geographical locations, some researchers have quantified WF weighted with water scarcity and/or pollution indicators to account for the environmental relevance of the water appropriation [6,7]. For convenience, we name the WF without weight as volumetric WF and with weight as weighted WF. When not specified, WF generally refers to either volumetric WF or weighted WF. There are many overlaps in the VW and WF concepts. Volumetric WF of a product is numerically equal to ‘VW content’. VW and WF can be categorized into green, blue and grey. ‘Green’ refers to the use of soil moisture, ‘blue’ refers to the use of withdrawn freshwater [8], and ‘grey’ concerns polluted water [5]. The term WF is usually used in the context where consumers or producers of products are concerned, whereas the term VW is mostly used in the context of international or interregional trade. There have been a large and still increasing number of studies on VW and WF issues since the advent of the two concepts. The technical approaches and the understanding of the relevant issues have evolved over time. This paper provides a critical review of the current knowledge and methodological development of the accounting for VW and WF. It highlights the up-to-date status of the studies with a focus on those of significance to the global water system. An outlook at the end provides prospects of the future VW and WF research in the context of increasing globalization of the world economy and consequently closer interconnections of water resources management across geographical boundaries. Current Opinion in Environmental Sustainability 2013, 5:599–606
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Figure 1
Methods
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VW and WF accounting methods and approaches.
Methodological development in VW and WF accounting Many methods have been developed for VW and WF accounting. Figure 1 provides a summary of the commonly used methods. In general, the approaches of the accounting can be categorized into two groups: bottom-up and top-down [9]. The complexity of the water system, the scope of the accounting, the nature of the issues tackled, and the data availability determine the selection of the approaches. Bottom-up approach
The bottom-up approach departs from the smallest unit feasible in assessing VW and WF and then aggregates each unit to desired scale and period. The amount of water required for the production of a unit of product, measured in physical or monetary term, is called ‘VW content’, expressed in m3/kg or m3/$. It can be viewed as the inversion of water productivity. VW content of a product is the ‘building block’ in accounting for VW flows and WF. The agricultural sector is the largest water user in the world and has a substantial impact on the global water system. A large number of VW and WF studies have focused on agricultural products, especially staple food crops due to their high water intensity in production and the importance for food security. Process-based crop growth models supported by GIS techniques have been commonly applied to estimate crop water consumptive use and VW content [10,11]. The crop modeling approach has provided a systematic tool to account for green and blue water consumptive uses in crop production and spatial variations in VW content, and enabled analyses of impacts of changes in various input factors, such as water availability, fertilizer application, as well as climate change, on crop yield and VW content. Current Opinion in Environmental Sustainability 2013, 5:599–606
While VW focuses primarily on water quantity, WF also emphasizes the environmental impact of water use [7,12]. One approach is the accounting for the grey WF [5], which refers to the volume of freshwater required to assimilate the load of pollutants based on natural background concentrations and existing ambient water quality standards. Another approach is to incorporate environmental impact assessment into the WF accounting, which typically weights the water uses with water scarcity/stress indicators [13,14]. Life Cycle Assessment (LCA) is an ISO standardized procedure to assess environmental impacts of a product or service over its whole life cycle. It is mainly a bottomup approach, although it may partially utilize input– output (IO)-derived values in the case of hybrid LCA. The LCA approach has been increasingly applied in the studies of VW and WF. In the LCA approach, the system boundary is defined in the first step (goal and scope definition) followed by data collection covering the information of processes and trade as well as specific emissions/resource uses of each process involved (life cycle inventory). The third step consists of assessing the overall resource consumption in respect to environmental damage (e.g. water appropriation). This step allows for comparing products from different regions which have resource uses under different environmental conditions (e.g. water scarcity). Top-down approach
The top-down approach departs from the highest level defined by the system boundary. The analysis then breaks down to lower levels according to the subsystem boundaries, for example, economic sectors, river basins, and countries. IO and multi-region IO (MRIO) models have been widely used in the current literature to account for WF and VW [15]. www.sciencedirect.com
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An IO table/model represents the monetary transactions of goods and services among different sectors of economic system. It provides a technique to specify how the substances flow among sectors through supplying inputs (including water) for the outputs (where VW is embedded) in the economic system. The IO approach accounts for VW and WF in economic sectors by distinguishing the direct water use and indirect water use. The direct water use coefficient (DWUC) refers to the amount of direct water intake to produce one monetary unit of production. DWUC is the conventional measure for the sectoral water use intensity. The total water use coefficient (TWUC) for one monetary unit of production reflects the water use throughout the whole supply chain, for example, from agriculture to textile to clothing. It provides a more complete picture of water use intensity. The MRIO analysis is a variant of IO analysis, operating on large databases combining the IO tables of many regions. While IO assumes that imported goods and services are being produced with the same technology as the regional/domestic technology in the same sector, MRIO endogenously combines regional/domestic technical coefficient matrices with import matrices from multiple regions or countries into one large coefficient matrix. Thus, MRIO captures trade supply chains between all trading partners as well as feedback effects [16].
