ContentslistsavailableatScienceDirect
Ecological
Indicators
j ou rn a l h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e /e c o l i n d
A
critique
on
the
water-scarcity
weighted
water
footprint
in
LCA
Arjen
Y.
Hoekstra
UniversityofTwente,P.O.Box217,7500AEEnschede,Netherlands
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received30October2015
Receivedinrevisedform6February2016 Accepted7February2016 Keywords: Waterfootprint Waterscarcity Waterstress Waterdepletion Waterpollution Lifecycleassessment ISO14046
Environmentalimpactassessment
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t
Thewaterfootprint(WF)hasbeendevelopedwithinthewaterresourcesresearchcommunityasa volu-metricmeasureoffreshwaterappropriation.Theconceptisusedtoassesswaterusealongsupplychains, sustainabilityofwaterusewithinriverbasins,efficiencyofwateruse,equitabilityofwaterallocationand dependencyonwaterinthesupplychain.WiththepurposeofintegratingtheWFinlifecycleassessment ofproducts,LCAscholarshaveproposedtoweighttheoriginalvolumetricWFbythewaterscarcityin thecatchmentwheretheWFislocated,thusobtainingawater-scarcityweightedWFthatreflectsthe potentiallocalenvironmentalimpactofwaterconsumption.Thispaperprovidesanelaboratecritiqueon thisproposal.Themainpointsare:(1)countinglitresofwaterusedifferentlybasedontheleveloflocal waterscarcityobscurestheactualdebateaboutwaterscarcity,whichisaboutallocatingwaterresources tocompetingusesanddepletionataglobalscale;(2)theneglectofgreenwaterconsumptionignores thefactthatgreenwaterisscarceaswell;(3)sincewaterscarcityinacatchmentincreaseswithgrowing overallwaterconsumptioninthecatchment,multiplicationoftheconsumptivewateruseofaspecific processoractivitywithwaterscarcityimpliesthattheresultantweightedWFofaprocessoractivitywill beaffectedbytheWFsofotherprocessesoractivities,whichcannotbethepurposeofanenvironmental performanceindicator;(4)theLCAtreatmentoftheWFisinconsistentwithhowotherenvironmental footprintsaredefined;and(5)theWaterStressIndex,themostcitedwaterscarcitymetricintheLCA community,lacksmeaningfulphysicalinterpretation.Itisproposedtoincorporatethetopicof fresh-waterscarcityinLCAasa“naturalresourcedepletion”category,consideringdepletionfromaglobal perspective.Sinceglobalfreshwaterdemandisgrowingwhileglobalfreshwateravailabilityislimited, itiskeytomeasurethecomparativeclaimofdifferentproductsontheglobe’slimitedaccessibleand usablefreshwaterflows.
©2016TheAuthor.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Thewaterfootprint(WF)conceptwasfirstpresentedatan
inter-nationalexpertmeetingonvirtualwatertradeinDecember2002in
Delft,theNetherlands(Hoekstra,2003).Theincreasingamountof
workonwateruseandscarcityinrelationtoconsumptionandtrade
hasledtotheemergenceofthefieldofWaterFootprintAssessment
(WFA).Methodologicaladvancesoverthepastdecadeincludethe
developmentofthefour-stepWFAmethodology(settingscopeof
analysis,accounting,sustainabilityassessmentandresponse
for-mulation;Hoekstraetal.,2009a,2011),thedevelopmentofgrey
WFguidelines(Frankeetal.,2013),theestimationofWFsathigh
spatialandtemporalresolution(MekonnenandHoekstra,2010),
theexplorationoftheevolutionoftheglobalvirtualwatertrade
network(Dalinetal.,2012),thedevelopmentofWFbenchmarks
E-mailaddress:a.y.hoekstra@utwente.nl
forcrops(MekonnenandHoekstra,2014),theestimationofblue
waterscarcityinriverbasinsbasedonblueWFs(Hoekstraetal.,
2012),theassessmentofwaterpollutionlevelsinriverbasinsbased
onnitrogenandphosphorus-related greyWFs(Liuet al.,2012;
MekonnenandHoekstra,2015),studyinginter-annualvariability
ofWFs(Sunetal.,2013),assessingWFuncertainties(Zhuoetal.,
2014),theexplorationoftheuseofremotesensing(Romaguera
etal.,2010)andthedevelopmentoffutureWFscenarios(Ercinand
Hoekstra,2014).ApplicationsoftheWFvarywidely,from
prod-uctassessments(Chapagainetal.,2006),sectorstudies(Mekonnen
etal.,2015),dietassessments(Vanhametal.,2013),national
stud-ies(Ercinetal.,2013),catchmentstudies(Zengetal.,2012)toglobal
assessments(HoekstraandMekonnen,2012a).
Since 2009 the life cycle assessment (LCA) community has
shown interest in the WF concept, because of its relevance in
comparingtheenvironmentalperformance ofproducts.TheWF
as developed and appliedwithin the water resourcesresearch
community has received criticism from the LCA community
for not appropriately accounting for differences in potential
http://dx.doi.org/10.1016/j.ecolind.2016.02.026
environmentalimpactofwaterusegivenregionaldifferencesin
waterscarcity.Therehasbeensomeexchangeoflettersbetween
bothcommunities (letterfromPfisterand Hellweg,2009; reply
fromHoekstraetal.,2009b;letterfromRidouttandHuang,2012;
replyfromHoekstraandMekonnen,2012b)andtherehavebeen
effortstocometofruitfulexchange(e.g.Boulayetal.,2013),which
howeverhavenotbeenverysuccessfulinbringingthetwo
com-munitiesclosertogether,asapparentforinstancefromtheletterby
PfisterandRidoutt(2014).Theyadvisethewaterresources
com-munityto“update”theWFAmethodologytobringitinlinewith
theirownLCAwork.Theproblem,however,isthattheLCA
commu-nityhastakenitsownpathofdevelopment,inadirectionthatcan
contributelittletoimprovewatermanagementandisthereforenot
veryinterestingforthewaterresourcescommunity.Thereisavast
bodyofliteratureinthefieldofWFA,whichhasevolvedovertime,
butshowsconsistencyandcoherence.Thewaterresources
com-munityhasbeenlittleresponsivetothedemandsfromtheLCA
communitytochangedirectionandadaptmethodstoperceived
LCAneeds.Thereisagoodreasonforthat:theso-called“better”
approachtowaterfootprintingasproposedintheLCAcommunity
isnotbetter.Itisnarrowlyfocussedonassessingpotential
environ-mentalimpactsofproducts,whilethebroaderissueofsustainable,
efficientandequitableallocationoflimitedfreshwaterresources
fromcatchmenttogloballevelremainsoutofscope.Thereisno
reasontochangeaWFAmethodthatisconsistentwithother
envi-ronmentalfootprintmethods(Giljumetal.,2011;Gallietal.,2012;
HoekstraandWiedmann,2014;Fangetal.,2014,2015)andsuitable
toaddressbigquestionsonwaterresourcesallocation(Hoekstra,
2013,2014)merelytofitthespecificgoalofLCA.Itshouldbethe
otherwayaround:theincorporationoffreshwaterintoLCAcould
betterbebasedoninsightsasdevelopedwithinthewaterresources
sciencecommunity.Thecurrentpaperaimstosupplyacritiqueon
thewater-scarcityweightedWFapproach,whichisatthecoreof
whatLCAauthorsproposeandatthebasisofthedisputewiththe
waterresourcesresearchcommunity.Theneedforthiscritiquehas
gainedurgencynowthattheideaofthewater-scarcityweighted
WFhasbeenadoptedinISO’sLCA-basedWFstandard(ISO,2014).
Section2explainstheconceptsanddefinitionsappliedbythe
two communities.Section3 contains theactualcritique onthe
water-scarcityweightedWFapproachaspromotedbyLCAauthors.
