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Accounting for a scarce resource: virtual water and waterfootprint in the global water systemHong 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.

Addresses1

Swiss Federal Institute of Aquatic Science and Technology,Ueberlandstrasse 133, 8600 Duebendorf, Switzerland2Department of Environmental Science, University of Basel, Petersplatz

1, 4003 Basel, Switzerland3Ecological Systems Design, Institute of Environmental Engineering

(IfU), ETH Zurich, 8093 Zurich, Switzerland4Global Water System Project (GWSP), International Project Office,

Walter-Flex-Strasse 3, 53113 Bonn, Germany

Corresponding author: Yang, Hong ([email protected])

Current Opinion in Environmental Sustainability2013, 5:599606

This review comes from a themed issue on Aquatic and marinesystems

Edited by Charles J Vorosmarty, 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

IntroductionAs the worlds available freshwater resources are limitedand unevenly distributed, it is important to quantify how

and where available water volumes are appropriated: forproducing certain commodities, for certain people [1].With commodities being traded across economic andhydrological system boundaries, the use and consumptionof water resources in one location can exert impacts onfreshwater resources in other locationswhere trade occurs.This interconnection renders the localwater resources andtheir 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 duringthe production process, measured over the whole supplychain [5]. Given the uneven spatial distribution ofglobal water resources and different environmentalimpacts of water use across geographical locations, someresearchers have quantified WF weighted with waterscarcity and/or pollution indicators to account for theenvironmental relevance of the water appropriation[6,7]. For convenience, we name the WF withoutweight as volumetric WF and with weight as weightedWF. When not specified, WF generally refers to either

volumetric

WF

or

weighted WF. There

are

many

over-laps in the VW and WF concepts. Volumetric WF of aproduct is numerically equal to VW content. VW andWF can be categorized into green, blue and grey.Green refers to the use of soil moisture, blue refersto the use of withdrawn freshwater [8], and greyconcerns polluted water [5]. The term WF is usuallyused in the context where consumers or producers ofproducts are concerned, whereas the term VW is mostlyused in the context of international or interregionaltrade.

There have been a large and still increasing number of

studies

on

VW

and

WF

issues

since

the

advent

of

the

twoconcepts. The technical approaches and the understand-ing of the relevant issues have evolved over time. Thispaper provides a critical review of the current knowledgeand methodological development of the accounting forVW and WF. It highlights the up-to-date status of thestudies with a focus on those of significance to the globalwater system. An outlook at the end provides prospects ofthe future VW andWF research in the context of increas-ing globalization of the world economy and consequentlycloser interconnections of water resources managementacross geographical boundaries.

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Methodological development in VW and WFaccountingMany methods have been developed for VW and WFaccounting. Figure 1 provides a summary of the com-monly used methods. In general, the approaches of theaccounting can be categorized into two groups: bottom-upand top-down [9]. The complexity of the water system,the scope of the accounting, the nature of the issuestackled, and the data availability determine the selectionof the approaches.

Bottom-up approach

The bottom-up approach departs from the smallest unitfeasible in assessing VW and WF and then aggregateseach unit to desired scale and period. The amount ofwater required for the production of a unit of product,measured in physical or monetary term, is called VWcontent, expressed in m3/kg or m3/$. It can be viewed asthe inversion of water productivity. VW content of aproduct is the building block in accounting for VWflows and WF.

The agricultural sector is the largest water user in theworld and has a substantial impact on the global watersystem. A large number of VW and WF studies have

focused on

agricultural

products,

especially

staple

foodcrops due to their high water intensity in production andthe importance for food security. Process-based cropgrowth models supported by GIS techniques have beencommonly applied to estimate crop water consumptiveuse and VW content [10,11]. The crop modelingapproach has provided a systematic tool to account forgreen and blue water consumptive uses in crop pro-duction and spatial variations inVW content, and enabledanalyses of impacts of changes in various input factors,such as water availability, fertilizer application, as well asclimate change, on crop yield and VW content.