Accounting for VW flows and WF in economic sectors and across regions (basins) Figure 2 depicts the spectrum of VW and WF accounting with respect to the complexity of the concerned systems, relevance to consumers, producers and policy makers, as well as significance in the global water system. From left to right, the complexity, policy relevance and significance in global water system tend to increase. From right to left,
the accuracy of data, consumer interests and producer relevance of the accounting tend to increase. Early studies of VW and WF were mostly concentrated on the left side of the spectrum, quantifying water uses of per unit of product and over production chain, such as agricultural commodities, a cup of coffee, a pair of shoes, a cotton shirt, and so on. These items are often directly related to daily consumptions of individual people and hence have high relevance to raising public awareness of water consumption. The focus of the recent studies has mostly on the right side of the spectrum, with attempts to increase policy relevance. Many studies have quantified VW and WF for economic sectors, catchments/basins, regions and nations and the globe. With the distinction of direct and indirect water uses, the IO and MRIO analysis enables quantification of water use intensities in individual economic sectors over their whole supply chains. Many studies have investigated sectoral water uses in the national and regional economic systems with the IO tables at country and regional levels [17,18–20]. Some have conducted key sector analysis considering both economic growth potential and water use intensities to determine the direction of sectoral transformations for sustainable growth in a country or region. Studies on China using the IO models on the relevant issues have been notably many in number. One focal region is the North China Plain where water scarcity is severe [2,21,22,23,24–26]. A common conclusion is that North China in spite of its poor water endowments virtually exports water to the other regions in the form of mainly agricultural products (as the region is the major wheat and corn producing area in China), while South China with abundant water resources imports VW from the rest of the regions. Given the computational and labor intensity of the MRIO approach, only few globally comprehensive and sectorally detailed models have been
Figure 2
Increase
Primary items
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Accuracy of values (data) Consumer interest Producer relevance Economic/ industrial sector
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Complexity of the accounting system Policy relevance Significance in global water system
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Spectrum of VW and WF accounting.
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developed so far. Two most recently developed global MRIOs are the World Input–Output Database (WIOD) [27] and the Eora MRIO tables [28], both with the coverage for the period 1990s and 2000s. Water accounting using global MRIO tables has showed substantial water savings globally through VW trade [29].
Incorporating environmental impact in VW and WF accounting It has been widely recognized that water uses can have different impacts on the environment, depending on where water is used, what type of water is used, and how much water is polluted. There has been an increasing effort to incorporate environmental impacts, primarily water pollution and water scarcity, in VW and WF assessment. Water scarcity index has been used as a weight for converting total water use into scarce water use [14]. It offers a way for incorporating water scarcity into MRIO analyses of global water uses, and characterizing national WFs and trade balances in terms of scarce water. Together with water exploitation index (WEI) established based on national statistics, the water stress metric can be built in the MRIO framework to identify countries with relatively high abstraction in relation to water availability. More generic modeling of water uses and related environmental impacts per country has been conducted, including uncertainty assessment with continental and global coverage for crops and power production [11,29,30]. Combined with MRIO data, the global average values with uncertainty induced by underlying variability can be used for crops with unknown origin or that are traded on stock markets, such as cotton or coffee [11]. Given the nature of the LCA analysis, the method has often been used in assessing environmental impact of water uses. A number of studies which combined trade and water stress assessment for specific products and services have been published recently [6,29,31,32]. They generally quantified WF with weights representing degrees of environmental impact. For example, studies on meat production indicated the importance of looking at water sources and related impacts when analyzing meat consumption, since pasture and grain fed production systems differ widely [12], which is also true for milk production [33]. A case study on large water transfers from Southern to Northern China has indicated positive net effects from a blue water scarcity perspective [26]. The result is in contrast with the common perception of the irrationality of the water transfer project viewed from the volumetric WF perspective as mentioned earlier. So far, studies which specifically quantify grey WF have mostly focused on nitrogen and phosphorus input from agriculture into rivers and other water bodies [34]. Accounting for grey WF from industries is generally absent due to the lack of data on pollution loads in individual sectors. Current Opinion in Environmental Sustainability 2013, 5:599–606
Usefulness, limitations and gaps of VW and WF accounting What has been learnt from the current studies?