Section4concludeswithareflectiononthewayforward.
2. Conceptsanddefinitions
2.1. ThevolumetricWFinwaterresourcesstudies
TheWFisameasureofconsumptiveanddegradativefreshwater
use.TheconsumptiveWFincludesagreencomponent,whichrefers
totheconsumption ofrainwater,anda bluecomponent,which
referstotheconsumption ofsurface-orgroundwater(Hoekstra
etal.,2011).TheinclusionofthegreenWFenablesthebroadening
ofperspectiveonwaterresourcesbeyondthehistoricalfocusof
waterengineersonbluewater(FalkenmarkandRockström,2004).
ThedegradativeWF,theso-calledgreyWF,representsthevolume
ofwaterrequiredtoassimilatepollutantsenteringfreshwater
bod-ies(Hoekstraetal.,2011), anideathatbuildsontheconceptof
dilutionwaterrequirementearlierappliedbyPosteletal.(1996).
TheWFofonesingleprocessstep(aunitprocess)isthebasic
buildingblockofallWFaccounts(Hoekstraetal.,2011).TheWFof
aproductisthesumoftheWFsoftheprocessstepstakento
pro-ducetheproduct.TheWFofabusinessisthesumoftheWFsofthe
finalproductsproducedbythebusiness,whichincludesthe
oper-ationalWFofthebusinessaswellasitssupply-chainWF.TheWF
ofaconsumeristhesumoftheWFsofallproductsconsumed.The
WFofnationalconsumptionisthesumoftheWFsofthecountry’s
inhabitants,whichincludesaninternalcomponent(theWFwithin
the national territory for making products that are consumed
withinthecountry)andanexternalcomponent(theWFinother
countriesformakingproductsimportedbyandconsumedwithin
thecountryconsidered).TheexternalWFofnationalconsumption
ismadepossiblebyimportofwater-intensivecommodities.This
tradeimpliesso-calledvirtualwaterflowsbetweenexportingand
importingcountries(Hoekstra,2003).Finally,thetotalWFwithina
certainarea(e.g.amunicipality,provinceorstate,orahydrological
unitlikeacatchmentarea)issumoftheWFsofallprocessestaking
placewithinthearea.
TheWFconceptintroducedsupply-chainthinkinginthefield
ofwatermanagementandishelpfulinanalysingthelinkbetween
humanconsumptionandtheappropriationoffreshwater(Hoekstra
etal.,2011).Theconceptisusedtoassesswaterusealongsupply
chains,sustainabilityofwaterusewithinriverbasins,efficiencyof
wateruse,equitabilityofwaterallocationandrelianceon
exter-nalwater suppliesordependence onwaterinthesupplychain.
Thesustainabilityofwaterusecanbeevaluatedbycomparingthe
WFwithinanareatothemaximumsustainableWFinthatarea
(Hoekstra,2014).Efficiencyofwaterusecanbeassessedby
com-paringtheWFofaspecificprocessorproducttoaWFbenchmark
forthatprocessorproduct,whichcanbebasedonbestavailable
technologyandpractice(MekonnenandHoekstra,2014;Chukalla
etal.,2015).Equitabilityofwaterusecanbediscussedby
com-paringthe WFsrelated totheconsumption levelsand patterns
ofdifferentcommunities(Seekell,2011;Hoekstra, 2014).Water
dependencyandsecuritycanbeassessedbyanalysingtheextentto
whichcompaniesorcommunitiesdependonunsustainablewater
useintheirsupplychain(Ercinetal.,2013).Commoninthevarious
typesofanalysisisthestudyofhowwatervolumesareallocated
tocompetingdemands.Countingwatervolumesisakeyelement,
whichexplainstheuneaseinthewaterresourcescommunityto
talkintermsof“weightedcubicmetres”ofwater,whichisseenas
keyintheLCAcommunity.
2.2. Thewater-scarcityweightedWFinLCA
CritiqueonthevolumetricWFandtheideatoweightconsumed
water volumes based onlocal water scarcity emerged in 2009
(Ridouttetal.,2009;Pfisteretal.,2009;RidouttandPfister,2010).
TheproposalhadenormoustractionwithintheLCAcommunity,
whichhadjuststartedtoaskhowwaterusecouldbeincorporated
intoLCA(Koehler,2008;MilàiCanalsetal.,2009).Therationale
isstraightforward.ThepurposeofLCAstudiesistoestimatethe
differentsortsofpotentialenvironmentalimpactattributableto
thelifecycleofaproduct,fromcradletograve(HellwegandMilài
Canals,2014).AnLCAisacomparativeanalysisofpotential
environ-mentalimpactsofalternativeprocessesorproducts,forinstance
whenusingalternativematerialsordesigns(Rebitzeretal.,2004).
Alifecycleinventory(LCI),whichcompilesnaturalresourcesuse
and emissionsfor eachprocess inthelifecycleof aproduct,is
followedbyalifecycleimpactassessment(LCIA),whichincludes
aselectionofthe“environmentalimpactcategories”ofinterest,
acalculationof“impactcategoryindicators”basedoninventory
datausing“characterisationfactors”(characterisation)and
option-allyacalculationof“impactcategoryindicatorresults”relativeto
referencevalues(normalisation)andagroupingand/orweighting
oftheresults(Penningtonetal.,2004).
Thecarbonfootprint(CF)isoneofthepopular“impactcategory
indicators”,fortheimpactcategoryofclimatechange.The
emis-sionsofdifferentgreenhousegasesareweightedbasedontheir
“globalwarmingpotential”(GWP)relativetocarbondioxide(e.g.
onekgofmethanehasamuchgreaterGWPthanonekgofcarbon
dioxide).Theweightingistechnicallycalled“characterisation”of
arethecharacterisationfactors.TheresultantCFisexpressedin
termsoftonnesCO2-equivalents.
Theideatocalculateawater-scarcityweightedWFintheLCIA
stagehasbeeninspiredbythewaytheCFhasbeenincorporated
intoLCAprocedures(PfisterandHellweg,2009;RidouttandPfister,
2010). Animplicit choice made – which is an important point
towhich Iwillcomebackin thenextsection–wasthat water
useitselfisnot interesting,butthatthefocusshouldbeonthe
potentialenvironmentalimpactofwateruse.Giventhatfocus,the
questionwashowto“characterize”waterconsumption,i.e.what
characterizationfactortouse.Thelogicfollowedwasthatonelitre
ofwaterconsumptioninawater-scarcebasinisworsethanthe
samewaterconsumptioninawater-abundantbasin,hencethe
pro-posaltousewaterscarcityasthecharacterizationfactorofwater
consumption.Usingwaterscarcityasacharacterizationfactorfor
waterconsumptionwaspresentedasananalogytousingGWPas
acharacterizationfactorofgreenhousegasemissions–whichisn’t
aproperanalogythough,asIwillshowinthenextsection.
TheabovewasallabouttheblueWF.ThegreenWFwasputaside
asirrelevant,becausethegreenWFdoesn’taffectrunoffand
there-forenot(blue)waterscarcity(PfisterandHellweg,2009;Ridoutt
and Pfister, 2010).Thegrey WF hasreceivedmixed responses,
becauseontheonehandthegreyWFhasbeenregardedasalready
includingpropercharacterization(becauseloadsofdifferent
chem-icalsaremadecomparablebycalculatingthewatervolumeneeded
toassimilatethem based onthedifference betweenmaximum
allowableandnatural concentrationof thechemical), whileon
theotherhandthereissomeuneasewiththegreyWFbecauseit
overlapswithsomeotherexistingenvironmentalimpactcategories
(likeeutrophication).