While VW focuses primarily on water quantity, WF alsoemphasizes the environmental impact of water use[7,12]. One approach is the accounting for the greyWF [5], which refers to the volume of freshwaterrequired to assimilate the load of pollutants based onnatural background concentrations and existing ambientwater quality standards. Another approach is to incorpor-ate environmental impact assessment into the WFaccounting, which typically weights the water uses withwater scarcity/stress indicators [13,14].

Life Cycle Assessment (LCA) is an ISO standardized

procedure to assess environmental impacts of a productor service over its whole life cycle. It ismainly a bottom-up approach, although it may partially utilize inputoutput (IO)-derived values in the case of hybrid LCA.The LCA approach has been increasingly applied in thestudies ofVW and WF. In the LCAapproach, the systemboundary is defined in the first step (goal and scopedefinition) followedbydata collection covering the infor-mation of processes and trade as well as specific emis-sions/resource uses of each process involved (life cycleinventory). The third step consists of assessing the over-all resource consumption in respect to environmentaldamage (e.g. water appropriation). This step allows for

comparing products from

different regions which haveresource uses under different environmental conditions(e.g. water scarcity).

Top-down approach

The top-down approach departs from the highest leveldefined by the system boundary. The analysis thenbreaks down to lower levels according to the subsystemboundaries, for example, economic sectors, river basins,and countries. IO and multi-region IO (MRIO) modelshave been widely used in the current literature to accountfor WF and VW [15].

600 Aquatic and marine systems

Figure 1

Methods

Bottom upapproaches

Top downapproaches

Rule ofthe

thumb

Cropmodeling

Aggregationover space and

supply chain

Input-Output

(IO)

Multi-regionInput-Output

(MRIO)

Life cycleassessment

(LCA)

Current Opinion in Environmental Sustainability

VW and WF accounting methods and approaches.

Current Opinion in Environmental Sustainability2013, 5:599606 www.sciencedirect.com

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An IO table/model represents the monetary transactionsof goods and services among different sectors of economicsystem. It provides a technique to specify how the sub-stances flow among sectors through supplying inputs(including water) for the outputs (where VW is

embedded)

in

the

economic

system.

The

IO

approachaccounts for VW and WF in economic sectors by dis-tinguishing the direct water use and indirect water use.The direct water use coefficient (DWUC) refers to theamount of direct water intake to produce one monetaryunit of production. DWUC is the conventional measurefor the sectoral water use intensity. The total water usecoefficient (TWUC) for one monetary unit of productionreflects the water use throughout the whole supply chain,for example, from agriculture to textile to clothing. Itprovides a more complete picture of water use intensity.

The MRIO analysis is a variant of IO analysis, operatingon large databases combining the IO tables of many

regions. While IO assumes that imported goods andservices are being produced with the same technologyas the regional/domestic technology in the same sector,MRIO endogenously combines regional/domestic tech-nical coefficient matrices with import matrices frommultiple regions or countries into one large coefficientmatrix. Thus, MRIO captures trade supply chains be-tween all trading partners as well as feedback effects[16].

Accounting for VW flows and WF in economicsectors and across regions (basins)

Figure 2 depicts the spectrum of VW andWF accountingwith respect to the complexity of the concerned systems,relevance to consumers, producers and policy makers, aswell as significance in the global water system. From leftto right, the complexity, policy relevance and significancein globalwater system tend to increase. From right to left,

the accuracy of data, consumer interests and producerrelevance of the accounting tend to increase.

Early studies ofVW andWFweremostly concentrated onthe left side of the spectrum, quantifyingwater uses of per

unit of

product

and

over

production

chain,

such

as

agricul-tural commodities, a cup of coffee, a pair of shoes, a cottonshirt, and so on. These items are often directly related todaily consumptions of individual people and hence havehigh relevance to raising public awareness of water con-sumption. The focus of the recent studies has mostly onthe right side of the spectrum, with attempts to increasepolicy relevance. Many studies have quantified VW andWF for economic sectors, catchments/basins, regions andnations and the globe.

