Role of watering practices in large-scale urban planning strategies to face the heat-wave risk in future climate

2018·2.1 MB·English

by  M. Daniel

UrbanClimate23(2018)287–308
ContentslistsavailableatScienceDirect
Urban Climate
journalhomepage:http://www.elsevier.com/locate/uclim
Role of watering practices in large-scale urban
planning strategies to face the heat-wave risk in
future climate
M.Daniela,⁎,A.Lemonsua,V.Viguiéb
aMétéo-France/CNRS,Grouped'étudedel'atmopshèremétéorologique,42avenueGaspardCoriolis,31057Toulousecedex,France
bCentreinternationalderecherchesurl'environnementetledéveloppement(CIRED),45bisavenuedelaBelleGabrielle,F-94736Nogentsur
Marnecedex,France
a r t i c l e i n f o a b s t r a c t
Articlehistory: Increasingheat-waveriskduetoregionalclimateevolutions,exacerbat-
Received31May2016 edbyurbanheatisland(UHI)effects,isamajorthreatfortheinhabi-
Receivedinrevisedform27October2016 tantsof manycities.Adaptivepoliciessuchasgreeningtheurban
Accepted5November2016 environmentareoftenproposedtolimitpopulationvulnerability,as
vegetationenablestoregulatethemicroclimatebyevapotranspiration.
Theefficiencyofsuchstrategiesdependsonwateravailabilityandraises
Keywords: theissuesofwatersupplyforirrigationandofvegetationefficiency.
Irrigation
Threevegetationwateringalternativesandascenarioofpavement
Pavementwatering
wateringarestudiedandcomparedusingParis(France)urbanareaas
Urbanheatislandeffect
a case study. With an evolution of the city based on “business as
Urbanclimate
Climatechange usual”trends,urbanclimatemodelingenablestoevaluatebothUHI
Adaptation and heat stress under heat-wave conditions in 2100. Vegetation
wateringisefficientinreducingairtemperatureandthermalstress,
butmostlyinresidentialareaswherevegetationdensityisimportant
enough.Pavementwateringisrelevantinthedenselybuiltcitycenter
onlywhereitimprovesthecoolingefficiencyandincreasesthewater
consumptionby2%only.Thecombinationofbothsolutionsprovides
thebestperformanceswithareduction(comparedtoanonirrigated
scenario)ofthemaximumtemperatureanomalyby0.8°C(2.6°C)dur-
ingtheday(night).
©2016ElsevierB.V.Allrightsreserved.
⁎ Correspondingauthor.
E-mailaddresses:[email protected](M.Daniel),[email protected](A.Lemonsu),[email protected](V.Viguié)
http://dx.doi.org/10.1016/j.uclim.2016.11.001
2212-0955/©2016ElsevierB.V.Allrightsreserved.
288 M.Danieletal./UrbanClimate23(2018)287–308
1.Introduction
Climatechangeandglobalwarmingduetogreenhousegasemissionsarealreadyobservedandareinten-
sifying(IPCC,2015).Oneoftheconsequencesisanincreaseofsummerheat-waves(IPCC,2015,Giorgi,2006).
ThishasbeenhighlightedbyDéquéetal.(2007)forFranceatthenationallevelbyLemonsuetal.(2014)for
theParisregion,whichcouldbeaffectedbymorethantendaysofheat-waveperyearonaveragebytheendof
the21stcentury.Thissevereheat,exacerbatedincitiesbytheurbanheatisland(UHI)effect(Basaraetal.,
2010;Tanetal.,2010;GabrielandEndlicher,2011;LiandBou-Zeid,2013),arealreadyaclearly-identified
publichealthconcern,whichissteadilygrowing(Lemonsuetal.,2013)andalarminginstitutionalstake-
holders(seeforinstance,StoneandRodgers,2001;Soleckietal.,2005;HaminandGurran,2009;Lambert-
Habibetal.,2013).
Tofacethisissue,itisessentialtodevise,evaluate,andimplementefficientclimatechangeadaptationand
UHImitigationstrategies.Thecountermeasuresusuallyproposedtoreduceairtemperatureinurbanenviron-
mentareeitherdesignedtomodulatethethermo-radiativeprocessesortheenergyexchangeswiththeatmo-
sphere.Thefirstonesarerelatedtoamodificationoftheradiativepropertiesofurbanfacetsandbuilding
envelopes,e.g.withreflectivematerials(SantamourisandKolokotsa,2013;Kolokotsaetal.,2011),orthecre-
ationofshadingareasbyplayingwithurbanformortrees(Potchteretal.,2006,SouchandSouch,1993,
Shashua-BarandHoffman,2000).Someothertechniquesaimatmodifyingthesurfaceenergybalancebyfa-
voringwaterevaporation,inordertorefreshtheambientair,whilereducingheatconvectionandstorage.
Thisispossiblewiththeuseofpervioussoilsandvegetation,orthroughthewettingofurbansurfaces.