3. Acritiqueonthewater-scarcityweightedWF
3.1. Historyandbackground
Thepointsofcritiquethatwillbeelaboratedinthenext
sec-tionsfollowfromacoreassumptionmadebyLCAauthorsregarding
whatconstitutestheessenceofthewateruseproblem.TheLCA
method aims to consider both input-related impact categories
(naturalresourcedepletion)andoutput-relatedimpactcategories
(pollution)(UdodeHaes,2002),butrecentLCAliteraturedoesn’t
makethisdistinctionandratherspeaksgenerallyabout
“environ-mentalimpactcategories”(HellwegandMilàiCanals,2014).The
issueofwateruseistreatedasacategoryofenvironmentalimpact,
wherebyinsufficientthoughthasbeenonwhatactuallyisthe
envi-ronmentalissue.Obviously,theissuecoversbothwaterdepletion
andpollution,whichmakesitaheterogeneouscategory.Butthe
LCAcommunityhasprimarilyjumpedontheissueofconsumptive
wateruse(giventhatwaterpollutionisalreadypartiallycovered
throughotherenvironmentalimpactcategorieslike
eutrophica-tion)andtherebyfocussedon“impactofwaterconsumption”.The
latterimpliesthatwater useinitselfisn’tregardedasthe
envi-ronmentalissue,butrather“environmentalimpactofwateruse”.
Theissueofwaterdepletion(whichfractionsoftheavailablegreen
andbluewater resourcesandwhich partoftheavailablewaste
assimilationcapacityarealreadyappropriated)isthusignored.
AlogicconsequencewasthattheLCAcommunitywasn’t
sat-isfiedwithametric ofjustwateruse(thevolumetric greenand
blueWFs)andstartedsearchingforametricthatcanrepresentthe
environmentalimpactofwateruse.Confusingly,LCAscholarstook
theexistingWF,whichhadbeendefinedasawateruseindicator,
andstartedtocriticiseitfornotbeingawateruseimpactindicator.
ThereasoningwasthattheWFconcepthadtobetransformedinto
anothermetric,toservethepurposeofawateruseimpact
indi-cator.Thisdevelopmenthasbeenunfortunate,becauseitwould
havebeenbetterifanothertermhadbeenchosen,e.g.aWFimpact
index,asproposedbytheWaterFootprintNetwork(Hoekstraetal.,
2011).It wouldhavepreventedthecurrentdisputeover
termi-nology.Therewasanimportantreasonfor thewaterresources
researchcommunitytostaywiththeWFasanindicatorofwater
use:theWFhadbeendevelopedandusedtofeeddiscussionsabout
sustainable,efficientandequitableallocationoflimitedfreshwater
resourcesandaboutresourcesecurity,giventhatmanycountries
dependonwaterresourcesoutsidetheirterritory.Theessenceof
allocatinglimitedwaterresourcesisaboutallocatinglitresamong
competinghumanpurposesandaboutallocatingwithin
sustaina-bilitylimits, respecting environmentalwater needs. If properly
allocated,withinsustainability limits,respecting environmental
waterneeds,theenvironmentalimpactswillremainwithin
accept-ablelimits.Theissueofenvironmentalimpactsisthuspartofthe
largerthemeofsustainable,efficientandequitableallocationof
limitedwaterresources.RedefiningtheWFconcepttoreferto
envi-ronmentalimpactofwateruseisnotinstrumentaltothislarger
theme.Allocationisaboutallocating litres,notaboutallocating
scarcity-weightedlitres.
3.2. TheenvironmentalrelevanceofwaterproductivityandWFs
inwater-richareas
Theproponentsofthewater-scarcityweightedapproachhave
persistentlypointedattheneedfor“environmentalrelevance”of
theWFindicator(PfisterandHellweg,2009;RidouttandHuang,
2012;BergerandFinkbeiner,2013).Thishasbeeninterpretedas:it
shouldreflectenvironmentalimpactofwateruse.Bluewater
con-sumptioninawater-scarcecatchmentisregardedtohavepotential
environmentalimpact,becauseitreducesrunoff andmayaffect
downstreamecosystemsandlivelihoods.Asimilaramountofblue
water consumption in a water-rich catchmenthas less impact
andwouldthereforehavesmallerenvironmentalrelevance.Green
waterconsumptiondoeshardlychangerunoff(sinceevaporation
fromafarmlandorproductionforestisinthesameorderof
magni-tudeasevaporationfromnaturalvegetation)andwouldtherefore
havenoenvironmentalrelevanceatall.Theproblemwiththis
rea-soning,however, is that theterm“environmental relevance” is
interpretedinatoonarrowsense.
Asubstantialcomponentofthesolutiontooverexploitationof
bluewaterresourcesinwater-scarceregionsistouse(greenand
blue)water resources inwater-rich regions more productively,
becauseproducing morewater-intensiveproductswhere water
is sufficient takesaway theneed toproduce those productsin
placeswherewaterisscarce.Improvinglandandwater
produc-tivityinrain-fedagricultureinallthoseregionswithsufficientrain
wouldreducetheneedforirrigatedagricultureinregionsthatare
basicallyunsuitableforcropproductiongiventhelimited
availabil-ityofwater(Rockströmetal.,2009).Byfocussingonbluewater
consumptioninwater-scarcebasins,oneoverlookstwoimportant
featuresofwater:(1)waterisaglobalresource:water-intensive
commoditiescanbetradedfromwater-richtowater-poorriver
basins,whichmeansthatwhereintheworldwaterisbeingused
andhowmuchispartlysubjecttotheworkingoftheglobal
econ-omy (Hoekstraand Hung, 2005; Hoff,2009; Vörösmartyet al.,
2015);and(2)bluewaterusecannotbeconsideredindependently
from green water use. Since water is a global resource, water
depletionhasaglobalcharacteraswell.Globalwater
availabil-ityisthesumofthewateravailabilityinthevariousbasinsinthe
world;someofthemcontributealottooverallavailability,others
onlyalittle.Everylitreofwaterconsumption–whetherit’sina
water-richorwater-poorriverbasinandwhetherit’sgreenorblue
water–willreducethewatervolumeremainingforotheruses
andthushasequalenvironmentalrelevance.Thefactthatgreen
inefficientlyused(i.e.lowwaterproductivityintermsof
produc-tionunitsperm3orlargeWFintermsofm3perproductionunit),is
highlyenvironmentallyrelevant,becausehereliespartofthe
solu-tiontotheproblemsinwater-poorareas:producingmorecrops
withthewaterinwater-richbasinsreducestheneedtoproducein
water-poorbasinsandthushelpstoreducethewaterconsumption
andscarcityinthosewater-poorbasins.Lookingatthecontribution
ofaproducttolocalwaterdepletionshouldn’tbethemerefocusin
aproduct-LCA.Comparingtheenvironmentalperformanceoftwo
cottonshirts,forinstance,requirestolookatboththetotal(green
andblue)waterconsumptionunderlyingeachshirtandthefraction
ofthetotaltakingplaceinriverbasinswhereoverallwater
con-sumptionlevelsaresohighthatminimumenvironmentalwater
needsarenolongermet.
Thethemeofwaterconsumptioncanbecomparedtothatof
landuse.Theenvironmentalissuearoundlanduseistwofoldas
well.Thefirstconcernisthatoveralllandusekeepsonrising,
caus-inggloballandscarcity;remindthe1.5Earthsweneedtosustain
ourcurrentglobaleconomy(Boruckeetal.,2013).Thesecond
con-cernisthatsomeformsoflanduse(e.g.urbanland)havelarge
localenvironmentalimpact(largerthanotherformsoflanduse,
likee.g.productionforest).Theissueofdifferentlocal
environmen-talimpactsofdifferentformsoflanduseisnoreasontoignorethe
concernoftotallanduse.Aproductwithlargerlandrequirement
toproduceitisofgreaterenvironmentalconcernthanasimilar
productwithsmallerlandrequirement.Thesameistrueforwater:
aproductwithlarger(volumetric)greenandblueWFtoproduce
itisofgreaterenvironmentalconcernthanasimilarproductwith
smallerWF.