The application of more systematic and sophisticated models in recent years has enabled comprehensive analysis of interconnections of water uses across economic sectors, administrative regions and hydrological systems (catchment and river basin) through the VW trade. Such accounting enhances the relevance of the information to water resources management which is primarily conducted in the context of economic sectors and regions, as well as river basins. For example, the study by Zhang et al. [23] explicitly quantified the changes in internal and external WF in Beijing between 2002 and 2007 with specification of economic sectors and factors contributing to the changes and identified the sources where the external WF originates. The results provide basis for the assessment of the effects of economic development and water policies in Beijing over the period studied. It also helps to formulate strategies for the future water resources management of the city. The effort to incorporating environmental impact into VW and WF accounting facilitated the assessment of damages or benefits (in much lesser circumstances) of human appropriation of water resources in specific locations and their repercussions to other locations. The LCA approach has demonstrated particular advantage by investigating the life cycle of water and pollutants in the systems concerned. The IO based approaches help identify high water intensity sectors by looking at not only the direct water use and wastewater discharge at the final stage of the process, but also the total water use and the environmental damage incurred in the entire supply chain of the concerned sector — with each stage of the production processes often located in different regions, basins and countries. Information/data gaps
Concerning data on VW and WF, major problems remain in quality and accuracy of the source data and modeled water uses in the production processes. While many agricultural processes are covered [11,35] and hydropower has global estimates too [29,36], there is a large lack in knowledge of water consumption and use in the industrial sectors. For the trade data, the aggregation of products into sectors often mixes products with very different water productivities. Also problematic is the inconsistency between sectors reported by different countries. Lacking up-to-date high quality data has often impeded the studies to provide timely information on VW and WF status. IO tables typically have a long time-lag due to heavy task in data collection and processing. For example, the complete set of the provincial level IO tables in China is only available up to 2007. Given the rapid economic development of the country, the usefulness of the results www.sciencedirect.com
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using the 2007 data is largely discounted. Furthermore, its processing step is more a ‘black-box’, not allowing for transparent reporting. The problem of lacking detailed data is especially true for global IO tables. For example, the existing MRIO tables based on GTAP distinguish 129 regions, each breaks down into typically 57 sectors [11]. Some of the regions in these tables are single countries; some are groups of countries. Many areas of critical water problems in developing countries are not distinguished. In addition, the global MRIO databases do not distinguish differences within large countries such as China or the USA.
starts. The reason is that it is not possible and feasible to put labels of WF concerning the green, blue, grey components and spatial and temporal dimensions of the water uses in a product. Even if this were done, it may do little good to the environment and ecosystem in the water scarce regions before we are sure that the alternative water uses in these regions are less harmful. Gaining complete information on the complicated interconnections of water uses in different locations remains a great challenge to the scientific community; even greater is to convey the information to the policy makers and general public for appropriate actions.
Mainly owing to the data constraint, most IO based studies have conducted static analysis which only provides ‘snapshot information’ using the data of a single year or an average over a period of time. Dynamic studies investigating changes in WF at different times are rare. Also, the existing studies have mostly only provided descriptions on ‘what the situation is’, but no explanation on ‘how the situation is shaped’. Studies investigating the driving forces of the changes remain very limited, with a notable exception of the study by Zhang et al. [23].
Another pitfall is related to the impact of the shift of a region’s WF to external sources on the destination regions, countries and river basins. As shown in the study by Zhang et al. [23], Beijing has shifted its WF to other regions, including water scarce regions, by increasing external WF. An intuitive conclusion would be that the shift would increase the water pressure on the destination regions (e.g. the two peer reviewers of Zhang’s paper both raised this concern). However, a further scrutiny would suggest that the actual effects of the shift depend on whether it leads to improvement of water use efficiency in the destination regions and whether the total water use will increase or decrease. Much effort is needed to investigate these issues on individual case basis. A general conclusion cannot be established.