With the distinction of direct and indirect water uses, theIO andMRIO analysis enables quantification ofwater useintensities in individual economic sectors over their

whole supply chains. Many studies have investigatedsectoral water uses in the national and regional economicsystems with the IO tables at country and regional levels[17,1820]. Some have conducted key sector analysisconsidering both economic growth potential and wateruse intensities to determine the direction of sectoraltransformations for sustainable growth in a country orregion. Studies on China using the IO models on therelevant issues have been notably many in number. Onefocal region is theNorth China Plainwhere water scarcityis severe [2,21,22,23,2426]. A common conclusion isthat North China in spite of its poor water endowmentsvirtually exports water to the other regions in the form of

mainly agricultural products (as the region is the majorwheat and corn producing area in China), while SouthChina with abundant water resources imports VW fromthe rest of the regions.Given the computational and laborintensity of the MRIO approach, only few globally com-prehensive and sectorally detailed models have been

VW and WF in the global water system Yang, Pfister and Bhaduri 601

Figure 2

Primaryitems

Administrativeregion(s)

Multipleregions

Catchment& riverbasin

Economic/industrial

sector

Productionchain

Increase

Increase

Accuracy of values (data)

Consumer interest

Producer relevance

Complexity of the

accounting system

Policy relevance

Significance in global

water system

Current Opinion in Environmental Sustainability

Spectrum of VW and WF accounting.


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developed so far. Two most recently developed globalMRIOs are the World InputOutput Database (WIOD)[27] and the Eora MRIO tables [28], both with thecoverage for the period 1990s and 2000s. Water account-ing using global MRIO tables has showed substantial

water savings

globally

through

VW

trade

[29].

Incorporating environmental impact in VW andWF accountingIt has been widely recognized that water uses can havedifferent impacts on the environment, depending onwhere water is used, what type of water is used, andhowmuch water is polluted.There has been an increasingeffort to incorporate environmental impacts, primarilywater pollution andwater scarcity, in VW andWF assess-ment.

Water scarcity index has been used as a weight forconverting total water use into scarce water use [14]. It

offers a way for incorporating water scarcity into MRIOanalyses of global water uses, and characterizing nationalWFs and trade balances in terms of scarce water.Together with water exploitation index (WEI) estab-lished based on national statistics, the water stress metriccan be built in theMRIO framework to identify countrieswith relatively high abstraction in relation to water avail-ability. More generic modeling of water uses and relatedenvironmental impacts per country has been conducted,including uncertainty assessment with continental andglobal coverage for crops and power production[11,29,30]. Combined with MRIO data, the global aver-age values with uncertainty induced by underlying varia-

bility can be used for crops with unknown origin or thatare traded on stock markets, such as cotton or coffee [11].

Given thenatureof theLCAanalysis, themethodhasoftenbeenused inassessingenvironmental impactofwateruses.Anumberof studieswhich combined tradeandwater stressassessment for specific products and services have beenpublished recently [6,29,31,32]. Theygenerally quantifiedWF with weights representing degrees of environmentalimpact.Forexample, studiesonmeatproduction indicatedthe importance of looking at water sources and relatedimpacts when analyzing meat consumption, since pastureandgrain fedproduction systemsdifferwidely [12], which

is also

true

for

milk

production

[33].

A

case

study

on

largewater transfers from Southern to Northern China hasindicated positive net effects from a blue water scarcityperspective [26].The result is in contrastwith thecommonperception of the irrationality of the water transfer projectviewed from the volumetricWF perspective as mentionedearlier. So far, studieswhich specifically quantify greyWFhave mostly focused on nitrogen and phosphorus inputfrom agriculture into rivers and other water bodies [34].Accounting for greyWF from industries isgenerally absentdue to the lack of data on pollution loads in individualsectors.

Usefulness, limitations and gaps of VW andWF accountingWhat has been learnt from the current studies?

The application of more systematic and sophisticatedmodels in recent years has enabled comprehensive

analysis

of

interconnections

of

water

uses

across

economicsectors, administrative regions and hydrological systems(catchment and river basin) through the VW trade. Suchaccounting enhances the relevance of the information towater resources management which is primarily con-ducted in the context of economic sectors and regions,as well as river basins. For example, the study by Zhanget al. [23] explicitly quantified the changes in internaland external WF in Beijing between 2002 and 2007 withspecification of economic sectors and factors contributingto the changes and identified the sources where theexternal WF originates. The results provide basis forthe assessment of the effects of economic developmentand water policies in Beijing over the period studied. It

also helps to formulate strategies for the future waterresources management of the city.