Iwill illustrate theinsufficiency of thegeographic focus on
water-scarcebasinswithasimpleexample.Supposethe
hypothet-icalcaseoftworiverbasins,withthesamesurface(Table1).BasinA
isrelativelydry,withawateravailabilityof50waterunitsperyear.
Farmersinthebasinconsume100waterunitsperyeartoproduce
100cropunits.TheWF(100)thusexceedsthemaximum
sustain-ablelevel(50).BasinBhasmorewater:250waterunitsperyear.
Farmersinthisbasinconsume200waterunitsperyear,toproduce
100cropunits,thesameamountasinbasinA,butusingtwotimes
morewaterpercropunit.InbasinB,theWF(200)remainsbelow
themaximumlevel(250),sothisissustainable.Accordingtothe
logicofLCAauthors,theenvironmentalperformanceisgoodfor
thecropsoriginatingfrombasinBandbadforthosefrombasinA.
Fromageographicperspective,thisistrue:theWFofcrop
produc-tioninbasinAneedstobereduced,thatseemstobethecrux.From
aproductperspective,however,weobservethattheWFpercrop
unitinbasinBistwotimeslargerthaninbasinA.Ifthefarmers
inbasinBwouldachievethesamewaterproductivityasinbasin
A,theywouldproducetwiceasmanycropswithoutincreasingthe
totalWFinthebasin.IffarmersinbasinAcannoteasilyfurther
increasetheirwaterproductivity,theonlysolution–inorderto
maintainglobalproduction–istobringdowntheWFinbasinAto
asustainablelevelbycuttingproductionbyhalf,whileenlarging
productioninbasinBbyincreasingthewaterproductivity.When
inbasinBthesamewaterproductivityisachievedasinbasinA,
globalproductionwould increasewhilehalving thetotal WFin
basinAandkeepingitatthesamelevelinbasinB.Thefactthat
cropsinbasinBhadavolumetricWFoftwicethatinbasinAwas
thushighlyenvironmentallyrelevantinformation.
3.3. Theneglectofgreenwateruse
TheLCAcommunityhasthusfarneglectedgreen water
con-sumptionasarelevantresourceusemetric.AccordingtoPfister
andHellweg(2009),greenwaterconsumptioninagricultural
pro-ductioncanbeneglectedifgreenwaterconsumptioninthecrop
fieldiscomparabletothatbytheoriginalnaturalvegetation,which
isgenerallythecase.RidouttandPfister(2010)arguethatgreen
waterconsumptiondoesn’tcontributetowaterscarcityandthat,
duetotheinseparabilityofgreenwaterandland,the
consump-tionofgreenwaterisbetterconsideredinthecontextoflanduse
impacts.This,however,reflectsalimitedviewontheissueof
sus-tainablewaterresourcesuse.Itistruethatrunoff(bluewater)will
notchangesignificantlyasaresultofgreenwaterconsumptionand
thatgreenwaterresourcesareinseparablylinkedtoland.Itisnot
right,though,tosaythatgreenwaterresourcesarenotscarce.Itis
verycommonthatfarmersstructurallysufferfromshortageofrain.
Conflictsoverbluewaterallocationamongfarmersoccurprecisely
forthereasonthatgreenwaterresourcesareinsufficient.Green
watershortageinagricultureisinfactthereasonforagriculture’s
bluewaterdemandandthereforethedriverofbluewaterscarcity.
Muchofthetroublearoundbluewaterscarcityrelatestothe
historical focus of engineers and policy makers on blue water
resources exploitation and theneglectof green water use. The
insightthatgreenandbluewaterresourcesuseshouldbe
consid-eredincombinationemergedinthewaterresourcescommunity
in the second half of the 1990s (Falkenmark, 1997) and has
receivedincreasingattentionsince (Falkenmark andRockström,
2004,2006).Whenemphasizingthatgreenwaterconsumptioncan
beignoredinanLCAbecauseitcannotbeconsideredasan
addi-tionallosstothewatershed,PfisterandHellweg(2009)actually
arguethatgreenwaterconsumptionisnotbluewater
consump-tion,whichisrightofcourse,butwhichbetraystheirpreoccupation
withtheidea thatbluewaterconsumption istheonlyrelevant
thing.RidouttandPfister(2010)areexplicitinthisrespectby
argu-ingthatgreenwaterresourcesconsumptionisnotrelevantbecause
itdoesn’tcontributetobluewaterscarcity.
Green water resources are often not perceived as scarce,
becauseraincomesforfree,butactuallytheyare(Savenije,2000;
Falkenmark,2013).Therearealternativecompetingusesforgreen
water(e.g.productionoffoodcrops,feedforanimals,energycrops,
fibrecropsortreesfortimberandpaper)andthereisaconflict
between appropriatinggreen water resourcesfor the economy
versusleavingthemfornaturalvegetation(Schynsetal.,2015).
Competingdemandsfora limitedresourcedefinestheresource
asscarce.Whenallavailablegreenwaterresourcesarefullyused
wecansaythattheresourceisdepleted.Thisisthecaseinmany
regionsoftheworld,wherehardlyanylandandassociatedgreen
waterisleftfornaturalvegetation.It’snotsufficienttofocusonland
Table1
Exampleofhowoverexploitationinawater-stressedriverbasin(A)canbesolvedbyincreasingwaterproductivityinawater-abundantbasin(B).
Parameter Unit Currentsituation Possiblesolution
BasinA BasinB BasinA BasinB
MaximumsustainableWF Waterunitsperunitoftime 50 250 50 250
(Volumetric)WF Waterunitsperunitoftime 100 200 50 200
Production Productunitsperunitoftime 100 100 50 200
WFperproductunit Waterunitsperproductunit 1 2 1 1
Waterproductivity Productunitsperwaterunit 1 0.5 1 1
appropriationandneglectgreenwaterconsumptionasproposed byRidouttandPfister(2010),becausewecannotdisconnectgreen
andbluewaterresources,ignoretheformerandfocusonthelatter.
Thebiggerissueisfreshwaterscarcityingeneral,i.e.competition
overprecipitation,theundifferentiatedformoffreshwater,which
willpartitioninagreenflow(evaporation)andblueflow
(ground-waterrecharge/surfacerunoff)(FalkenmarkandRockström,2006).
Theworld’slargestconsumerofbluewater,i.e.irrigated
agricul-ture,usesalotofgreenwateraswell.Greenandbluewaterscarcity
and depletionin a catchment arestrongly connected. The
rea-sonwhycropsareirrigatedisthattherainisinsufficienttogive
agoodcropyield.Inallcatchmentswithsignificantbluewater
scarcityasaresultofbluewaterconsumptionin irrigated
agri-culture,greenwaterresourcesarescarceaswell,otherwisethere
hadn’tbeenthedemandforirrigation.Onecannotgetagood
pic-tureofwaterscarcityifthefocusisonbluewaterresourcesalone.
Ifrain-fedagricultureproducesmore(closingtheso-calledyield
gapandincreasinggreenwaterproductivity),thereislessneedfor
irrigatedagriculture,thusreducingbluewaterscarcity.Anessential
componentinsolvingtheoverconsumptionofbluewaterresources
andassociatedenvironmentalimpactsinwater-scarceareasisto
usegreenwaterresourcesmoreproductivelyinwaterabundant
areas,becauseifwater-intensiveproductsareproducedinareas
wheresufficientwaterisavailable,thereisnofurtherneedto
pro-ducethoseproductsinareaswhereinsufficientwaterisavailable
(Hoekstra,2014).AlargegreenWFofacrop(inlitre/kg)
repre-sentslowgreenwaterproductivity(kg/litre)andshouldtherefore
becountedinLCAasworsethanasmallgreenWF.Ignoringthis
aspectmeansthatanessentialelementin(indirect)environmental
impactisoverlooked.