Urbangreening,i.e.addingvegetationinthecity,isasolutionmoreandmorestudiedandpromotedatdif-
ferentspatialscales(Gaffinetal.,2012;Rosenzweigetal.,2011;Bowleretal.,2010):greenbeltsaroundthe
city(Massonetal.,2013b,Adachietal.,2014),urbanparks(Potchteretal.,2006),orgreenbuildingenvelopes
(WarkandWark,2003;AlexandriandJones,2008;Jaffaletal.,2012;Malysetal.,2014).Pavementwateringis
amorerecentapproachthathasbeensofarinvestigatedessentiallythroughexperimentalworks(Takahashi
etal.,2010;Hendeletal.,2015).Nonetheless,thesevariousmethodsusedtomagnifyevaporationarehighly
dependentonwateravailability.Theymayrequireanimportantwatersupplythatmustbequantifiedtoob-
jectivelyevaluatethestrategy'sviability.Totheauthors'knowledge,onlyfewworksraisethisissue.Shashua-
baretal.(2011),especially,proposedanindicatorofcoolingefficiency,definedastheratiobetweentheday-
timecoolingandtheequivalentlatentheatfromtheevapotranspirationofplants.Basedonmeasurements,
differentvegetationcombinationshavebeenevaluated(Shashua-Baretal.,2011;Middeletal.,2012).
Withtheprospectofacceleratingclimatechange,especiallyillustratedbyanincreaseandintensification
ofheatwaves,associatedwiththeexpansionofcitiesandtheirpopulations,thepresentpaperaimsatinves-
tigatingthepossiblestrategiesforcities'evolutionandadaptationatlargescale.Specialattentionispaidhere
totheissueofcitygreening,andmoreimportantlytheassociatedwateringpractices,withtheambitionof
assessingtheurbanplanningstrategiesonthebasisofadditionalconsiderationsthatarerarelytakenintoac-
count.However,thequestionofwaterresourcesandoftheirsustainablemanagement,fromtheglobalscale
tothelevelofmunicipalitiesandlocalauthorities,istodayaprevalentandessentialissue.
2.Paris2100asacasestudy
2.1.Previousstudies
TheParisurbanarea(France)isusedhereasacasestudy,consideringthatParisconurbationisoneofthe
mostpopulatedurbanareasintheEuropeanUnionwithmorethan12millioninhabitantsin2012according
totheINSEEFrenchnationalstatisticsinstitute.IthasbeenparticularlyaffectedbytheEuropeanheat-waveof
summer2003withabout5000casualties(Hémonetal.,2003),andhasbeenthesubjectofseveralstudies
aboutUHImitigationstrategies(seeespeciallyMassonetal.,2013b;Lemonsuetal.,2013,2015;deMunck
etal.,2013;deMunck,2013;Kounkou-Arnaudetal.,2014).
TheworksofMassonetal.(2013b)haveshownthaturbanplanningatlargescalehasamajorroletoplay
ontheUHIregulation.Inthiscase,theplantingofforestsandvegetablecropsaroundthecityleadsuptoa2°C
decreaseofairtemperatureinsidethecityincaseofheatwaves.Moregenerally,theimplementationofgreen
orbluebeltsmayinfluencelocal-scalemeteorologyandtheflowsaroundandabovethecity,andconsequent-
lyrefreshthecityduringsummerperiods.Themodelingofthe2003heatwaveoverPariswiththeTEBurban
M.Danieletal./UrbanClimate23(2018)287–308 289
climatemodelcoupledtotheMeso-NHatmosphericmodel(Laforeetal.,1998)hasenabledtoassessvarious
actionleversatcityscale,moreespeciallytheimpactofgreeningandpavementwatering(deMunck,2013;
Kounkou-Arnaudetal.,2014).DeMunck(2013)hasinvestigatedthecoolingpotentialofgreenroofsand
ground-basedvegetation,onthebasisofrealisticscenariosforwhichvegetationisimplementedinthe
existingcityonlywherepossible.Theyhaveshowedthatvegetationwatersupplyisessentialinordertoob-
tainacoolingeffect.Inaddition,Kounkou-Arnaudetal.(2014)havetestedtheinnovativesolutionsofpave-
mentwateringusingasimilarnumericalsetup.Theyobtainedaslightdecreaseinairtemperatureinthe
streetsofabout0.5°Cintheafternoon,significantlysmallerthanthe2°Ccoolingobtainedwithwatered
ground-basedvegetation.