The effort to incorporating environmental impact intoVW and WF accounting facilitated the assessment ofdamages or benefits (in much lesser circumstances) ofhuman appropriation of water resources in specificlocations and their repercussions to other locations.The LCA approach has demonstrated particularadvantage by investigating the life cycle of water andpollutants in the systems concerned. The IO basedapproaches help identify high water intensity sectorsby looking at not only the directwater use andwastewater

discharge at the final stage of the process, but also thetotal water use and the environmental damage incurred inthe entire supply chain of the concerned sector witheach stage of the production processes often located indifferent regions, basins and countries.

Information/data gaps

Concerning data on VW and WF, major problems remainin quality and accuracy of the source data and modeledwater uses in the production processes. While manyagriculturalprocesses are covered [11,35] and hydropowerhas global estimates too [29,36], there is a large lack inknowledge ofwater consumption and use in the industrial

sectors. For

the

trade

data,

the

aggregation

of

productsinto sectors often mixes products with very differentwater productivities. Also problematic is the inconsis-tency between sectors reported by different countries.

Lacking up-to-date high quality data has often impededthe studies to provide timely information on VW andWFstatus. IO tables typically have a long time-lag due toheavy task in data collection and processing.For example,the complete set of the provincial level IO tables inChinais only available up to 2007. Given the rapid economicdevelopment of the country, the usefulness of the results



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using the 2007 data is largely discounted.Furthermore, itsprocessing step is more a black-box, not allowing fortransparent reporting. The problem of lacking detaileddata is especially true for global IO tables. For example,the existingMRIO tables based onGTAP 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 waterproblems in developing countries are not distinguished.In addition, the global MRIO databases do not dis-tinguish differences within large countries such as Chinaor the USA.

Mainly owing to the data constraint, most IO basedstudies have conducted static analysis which only pro-vides snapshot information using the data of a singleyear or an average over a period of time.Dynamic studiesinvestigating changes in WF at different times are rare.Also, the existing studies have mostly only provided

descriptions on what the situation is, but no explanationon how the situation is shaped. Studies investigating thedriving forces of the changes remain very limited, with anotable exception of the study by Zhang et al. [23].

An aspect that has beenmostly neglected in VW andWFaccounting is about climate change and uncertainty con-sideration of all involved steps. Uncertainties can stemfrom the input data which are often very coarse andsubject to high level of aggregation, the analysis pro-cedure which often involves many assumptions, andthe rapid change in the system status and the boundaryconditions.Climate change is expected to have significant

impact on hydrological regimes on both temporal andspatial dimensions. This will impose impacts oneconomic activities, particularly agriculture, and con-sequently VW and WF associated.

Interpretation pitfalls

The interpretation of the generated data on VW and WFis often contentious and subject to acute debate. Manystudies have found that regional and international tradepatterns of VW can be little explained by water scarcitypatterns. By now, a consensus has been more or lessestablished in the VW and WF community that theconcepts and the relevant data alone are not sufficient

to support

the

decision

making

on

optimal

water

uses

andVW trade [1,37]. Yet,many still consider themismatchof water endowments and VW trade as inefficient use ofwater resources without any further analysis of otherinvolved factors influencing water use decisions(economic, comparative advantages, political and culturalreasons).

One consequence of this water centric conclusion is thesuggestion to label WF in products to draw consumersconsciousness of their direct and indirectwater consump-tion. However, this attempt is doomed to fail before it

starts. The reason is that it is not possible and feasible toput labels of WF concerning the green, blue, grey com-ponents and spatial and temporal dimensions of thewateruses in a product. Even if this were done, it may do littlegood to the environment and ecosystem in the water

scarce regions

before

we

are

sure

that

the

alternativewater uses in these regions are less harmful. Gainingcomplete information on the complicated interconnec-tions of water uses in different locations remains a greatchallenge to the scientific community; even greater is toconvey the information to the policy makers and generalpublic for appropriate actions.