3.4. Squaringthefootprintandbeingchargedforthefootprintof
others
Theideaofawater-scarcityweightedWFleadstothesurprising
andundesirablesituationinwhichtheWFofaspecificwater
con-sumerorcompanywillinherentlybeafunctionoftheWFofothers.
Wewillthusfacetheconfusingsituationinwhichanincreasingor
decreasingWFofa specificactivity,product,consumeror
com-panymaytelllittleaboutthechangedenvironmentalperformance
ofthatactivity,product,consumerorcompanybutratheraboutthe
changedenvironmentalperformanceofothers.Thisstrange
impli-cationofthewater-scarcityweightedWFcaneasilybeillustrated.
Thewater-scarcityweightedWFofanactivityorproduction
processiinacertaincatchment(WF∗
i)canbecalculatedby
mul-tiplyingthevolumetricWFofthatactivityorproductionprocess
(WFi)bythewaterscarcity(WS)inthecatchment:
WF∗
i =WFi×WS=WFi×WFWAt =WFi×
ni=1WFi
WA
wherebyWSis theratioof thetotal volumetricwater footprint
(WFt)inthecatchmenttothewateravailability(WA).WFtisequal
totheaggregatevolumetricWFsofallactivitiesninthecatchment.
Thisapproach hastwooddimplications.Thefirstisthatthe
overallWS-weightedwaterfootprintinacatchment(WF∗
t)willbe definedas: WF∗ t =WFt×WS=WFt×WFWAt =(WFt) 2 WA
ThereisnologicindefiningtheWFwithinacatchmentasthe
squareofthetotalwaterconsumptioninthecatchmentdividedby
wateravailability.Ifthis footprint-squareapproachwerecopied
tothecarbonfootprint(CF)conceptwewouldgetaCFdefined
as something that increases withthe square of the volume of
greenhousegasemissions,whichisobviouslyanoddapproach.It’s
equallyoddtodothisforwater.
Thesecondoddimplicationisthat,whenWS-weighted,theWF
ofaconsumerorcompanywillnotonlygoupifaconsumeror
companyincreasesitsownwaterconsumption,butalsoifother
consumersorcompaniesincreasetheirwaterconsumption.
Imag-ineananalogousCFdefinitionwherebytheCFofacompanygoesup
whilethecompanyfactuallyreducesitsgreenhousegasemissions
becausetheincreasinggreenhousegasemissionsofotherscountin
theCFofthiscompanyaswell.ThiswouldmaketheCFuselessfor
thecompanyasanindicatorofitscontributiontoglobalwarming.
Exactlythesameisthecaseforwater:theWFbecomesuselessfor
acompanyasanindicatorofitscontributiontoWSifthe
indica-torisaffectedbythecontributionsofotherstoWS.Nonetheless,
theISOstandard forWFprescribescompaniestocalculatetheir
WFbasedonaWS-weightedapproach(ISO,2014).Wethushave
gotastandardwherebytheWFofanactivityinacatchmentwill
dependontheWFofotheractivitiesinthecatchment.Ironically,the
WFofacompanywillinevitablyincreaseiftheWFsofother
compa-niesincrease,punishedforthebadenvironmentalperformanceof
others.
Ifwewantaproxyforpotentialenvironmentalimpactofwater
consumptiononrunoffinacatchment,waterscarcity(or“relative
water scarcity” ifWS metricsfor differentcatchments are
nor-malizedbasedontheWSinonespecificriverbasinorcountryas
proposedbyPfisterandHellweg,2009)isnotaproper
character-izationfactor.The“runoffimpactpotential”ofonelitreofwater
consumptionislargerinacatchmentwithrelativelysmallnatural
wateravailability(WA)thaninacatchmentwithrelativelylarge
naturalWA.Therefore,“relativewateravailability”isabetter
met-ricfor“runoffimpactpotential”than“relativewaterscarcity”.In
LCAterminology:ifvolumetricWFsaretobeinterpretedinterms
oftheirpotentiallocalenvironmentalimpact,thentheybetterbe
multipliedwithacharacterizationfactorthatreflectsrelativeWA
thanwithafactorthatreflectsWSorrelativeWS.
Itisproposedheretoabandontheideaofweightingbasedon
WSasproposedbyRidouttandPfister(2010)andotherLCAauthors
andasprescribedbyISO(2014),becausetheideaisbasedona
fun-damentalerrorinlogic.WeightingvolumetricWFsmoreheavily
ifWS increasesis similartofollowinga logicof weightingone
tonneofgreenhousegasemissionsmoreheavilyifglobal
warm-ingprogressesorweightingonehectareoflandusemoreheavily
iflandbecomesscarcer.Amoresoundwayofgettingaproxyfor
potentialenvironmentalimpactofwaterconsumptionindifferent
catchmentsistoweightvolumetricWFsbydividingthemby
rela-tiveWAinthecatchmentsconsidered(insteadofmultiplyingthem
withrelativeWS).
Thewater-availabilityweightedwaterfootprint (WF∗∗
i )ofan
activityiinacatchmentcanbedefinedas:
WF∗∗
i =WA/WAWFi
ref
wherebyWArepresentsthewateravailabilityinthecatchmentand
WArefthewateravailabilityinareferencecatchment.Dividingby
WArefisdoneinordertonormalizethevalueofWA.
The water-availability weighted WF of all water-consuming
activitiesinacatchmentis:
WF∗∗
t =WA/WAWFt
ref
Thedifferencesbetweenthevolumetric,WS-weightedand
WA-weightedWFsareillustratedinTable2,whichincludescalculation
examplesforthreebasins,XtoZ,attwopointsintime.We
con-sideronespecificactivityA,takingplaceineachbasin,everywhere
withavolumetricWFof1waterunitperunitoftime.Inallthree
basins,thevolumetricWFofactivityAisassumedtodecreaseby
10%fromttot+1.WealsoconsiderthetotalWFofallactivities
Table2
Calculationofthevolumetric,water-scarcityweightedandwater-availabilityweightedblueWFinthreehypotheticriverbasinsattwopointsintime.
BasinX BasinX BasinY BasinY BasinZ BasinZ
Timet Timet+1 Timet Timet+1 Timet Timet+1
Wateravailability Wateravailability(WA) 100 100 100 100 200 200
RelativeWAa 1 1 1 1 2 2
Waterscarcity Waterscarcity(WS) 0.5 0.6 0.25 0.3 0.25 0.3
RelativeWSa 1 1.2 0.5 0.6 0.5 0.6
WFofactivityA VolumetricWF 1 0.9 1 0.9 1 0.9
WS-weightedWFb 1 1.08 0.5 0.54 0.5 0.54
WA-weightedWFb 1 0.9 1 0.9 0.5 0.45
TotalWFinthebasin VolumetricWF 50 60 25 30 50 60
WS-weightedWFb 50 72 12.5 18 25 36
WA-weightedWFb 50 60 25 30 25 30
aWAandWSinbasinXattimetarechosenasthereference. bExpressedintermsofBasinXwaterequivalentsasattimet.
basinisassumedtoincreaseby20%fromttot+1.Wecanmakefour
observationsfromthenumericalexamplesinthetable. First,in
allthreebasinstheWS-weightedWFofactivityAincreasesover
timewhileactualwaterconsumptionofactivityAdecreases,which
illustratestheinappropriatenessofthemetric asanindicatorof
theindividualcontributionofanactivitytopotential
environmen-talimpact.Second,inallthreebasinsthetotalWS-weightedWF
doesn’tincreaselinearlywithincreasingwaterconsumptioninthe
basin(factor1.2)butexponentially(factor1.44),whichlacksany
logic.Third,ifwecomparebasinsXandY,whicharesimilarbasins
butonlydifferintermsofthefractionoftheavailablewateralready
consumed(thetotalvolumetricWFinbasinXistwotimesbigger
thaninbasinY),weseethattheWS-weighted WFofactivityA
inbasinYishalfofthatinbasinX,whilewetalkaboutthesame
activitywiththesamewaterconsumptionintwobasinsnaturally
endowedwiththesameamountsofwater.Morelogically,the
WA-weightedWFofactivityAisthesameinbothbasins.Finally,when
comparingbasinsXandZweseethatwateravailabilityinZistwo
timesthewateravailabilityinX,whilethetotalvolumetricWFsin
bothbasinsarethesameandincreasingovertimeatthesamerate.