An aspect that has been mostly neglected in VW and WF accounting is about climate change and uncertainty consideration of all involved steps. Uncertainties can stem from the input data which are often very coarse and subject to high level of aggregation, the analysis procedure which often involves many assumptions, and the rapid change in the system status and the boundary conditions. Climate change is expected to have significant impact on hydrological regimes on both temporal and spatial dimensions. This will impose impacts on economic activities, particularly agriculture, and consequently VW and WF associated. Interpretation pitfalls
The interpretation of the generated data on VW and WF is often contentious and subject to acute debate. Many studies have found that regional and international trade patterns of VW can be little explained by water scarcity patterns. By now, a consensus has been more or less established in the VW and WF community that the concepts and the relevant data alone are not sufficient to support the decision making on optimal water uses and VW trade [1,37]. Yet, many still consider the mismatch of water endowments and VW trade as inefficient use of water resources without any further analysis of other involved factors influencing water use decisions (economic, comparative advantages, political and cultural reasons). One consequence of this water centric conclusion is the suggestion to label WF in products to draw consumers’ consciousness of their direct and indirect water consumption. However, this attempt is doomed to fail before it www.sciencedirect.com
Incorporating green water and water pollution in VW and WF accounting
In most VW and WF accounting for entire economic systems, water uses concern only blue water. Green water or soil moisture is not considered. This is because except for the agricultural sector and the sectors to which agriculture, livestock or forestry provides raw materials, all other sectors exclusively use blue water. Including green water would greatly increase the share of agricultural water use and thus derive biased conclusions in assessing the value of water use across different sectors. On the other hand, green water is an extremely valuable resource with large potential in biomass production and should be included in the analysis. One option is to consider green WF with distinction from blue water uses. This way of accounting helps gain a complete picture of water appropriations in the economic systems. The information is important for water planning and management. Green water is the main water source of agricultural production, while irrigation is the single largest withdrawn water user in most of the countries. Enhancing the green water use efficiency is closely associated with improving land management. For a given region and country, this could reduce the demand for blue water for irrigation. However, it should to be noted that there is no clear boundary between green and blue water in the hydrological system, since blue water can become Current Opinion in Environmental Sustainability 2013, 5:599–606
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green water in case of river floods or capillary rise from groundwater. A complete separation of green and blue WF may be inappropriate. Finding an appropriate way to harmonize the blue and green WF accounting is both scientifically and practically important. The lack of information has generally impeded the inclusion of grey water in WF assessment involving multiple industrial sectors. The grey WF consists of many different pollutants that have adverse effects on the environment. This is the issue where LCA can make a greater contribution. The development of MRIO data including a more complete set of resource use and effluence for each sector and region is necessary. For impact assessment, more sophisticated approaches based on risk assessment may enhance the credibility of grey WF accounting compared to the currently applied critical load approach [12]. One more critical problem which has not drawn adequate attention is the conceptual basis of grey WF. In reality, water required for diluting polluted water to meet the certain standards is not available in many regions. In this case, the grey WF accounted does not exist. In the global context, there is a possibility that the world’s currently accessible water would not be enough to dilute the polluted water to meet the prevailing standards, leaving the global assessment of grey WF little meaningful. This situation calls for the development of a more pertinent indicator that can better reflect the environmental impacts of water uses of human society.