Another pitfall is related to the impact of the shift of aregions WF to external sources on the destinationregions, countries and river basins. As shown in the studyby Zhang et al. [23], Beijing has shifted its WF to otherregions, including water scarce regions, by increasingexternal WF. An intuitive conclusion would be that the

shift would increase thewater pressure on the destinationregions (e.g. the two peer reviewers of Zhangs paper bothraised this concern). However, a further scrutiny wouldsuggest that the actual effects of the shift depend onwhether it leads to improvement ofwater use efficiency inthe destination regions and whether the total water usewill increase or decrease. Much effort is needed to inves-tigate these issues on individual case basis. A generalconclusion cannot be established.

Incorporating green water and water pollution inVW and

WF accounting

In most VW and WF accounting for entire economic

systems, water uses concern only blue water. Greenwater or soil moisture is not considered. This is becauseexcept for the agricultural sector and the sectors towhich agriculture, livestock or forestry provides rawmaterials, all other sectors exclusively use blue water.Including green water would greatly increase the shareof agricultural water use and thus derive biased con-clusions in assessing the value of water use acrossdifferent sectors. On the other hand, green water isan extremely valuable resource with large potential inbiomass production and should be included in theanalysis. One option is to consider green WF withdistinction from blue water uses. This way of account-

ing helps gain

a

complete picture

of

water

appropria-tions in the economic systems. The information isimportant for water planning and management. Greenwater is the main water source of agricultural pro-duction, while irrigation is the single largest withdrawnwater user in most of the countries. Enhancing thegreen water use efficiency is closely associated withimproving land management. For a given region andcountry, this could reduce the demand for blue waterfor irrigation. However, it should to be noted that thereis no clear boundary between green and blue water inthe hydrological system, since blue water can become



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green water in case of river floods or capillary rise fromgroundwater. A complete separation of green and blueWF may be inappropriate. Finding an appropriate wayto harmonize the blue and green WF accounting is bothscientifically and practically important.

The lack of information has generally impeded theinclusion of grey water in WF assessment involvingmultiple industrial sectors. The greyWF consists ofmanydifferent pollutants that have adverse effects on theenvironment. This is the issue where LCA can make agreater contribution. The development of MRIO dataincluding a more complete set of resource use and efflu-ence for each sector and region is necessary. For impactassessment, more sophisticated approaches based on riskassessment may enhance the credibility of grey WFaccounting compared to the currently applied critical loadapproach [12].

One more critical problem which has not drawn ad-equate attention is the conceptual basis of grey WF. Inreality, water required for diluting polluted water tomeet the certain standards is not available in manyregions. In this case, the grey WF accounted does notexist. In the global context, there is a possibility thatthe worlds currently accessible water would not beenough to dilute the polluted water to meet the pre-vailing standards, leaving the global assessment of greyWF little meaningful. This situation calls for the de-velopment of a more pertinent indicator that can betterreflect the environmental impacts of water uses ofhuman society.

Outlook to future water accounting in thecontext of global water systemImprove data quality and accessibility at all levels

Given the data problems specified earlier, there is a greatneed for improving the completeness and reliability ofdata forwater availability,water uses,water pollution andtrade of commodities on all geographical scales. Newdevelopments should better utilize the hydrological, agro-nomic, economic and trade models to improved dataquality with higher spatial and temporal resolutionsand refine analysis on the relevant dimensions. In

addition, models

in

environmental

sciences

and

technol-ogies should also be utilized to help identification andmonitoring of water pollution and quantification ofenvironmental impacts.

Given the anticipated impact of climate change on wateravailability, water uses and economic development, incor-porating climate change projections in VW and WFstudies is important to facilitate the development offuture water policies to support sustainable economicdevelopment and improvement of human and natureswell-beings.