Overtime,therelativeWAinZ(comparedtoX)remainsconstant,
whiletherelativeWSinZincreases.Asaresult,theWS-weighted
WFofactivityAinZincreaseseventhoughtheactualwater
con-sumptionofactivityAdecreases.TheWA-weightedWFofactivity
AinbasinZdecreaseswiththesamerateasthevolumetricwater
consumptionoftheactivity.IntheWA-weightedcase,1unitof
waterconsumptioninZisequivalenttohalfaunitofwater
con-sumptioninX,becausewateravailabilityinZistwotimesbigger
thaninX.
ItshouldbenotedherethattheLCAliteraturereferstovarious
alternativeWSindicatorsthatcouldbeusedtoweightconsumed
watervolumes(JeswaniandAzapagic,2011;Kouninaetal.,2013;
Boulayetal.,2015a,2015b,2015d).Theaboveargumenthasbeen
builtontheassumptionthatWSisdefinedasthetotal
volumet-ricWFdividedbythewateravailabilityinthecatchment.Many
WSindicatorsthathavebeenproposedwithintheLCAcommunity
lookdifferent,includingforinstancetheWaterStressIndex(WSI)
ofPfisteretal.(2009)ortherecentlyproposedinverseofthe
Avail-ableWaterRemaining(AWaRe)perm2,withtheavailablewater
remainingbeingmeasuredasthetotalwateravailabilityina
catch-mentminusthehumanandenvironmentalwaterdemands(Boulay
etal.,2015d).Onemaywonderwhethertheargumentagainstthe
WS-weightedWFholds ifthesevariousotherdefinitions ofWS
areapplied.Thisiscertainlythecase,sinceanymetricofWSwill
increaseifthevolumetricWFinabasinincreases.Thisisalsothe
caseforPfister’sWSI,althoughtheeffecthereisobscuredbythe
complexityofthatindex(seeSection3.6),orBoulay’sinverseof
AWaReperm2.WhateverWSindicatorisused,itwillpositively
relatetothevolumetricWFinthecatchment,withtheinevitable
effectthattheWS-weightedWFofaspecificactivityorprocesswill
increaseifotheractivitiesorprocessesconsumemorewater.
Anothernoteistobemadeonthemeasurementofwater
avail-ability (WA). One can measure total runoff (Vörösmartyet al.,
2000)ornaturalrunoffminusenvironmentalflowrequirements
(Hoekstraetal.,2011,2012),wherebythelatterisbetterbut
requir-ingmoredata.Allvariables–WA,WFandWS–canbemeasuredon
annualormonthlybasis.Obviously,measurementpermonthwill
capturetheintra-annualvariabilityinthethreevariables,which
willbelostincase ofmeasurement onannual basis.Therefore,
boththewaterresources(Hoekstraetal.,2011,2012;Wadaetal.,
2011)and LCAcommunity (Pfisterand Bayer, 2014)will easily
agreethatmonthlymeasurementistobepreferredoverannual
measurement.
3.5. Inconsistencywithotherfootprintdefinitions
SeveralLCAscholarshavepointedattheneedtoweightwater
consumption basedonlocalWS withtheargumentthatthis is
consistentwithcarbonfootprint(CF)accounting,whereemissions
ofgreenhousegasesareweightedbasedontheirglobalwarming
potential(Pfisterand Hellweg,2009; RidouttandPfister, 2010;
Kouninaetal., 2013;Boulayetal., 2015b).By multiplyingeach
consumedlitreofwaterbyalocalWSfactorbetweenzeroand1,
waterconsumptioncanbeexpressedinlitresofH2O-equivalents
(Boulayetal.,2015b).Onelitreofwaterconsumedinanareawith
aWaterStressIndexof0.5,whichreferstothethresholdbetween
‘moderate’and‘severe’waterstress,wouldthuscountas0.5litre
ofH2O-eq.ApartfromthefactthattheseH2O-equivalentshave
nomeaningfulphysicalinterpretation(unlikeCO2-equivalentsthat
dohaveameaning)andthefactthattheuseofdifferent,
alterna-tiveWSindicatorsleadstodifferentweightingsandthusdifferent
andincomparablesortsofH2O-equivalents(Boulayetal.,2015a),
thereisafundamentalerrorinreasoninghere.Thewater-scarcity
weightedWFisnotconsistentwiththegeneralfootprintconcept
atall.
Common to all environmental footprintsis that they
quan-tifythehumanappropriationof naturalcapitalasasourceora
sink(HoekstraandWiedmann,2014).Thefootprintthatwasfirst
introducedistheecologicalfootprintandmeasuresthe
appropri-ationoflandasaresourceandthelandneededforwasteuptake
(CO2absorption)(WackernagelandRees,1996).The(volumetric)
WFmeasuresboththeconsumptionoffreshwaterasaresource
(the green andblue WF)and theuseof freshwater to
assimi-latewaste(thegreyWF)(HoekstraandMekonnen,2012a).TheCF
measures emission of greenhouse gases to the atmosphere
materialextraction(Wiedmannetal.,2015).Inallcases,footprints
measurethevolumeofresourceuseand/oravolumeofemission,
andassuchrepresentacertainpressureexertedbyhumansonthe
environment.Innoneofthecasesthefootprintstellsomething
abouttheresultantimpact.Footprintsbecomemeaningfulwhen
evaluatedagainstmaximumsustainablelevels,whichrelatetothe
carryingorassimilationcapacityoftheenvironment.
TheCFhasbeenadoptedinLCAstudiesasaproxyforimpact
(asamid-pointimpactindicator),whichhasbeenamajorsource
ofconfusion,sincemanyscholarshavestartedtoconsiderCFas
anindicatorofimpactandexpecttheWFtofulfilthatroleaswell
(PfisterandHellweg,2009;RidouttandPfister,2010).However,the
CFcanonlybeinterpretedasapressureindicator,becauseitsimply
measuresgreenhousegasemissions–indeedinCO2-equivalentsto
bringthedifferenttypesofemissionunderonecommon
denom-inator –and tells nothingabout theresultantimpacts, suchas
changingspatial patternsof temperature, evaporationand
pre-cipitationoraboutmeltingglaciersandicecapsorsealevelrise,
letalonesomethingaboutfinalimpactsonhumanwell-beingor
ecosystemintegrity.TheideaofCFasanimpactindicator,
how-ever,hastakenholdandhasledtotheclaimthatWFshouldshow
impactaswell.Thesteptowardsweightingwaterconsumption
basedonlocalWSthenseemedalogicalstep.Asaconsequence,
however,theWFwouldbecomeinconsistentwiththegeneralidea
offootprintsasmeasuresofresourceuseand/orwastegeneration.