Outlook to future water accounting in the context of global water system Improve data quality and accessibility at all levels
Given the data problems specified earlier, there is a great need for improving the completeness and reliability of data for water availability, water uses, water pollution and trade of commodities on all geographical scales. New developments should better utilize the hydrological, agronomic, economic and trade models to improved data quality with higher spatial and temporal resolutions and refine analysis on the relevant dimensions. In addition, models in environmental sciences and technologies should also be utilized to help identification and monitoring of water pollution and quantification of environmental impacts. Given the anticipated impact of climate change on water availability, water uses and economic development, incorporating climate change projections in VW and WF studies is important to facilitate the development of future water policies to support sustainable economic development and improvement of human and nature’s well-beings. Current Opinion in Environmental Sustainability 2013, 5:599–606
Integrated assessment to enhance policy relevance for national and global water governance
Although the VW and WF data alone are not sufficient to support the decision making, it is important to incorporate them in the integrated analysis to tackle complex problems relating to water resources management today and in the future. Several aspects can be highlighted in this regard. Inter-regional VW trade in dealing with regional water scarcity. Many countries of water scarcity have significant regional variations in natural (including water endowments) and socio-economic conditions. VW trade provides one option to alleviate regional water scarcity [38]. Studies of different scenarios concerning economic structural (particularly crop patterns) adjustments and land uses may be conducted to assess their impacts on regional economy and water uses. The results allow policy makers to foresee the outcomes of each of the options concerning the adjustment of economic structures and promoting VW trade. However, the feasibility of each of the structure adjustment options must be further assessed by considering the socio-economic conditions, opportunity costs, environmental impacts and other trade-offs. Inter-basin water transfer versus VW transfer. Inter-basin water transfers have been conducted in many areas in the world, especially in high population density areas. The trend is expected to continue with the economic development and population growth. The rationale of VW and real water transfers is an issue of debate in the political arena and the scientific community. A comprehensive assessment of trade-offs taking into consideration the natural and socio-economic conditions is necessary to reach a conclusion. The information on VW embodied in the trade is needed for such an assessment. Implications of the local/regional water policies for the trade partners. Given the interconnections of local water uses with other regions in the global water system, VW and WF assessments need to pay attention to the repercussions of the local/regional water policies to other regions/ countries and river basins. For example, the EU Water Framework Directive (WFD) of 2000 was a response to the growing pressure on European water resources by pollution, overexploitation and endangered wetlands [39]. The implementation of WFD is conducive to the European waters and related ecosystems. However, the impact may not always be positive when viewed in a broader geographical context. The implication for the trade between the EU and the Middle Eastern and North Africa (MENA) countries is a case in point. The problems associated with agricultural water use in the EU could be exported, at least partially, to the water scarce MENA countries due to the possible increase in import of the EU countries from this region. www.sciencedirect.com
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Overall, with increasing domestic demand for food and water, as well as tightening environmental and resource protection policies, water-rich food export countries (mostly developed countries) are likely to reduce the amount of VW export [40]. This could leave water poor import-dependent countries without enough water to sustain their populations. For these countries, improving water use efficiency through enhancing productivities should be taken as a long-term strategy to reduce the vulnerability to water scarcity.
Conclusion (1) The current studies of VW and WF have made substantial progress in the elaboration of the interconnections of water uses in production and consumption of final products in the local, regional, national and international economic systems. (2) The methodological progresses have allowed more sophisticated but also complex quantification of VW and WF and assessment of their environmental impacts. (3) There remains a lack of policy relevance of VW and WF accounting to support water resources management and governance at all geographical levels. (4) Issues on climate change impact and uncertainties have generally been absent in VW and WF accounting and should be incorporated in the future. (5) Conceptual frameworks for incorporating green and grey WFs into VW and WF accounting are weak. There is a big challenge to harmonize the conceptual bases of blue, green and grey WFs and environmental impacts in water accounting. (6) The repercussions of local water management to the global water and economic systems through trade of goods and services require broader as well as in depth studies with joint efforts from different disciplines.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1. Hoekstra AY, Mekonnen MM: From water footprint assessment to policy. Proc Natl Acad Sci U S A 2012, 109:E1425. One of the major papers addressing the relevance of WF to water policies. 2. 3.
4.
Hoff H: Global water resources and their management. Curr Opin Environ Sustain 2009, 1:141-147. Allan JA: Policy responses to the closure of water resources: regional and global issue. In Water Policy: Allocation and Management in Practice. Edited by Howsam P, Carter RC. London, UK: Chapman and Hall; 1996:3-12. Yang H, Zehnder AJB: Virtual water—an unfolding concept in integrated water resources management. Water Resour Res 2007, 43 http://dx.doi.org/10.1029/2007WR006048.
5.
Hoekstra AY, Chapagain AK, Aldaya MM, Mekonnen MM: The WF Assessment Manual: Setting the Global Standard. London, UK: Earthscan; 2011, . This manual sets the benchmark for the assessment of WF and is widely followed in the WF studies.
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6.
Ridoutt BG, Pfister S: A revised approach to water footprinting to make transparent the impacts of consumption and production on global freshwater scarcity. Global Environ Change 2010, 20:113-120.
7.
ISO: ISO/DIS 14046: Water Footprint — Principles, Requirements and Guidelines. 2013.
Falkenmark M, Rockstro¨m J: The new blue and green water paradigm: breaking new ground for water resources planning and management. J Water Resour Plann Manage 2006, 132:129-132. This paper provides a new perspective in viewing water resources and management.
8.
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