Integrated assessment to enhance policy relevance for

national and global water governance

Although the VW and WF data alone are not sufficient tosupport the decisionmaking, it is important to incorporatethem in the integrated analysis to tackle complex pro-

blems relating

to

water

resources

management

today

andin the future. Several aspects can be highlighted in thisregard.

Inter-regional VW trade in dealing with regional water

scarcity. Many countries of water scarcity have signifi-cant regional variations in natural (including waterendowments) and socio-economic conditions. VW tradeprovides one option to alleviate regional water scarcity[38]. Studies of different scenarios concerningeconomic structural (particularly crop patterns) adjust-ments and land uses may be conducted to assess theirimpacts on regional economy and water uses. Theresults allow policy makers to foresee the outcomes

of each of the options concerning the adjustment ofeconomic structures and promoting VW trade. How-ever, the feasibility of each of the structure adjustmentoptions must be further assessed by considering thesocio-economic conditions, opportunity costs, environ-mental impacts and other trade-offs.

Inter-basin water transfer versus VW transfer. Inter-basinwater transfers have been conducted in many areas inthe world, especially in high population density areas.The trend is expected to continue with the economicdevelopment and population growth. The rationale ofVW and real water transfers is an issue of debate in the

political arena and the scientific community. A compre-hensive assessment of trade-offs taking into considerationthe natural and socio-economic conditions is necessary toreach a conclusion. The information on VW embodied inthe trade is needed for such an assessment.

Implications of the local/regional water policies for the trade

partners. Given the interconnections of local water useswith other regions in the global water system, VW andWF assessments need to pay attention to the repercus-sions of the local/regional water policies to other regions/countries and river basins. For example, the EU WaterFramework Directive (WFD) of 2000 was a response to

the growing

pressure

on

European

water

resources

bypollution, overexploitation and endangered wetlands[39]. The implementation of WFD is conducive to theEuropean waters and related ecosystems. However, theimpact may not always be positive when viewed in abroader geographical context. The implication for thetrade between theEU and theMiddleEastern andNorthAfrica (MENA) countries is a case in point.The problemsassociated with agricultural water use in the EU could beexported, at least partially, to the water scarce MENAcountries due to the possible increase in import of theEUcountries from this region.



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Overall, with increasing domestic demand for food andwater, as well as tightening environmental and resourceprotection policies, water-rich food export countries(mostly developed countries) are likely to reduce theamount of VW export [40]. This could leave water poor

import-dependent

countries

without

enough

water

tosustain their populations. For these countries, improvingwater use efficiency through enhancing productivitiesshould be taken as a long-term strategy to reduce thevulnerability to water scarcity.

Conclusion(1) The current studies of VW and WF have madesubstantial progress in the elaboration of the inter-connections of water uses in production and con-sumption of final products in the local, regional,national and international economic systems.

(2) The methodological progresses have allowed moresophisticated but also complex quantification of VW

and WF and assessment of their environmentalimpacts.

(3) There remains a lack of policy relevance of VW andWF accounting to support water resources man-agement and governance at all geographical levels.

(4) Issues on climate change impact and uncertaintieshave generally been absent in VW and WF account-ing and should be incorporated in the future.

(5) Conceptual frameworks for incorporating green andgrey WFs into VW and WF accounting are weak.There is a big challenge to harmonize the conceptualbases of blue, green and greyWFs and environmentalimpacts in water accounting.

(6) The repercussions of local water management to theglobal water and economic systems through trade ofgoods and services require broader aswell as in depthstudies with joint efforts from different disciplines.

References and recommended readingPapers of particular interest, published within the period of review,have been highlighted as:

of special interest of outstanding interest

1.

Hoekstra AY,MekonnenMM:From water footprint assessmentto policy. Proc Natl Acad Sci U S A 2012, 109:E1425.

Oneof themajor papersaddressing therelevance ofWFto water policies.

2. Hoff H: Global water resources and their management. CurrOpin Environ Sustain 2009, 1:141-147.

3. Allan JA: Policy responses to the closure of water resources:regional and global issue. In Water Policy: Allocation andManagement in Practice. Edited by Howsam P, Carter RC.London, UK: Chapman and Hall; 1996:3-12.