Comparingthefootprintsof two alternativeproductsmakes
alwayssense,becausethesizeofafootprinttellstheamountof
resourcesuseoremissionperunitofproduct.Fromaglobalpoint
ofview,onecanalwayssaythatthesmallerthefootprintthe
bet-ter(undertheconditionofothercircumstancesremainingequal;
ifthatisnotthecase,inevitabletrade-offsmaybeinvolved).This
istruefortheamountoflandusebehindaproduct,theamountof
greenhousegasemissions,andalsoforthevolumeofgreenenblue
waterconsumptionandthesizeofthegreyWF.Footprints
repre-senttheoverallpressureontheglobalenvironment.Impactswill
becomemanifestlocallyandmaydifferacrossregions.Alargeland
footprintperunitofaproductinabig,thinlypopulatedcountry
maymatterlittlefromalocalenvironmentalperspective.Similarly
alargeWFperunitofproductinawater-abundantcatchmentmay
matterlittlefromalocalpointofview.Onemayevenaskwhether
aCFandtheresultantglobalwarmingmattersalotforaregion
thathappenstobebetteroffthroughclimatechangeinsteadof
worse.Theissueisthatweneedtodifferentiatebetweenglobaland
localenvironmentalrelevance.LCAauthorshavemadetheimplicit
choiceinthecaseofwaterusetofullyignoretheglobalpressure
exercisedbyincreasingvolumetricwaterdemands.Inthisway,for
example,biofuelsproducedinwater-abundantareascompletely
disappearfromtheradarofenvironmentalconcern,while
actu-allythequicklyincreasingdemandforbiofuelsmaybeoneofthe
mostimportantdriversofwatershortagesinthefuture(
Gerbens-Leenesetal.,2012).Thesamecanbesaidfortheproductionof
animalproductsinwater-abundantareasorinregionswhere
live-stockmainlydependsonrain-fedgrassorfeedcrops.Thequickly
increasingdemandformeatanddairypercapitaisasignificant
driverbehindtheincreasingWFofhumanity(Liu andSavenije,
2008;ErcinandHoekstra,2014),withvariouslocalizedproblems
asaresult.ItisamajorerrorinLCAtoomitthevolumetricWF
ofproducts,becauseitwhitewashesproductsthatarecausingan
increasingpressureontheworld’sscarcefreshwaterresourcesand
shouldbeamajorenvironmentalconcern.
3.6. ThelackofphysicalinterpretationoftheWaterStressIndex
Whereastheprevioussectionsincludefundamentalcritiqueon
theweightingofWFs,therearealsosomeproblemsaroundthe
practicalproposalsthathavebeenmadeonwhichweightingfactor
touse, i.e.howtomeasurewaterscarcity(WS).Themostcited
methodtoestimateWSinLCAistheWaterStressIndex(WSI)by
Pfisteretal.(2009).TheydefineWSIpercatchmentasfollows:
WSI= 1
1+e−6.4×VFp×WTA×(1/0.01−1)
in which WTA representsthe annual withdrawal-to-availability
ratiointhecatchment(calculatedasthetotalannualgrosswater
withdrawal dividedbytheannual freshwater availability),VFa
fixedvariationfactorreflectingmonthlyandannualtemporal
vari-abilityofwateravailabilityinordertoaccountforincreasedscarcity
inwatershedswithirregularwateravailability,andpanexponent
equalling0.5forcatchmentswithstronglyregulatedflowsand1
forcatchmentswithoutstronglyregulatedflows.ThefactorVFis
definedasfollows: VF=
n i=1e ln(s∗ m,i) 2 +ln(s∗ y,i) 2 ×Pi n i=1Piwhereby Pi represents mean annual precipitation in grid cell i
withinthecatchment (whichis supposedlyschematized inton
gridcells), s∗
m,i thestandard deviationof monthlyprecipitation
ingridcelli,ands∗
y,i thestandarddeviationofannual
precipita-tionovera30-yrperiodingridcelli.Thismaylookimpressive
andadvanced,butinessencetheWSIisametricwithout
possi-blemeaningfulinterpretation.Thefactthattheargumentofthe
exponentialfunctionisnotdimensionlessinhibitsaphysical
inter-pretationoftheconstruct.Inaddition,thetwostandarddeviations
havedifferentunits:oneisinmm/month,whereastheotherisin
mm/year.It’simpossibletomeaningfullyaddthem.Somehowthe
metriccapturestheeffectoftemporalvariability,whichhasbeen
usedasanargumentthatWSIisabetterscarcityindicatorthan
theannualwithdrawal-to-availabilityratio(WTA),whichhasbeen
widelyusedasaWSindicatorinwaterresourcesliterature(e.g.
Vörösmartyetal.,2000).Ontheotherhand,theWSIequationis
calibratedsuchthataWSIof0.5isobtainedforaWTAratioof0.4,
whichhasinthepastoftenbeen(arbitrarily)usedasthethreshold
betweenmoderateandseverewaterstressinacatchment.Through
itsdefinition,WSIwillliebetween0.01and1.
TheWSIhasbeenembracedbytheLCAcommunityasauseful
metrictobeusedasaweightingfactorinthecalculationof
water-scarcityweightedWFs.Recently,PfisterandBayer(2014)published
animprovedversionoftheWSI,whichhoweverissimilarasthe
aboveone,thoughcalculatednowonamonthlyratherthanannual
basis,withthesuggestionthatitthuscapturesWSevenbetter.It
isdifficulttocriticizetheWSIbecauseithasnopretendedphysical
meaning,sothereisnowayofcheckingwhetheritmakessense.Itis
difficult,though,toseewhywewouldrelyonametricthatis
essen-tiallyameaninglessconstruct.ItwouldbemoreusefuliftheLCA
communitywouldrelyonadvancedwaterstressandWS
indica-torsbeingdevelopedwithinthewaterresourcescommunity.Wada
etal.(2011),forinstance,computedwaterstressatahighspatial
resolutiononamonthlybasisaswaterconsumptionoverwater
availability.Hoekstraetal.(2012)tookasimilarapproach,butalso
accountedforenvironmentalflowrequirementswhenestimating
wateravailability.
Ridoutt and Pfister (2013) presented a new WF calculation
methodintegratingconsumptiveanddegradativewateruseintoa
singlestand-aloneweightedindicatorthatmakesthingsevenmore
obscurethantheWSI.Inthenewmethodtheyproposedto
calcu-lateconsumptivewaterusebymultiplyingwaterconsumptionby
WSI(witharesultinH2Oequivalents),computedegradativewater
usebyconverting“ReCiPepoints”earnedbasedonemissionsto
waterintoH2Oequivalents,andtofinallyaddthetwo
equivalents).Thisallisproblematicparticularlybecausethereis
nowaytovalidatewhethertheresultingfigurecorrectly
repre-sents“potentialenvironmentalimpact”.Sincethemetriccannot
beinterpretedinanyphysicalwayandsincetheoutcomescannot
beempiricallytested,nothingelseremainsthanameaningless
con-struct.AsimilarcritiqueholdsfortheWaterImpactIndexbyBayart
etal.(2014),anotherefforttoexpresstheenvironmentalimpactof
consumptiveanddegradativewateruseinasinglemetric.
Thefactthattheresulting“impactcategoryindicators”from
Pfisteretal.(2009),RidouttandPfister(2013)andothershaveno
empiricalinterpretationbecomesevenworsegiventheambition
oftheLCAcommunitytotranslatetheH2OequivalentWFintoits
impactonhumanhealth(consideringdisabilityadjustedlifeyears)
andbiodiversityorecosystemquality(Pfisteretal.,2009;Bayart
etal.,2010;BergerandFinkbeiner,2010).Itisalreadyimpossible
toknowhowreducedgroundwaterlevelsand riverflowsaffect
humansandecosystems,giventhemultitudeof contextual
fac-torsthatplayarole,itiscompletemadnesstoestablisharelation
betweenthemeaninglessH2OequivalentWFinacatchmentand
itsimpactsonhumanhealthandbiodiversity.Andthis,though,is
preciselywhatseveralLCAauthorsproposetodo(Boulayetal.,
2011,2015a,2015b).Thepretentionistoassessthecostofwater
consumptionintermsofdisabilityadjustedlifeyearsperunitof
H2Oequivalentspercatchment.Thismakesnosenseatall,because
itisimpossibletoisolatetheimpactoflocalwaterdepletionon
localhumanhealth(givene.g.copingcapacity,thepossibilityto
import), let alone that one can establish a relation using
non-empiricalmetrics.Therehasbeennostudyevershowingempirical
evidenceof somegeneralizedrelationbetweenWS and human
healthincatchments,whichistobeexpected,becausedrinking
waterrequirementsaregenerallyrelativelysmallandthus
diffi-culttobeaffectedbylocalWS.EveniflocalWSaffectspublicwater
supplies,peoplemaystillbeabletocopeiftheycanaffordtobuy
importedwater.Furthermore,eventhoughWSinacatchmentcan
easilyaffectfoodharvests,thisdoesn’tnecessarilyleadto
malnu-tritionoflocalpopulations,sincepeoplemaystillbeabletoget
foodfromelsewhere.TherearetoomanypathwaysbetweenWS
andhumanhealth,withtoomanyothervariablesinbetween,to
findasingleequationthatrelatesbothfactors.
WhereastheLCAcommunitytriestobuildconsensusonthe
development of a stress-based indicator for LCA-based impact
assessmentofwaterconsumption(Boulayetal.,2015c),itis
proba-blybettertotakeastepback,soastofirstthoroughlyreconsiderthe
soundnessoftheideaofawater-scarcityweightedWFindicator,
andtoassessthefeasibilitytodevelopgeneralizedrelationships
betweenwateruse,WSandwaterpollutionversushumanhealth
andecosystemqualitythatcanbeempiricallytested.Onemust
admitthatexpressingtheenvironmentalimpactof productsin
termsofhumanhealthdamageandecosystemdegradationinthe
formofsinglemetricsasaimedforintheLCAmethodologymay
runagainstthelimitsofwhatispossible,giventhecomplexityof
thesocio-ecologicalsystem.
4. Thewayforward
Thefactthatwaterscarcityisamajorenvironmentalconcern
isareasontogetthewaterscarcityissuewellintoLCA.AsIhave
argued,the(volumetric)green,blueandgreyWFsareallequally
relevantfromanallocationanddepletionpointofview.Onelitre
ofgreenorbluewaterallocatedforconsumptionforonepurpose
isnotavailableforacompetingpurpose,andonelitreofgreenor
bluewaterallocatedtohumanuseisnotavailablefornature.Green
andblueWFsthussubtractfromthesupplycapacityleft.Similarly,
ifoneactivityhasagreyWFofonelitreandthusconsumespart
ofthetotalassimilationcapacityofawaterstream,thissubtracts
fromtheassimilationcapacityleftfor otherpollutingactivities.
Sincewaterisaglobalresource,everylitreofwaterconsumption
orpollutioncounts.TheessenceofgrowingWSisnot
environmen-talimpact,butincreasingglobalresourceusegivenlimitedglobal
resourceavailability,withheterogeneouslyspreadlocal
environ-mentalimpactsasaby-productinplaceswherelocalresourceuse
exceedslocalmaximumsustainablelevels.Whenconsideringthe
contributiontowaterscarcityorwaterdepletion,itiskeyto
con-siderhowmuchwaterunitsareusedperproduct,whereverthat
happens.Ifonly100unitsofgreenandbluewateraresustainably
available,80maybeavailableinwater-abundantareasand20in
water-poorareas.Thereisnoreasontonotcountcertaintypesof
wateruse(likegreenwater)orcountcertaintypesofwateruse
less(likebluewaterinwater-abundantareas).Onlybyconsidering
allformsofwateruseandallformsofwateravailability,itwillbe
possibletogetapictureofdepletion.
Itwouldbeusefultoincorporatethetopicoffreshwaterscarcity
inLCAasa“naturalresourcedepletion”category.Thisisan
unex-ploreddirectionasyet,seeforinstancethetreatmentoffreshwater
depletionin thereview by Klinglmairet al. (2014).Freshwater
depletionshouldbeconsideredfromaglobalperspective,since
freshwateris aglobalresource, withgrowingglobalfreshwater
demandwhileglobalfreshwateravailabilityislimited.This
limi-tationisdeterminedbythelimitedglobalfreshwaterrenewalrate
(precipitationoverland),theunevenspatialandtemporal
distribu-tionofwateravailability,thelimitedtransportandstorage
possibil-ities,theneedtoletpartofthenaturalwaterflowsuntouched,and
theimpossibilitytousepartofthenaturalflows(e.g.astheyflow
inunaccessibleareasorintimeswherethereistoomuchrather
thantoolittlewater).Giventhelimitedaccessiblefreshwaterflows
globallyavailableforproductiveuses,itisimportanttomeasure
(volumetric)WFsofproducts,tomeasurethecomparativeclaimof
differentproductsonthoselimitedfreshwaterflows.
Whenlooking at thepotentiallocal environmental impactof
wateruseinthefulllifecycleofaproduct,itmakessensetofocus
ontheblueandgreyWF,becausetheformermayleadto
ecosys-temimpactsasaresultofrunoffmodificationandthelattermay
impactonecosystemsifpollutionlevelsgettoohigh.Ridouttand
Pfister(2010)arerightintheirargumentthattheenvironmental
impactofthegreenWFcanaswellbeconsideredinthecontext
ofthelanduseimpactcategory.Theimpactindicator
represent-ingthelocalenvironmentalimpactofablueWFcouldbebasedon
theideaofwater-availabilityweightingasproposedinthispaper.
Inotherwords,theblueWFpercatchmentisweightedbasedon
thecarryingcapacitypercatchment,whichdependsonbluewater
availability(runoffminusenvironmentalflowrequirements).The
impactindicatorrepresentingthelocalenvironmentalimpactof
agrey WFcouldbebasedona similarapproach, e.g.weighting
thegrey WF per catchmentbased onassimilationcapacity per
catchment,whichdependsontheamountofrunofftoassimilate
agreyWF.
Ihavearguedthatweightingwaterconsumptionbasedon
rel-ative WAper catchmentgives a betterproxy ofpotential local
environmentalimpactofwaterconsumptionthanweightingbased
onrelativeWSpercatchment,butonemayretaindoubtsaboutthe
usefulnessofweightedmetricsaltogether,giventhelackof
physi-calmeaningofsuchconstructs.ThedifficultyremainsthatLCAaims
tocomparedifferentsortsofpotentialenvironmentalimpacts–
indeedcomparingapplesandpears,liketheimpactofwaterusein
onebasintotheimpactofwateruseinanotherbasin,ortheimpact
ofwaterconsumptiontotheimpactofwaterpollution.Weighted
metricsmayhavetheirspecificusewithinaproductLCA,butone
shouldbeextremelycarefulinapplyingsuchmetricsoutsidethat
context.
The critique in this paper does not concern LCA in itself,
consumptionandwaterscarcitywithinanLCA.WaterFootprint
Assessment(WFA)andLCAservedifferentpurposesandemploy
differentmethods,butbothcanusetheWFconcept.Itis
confus-ingifthefieldsemploydifferentdefinitionsoftheconcept,and
asarguedhere,theoriginalvolumetricdefinitionis mostuseful
andtheonlyoneconsistentwiththeecological(land)andcarbon
footprintconcepts.
Acknowledgement
ThisresearchhasbeenfundedbytheUniversityofTwente.
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