4. Yang H, Zehnder AJB: Virtual wateran unfolding concept inintegrated water resources management. Water Resour Res2007, 43 http://dx.doi.org/10.1029/2007WR006048.

5.

Hoekstra AY,Chapagain AK,Aldaya MM, MekonnenMM: TheWFAssessment Manual: Setting the Global Standard. London, UK:Earthscan; 2011, .

This manual sets the benchmark for the assessment ofWF and is widelyfollowed in the WF studies.

6. Ridoutt BG, Pfister S:A revised approach to water footprintingto make transparent the impacts of consumption andproduction on global freshwater scarcity. Global EnvironChange 2010, 20:113-120.

7. ISO: ISO/DIS 14046: Water Footprint Principles, Requirementsand Guidelines. 2013.

8.

Falkenmark M, Rockstrom J: The new blue and green waterparadigm: breakingnewground forwater resources planningand management. J Water Resour Plann Manage 2006,132:129-132.

This paper provides a new perspective in viewing water resources andmanagement.

9. FengKS, Chapagain A, Suh S,Pfister S, Hubacek K:Comparisonof bottom-up and top-down approaches to calculating thewater footprints of nations. Econ Syst Res 2011, 23:371-385.

10.

Liu JG, Zehnder AJB, YangH:Global cropwater use and virtualwater trade: the importance of greenwater. Water Resour Res2009, 45 http://dx.doi.org/10.1029/2007WR006051.

This study provides a systematical assessment of green and blue wateruses in crop production and the importance of green water in the globalvirtual water trade.

11. Pfister S, BayerP, Koehler A, Hellweg S: Environmental impactsof water use in global crop production: hotspots and trade-

offs with land use. Environ Sci Technol2011, 45:5761-5768.12.

Ridoutt B, Pfister S: A new WF calculation method integratingconsumptive and degradative water use into a single stand-alone weighted indicator. Int J Life Cycle Assess 2013,18:204-207.

This paper proposes an alternative approach to incorporate water scar-city in the assessment of WF.

13. Pfister S, Koehler A, Hellweg S: Assessing the environmentalimpactsof freshwaterconsumption inLCA. EnvironSci Technol2009, 43:4098-4104.

14. Lenzen M, Bhaduri A, Moran D, Kanemoto K, Bekchanov M,Geschke A, Foran B: The role of scarcity in global VW flows.ZEF-Discussion Papers on Development Policy. 2012. No. 169.http://papers.ssrn.com. Download 11 August, 2013.

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Duarte R, Yang H: Inputoutput and water: introduction to thespecial issue. Econ Syst Res 2011, 23:341-351.

This paperprovides a summary of thestate-of-theart of theapplicationofinput

outputmodels in assessingwaterandenvironmental impacts in the

national economic systems.

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Wiedmann T: A review of recent multi-region inputoutputmodels used for consumption-based emission and resourceaccounting. Ecol Econ 2009, 69:211-222.

This paper provided a critical overview of the application of inputoutputmodels for accounting for emission and resource uses.

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Cazcarro I, Duarte R, Sanchez-Choliz J:Water flows in theSpanish economy: agri-food sectors, trade and householdsdiets in an input output framework. Environ Sci Technol2012,46:6530-6538.

This paper links the water use directly with the staple food, the primarywater consumption of people, with water flows in the economic system.

18. Antonelli M, Roson R, Sartori M: Systemic inputoutputcomputation of green and blue virtual water flows with anillustration for theMediterranean region.WaterResour Manage

2012, 26:4133-4146.19. Yu Y, Hubacek K, Feng KS, Guan DB: Assessing regional and

global water footprints for the UK. Ecol Econ 2012,69:1140-1147.

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21. Zhao X, Chen B, Yang ZF: National WF in an inputoutputframeworka case study of China 2002. Ecol Model2009,220:245-253.

22. Zhao X, Yang H, Yang ZF, Chen B, Qin Y: Applying the inputoutputmethod to accountfor water footprint andvirtualwatertradein theHaihe riverbasin inChina. Environ Sci Technol2010,44:9150-9156.


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