2.2.TheVURCAproject
Thisstudyisinthelineofthepreviousworksbutaimsatadoptingamoreprospectiveandintegrativeap-
proachbyaccountingforevolutionofthecityitselfandmodificationofclimate.Thiswastheobjectiveofthe
VURCAnationalresearchprojectwhichambitionedtoassessthevulnerabilityofParistofutureheatwaves
andtoproposeadaptationstrategies.Byusinganinterdisciplinaryandsystemicmodelingsystemforthe
city,wehadbeenabletoprovidequantitativeestimatesofthepotentialbenefitsofvariousstrategiesforevo-
lutionofcityandofsomepractices.Theefficiencyofstrategieshasbeenassessedthroughdiagnosticsorin-
dicators that cover different thematic areas: urban climate, thermal comfort, energy consumption of
buildings,andwaterconsumption.Startingfromthemultiplescenariosstudied,afirststudyhasbeencon-
ductedontheimpactofurbanexpansionscenariosonurbanheatislandandheatstressinParis(Lemonsu
etal.,2015).Theresultshavehighlightedthattheexposureofinhabitantstoheatislowerincaseofacity
lesscompactandgreener.
However,thestartingassumptionwasthaturbanparksandgardensweresystematicallywatered,which
isnotnecessarilyrealisticunderheatwavecondition.Amorespecificanalysisisconductedhereinorderto
comparevariousstrategiesofwateruseeitherforvegetationirrigationorpavementwatering.Theobjective
istoevaluatetheirefficiencybyputtinginperspectivethegainsintermsofUHImitigationandthermalcom-
fortimprovement,andtheinducedwaterconsumption.Inaddition,onecanexpectthatthisefficiencyvaries
dependingonintensityanddurationofheatwaves.
AfteradescriptionoftheglobalmethodologyinSection3,thispaperinvestigatestheimpactsofdifferent
scenariosofvegetationirrigationorpavementwateringonevaporation(Section4)andUHIpatterns(Section
5).Section6examinestheefficiencyofeachscenariointermsoftemperatureandthermalcomfort,andfinally
Section7discussesthevariabilitiesanduncertaintiesofourresults,andSection8concludes.
3.Methodology
3.1.Generalmodelingmethodology
ThestudypresentedhereusesthemethodologydevelopedwithintheframeworkoftheVURCAresearch
project,describedindetailinLemonsuetal.(2015).TheurbanclimateofParisissimulatedusingaphysically-
basedurbancanopymodel.Duetorecentdevelopments,thismodelissophisticatedenoughtosimulate,at
thecityscale,airtemperaturesinsideandoutsidethebuildings,indexesofthermalcomfort,air-conditioning
systems(energyconsumption,associatedheatreleaseandpotentialfeedbackonairtemperatureoutside),as
wellasevapotranspirationofplantsinurbanenvironment.
Insteadstudyingapastheatwavetoillustratewhatcouldbethefutureevents(Beniston,2004;Vautardet
al.,2007),asetofsyntheticortheoreticalfutureheatwaveshasbeenbuilt.Theseeventsvaryinintensity
(dailymaximumtemperature)andinduration(numberofdays)withvariationrangesdefinedfromtheanal-
ysisofregionalclimatemodelprojections(Lemonsuetal.,2014).
Byaddressingtheclimatechangeissueincities,andbyworkingonlongtimeframesuptotheendofthe
21stcentury,itisimportanttoalsotakeintoaccounttheevolutionofcitycharacteristics.Thesedifferentpro-
cessesthatinteractlocallycansignificantlymodifytheurbantemperatures,energydemand,orwatercon-
sumption,comparedtopresentsituation.Consequently,amongtheensembleofcityscenariosbuiltinthe
VURCAproject,weusehereareferenceprospectivescenariowhichassumesevolutionsforcitycharacteristics
thatfollowcurrenttrends(detailedhereafter).
290 M.Danieletal./UrbanClimate23(2018)287–308
3.1.1.Heat-waveandurbanclimatemodeling
TheurbanclimateofParisissimulatedusingthephysically-basedurbancanopymodel,TownEnergyBal-
ance(TEB),(Masson,2000;HamdiandMasson,2008).Thismodelsimulatesatthesametime,meteorological
conditions,atstreet-levelandinsidebuildings,throughaspecificcoupledbuildingenergymodel(Buenoetal.,
2012;Pigeonetal.,2014).Moreover,usingnear-surfaceairtemperature,humidity,windspeed,aswellasin-
comingsolarandinfraredradiation,themodelalsocomputestheUniversalThermalClimateIndex(UTCI),
(Fialaetal.,2012).Thisisamodelingoftheestimatedthermalcomfortofthehumanbody(seeCOSTAction
730,http://www.utci.org).ThisindexiscomputedbyTEBmodel,bothinsidethebuildingsandinthestreets,
ateachgridcellofthemodelingdomain.
Inordertobetterrepresenturbangreenareasandcatchtheinteractionsbetweenbuild-upandnatural
covers(Lemonsuetal.,2012),TEBiscoupledwiththeInteractionSoilBiosphereAtmosphere(ISBA)model
(NoilhanandPlanton,1989).ISBAisaSoilVegetationAtmosphereTransfermodelthatcomputesthesurface
radiative(directanddiffusesolarradiationandinfraredradiation)andenergy(latent,sensibleandstorage
heatfluxes)budgetsfornaturalsoilsandvegetation.Italsosimulateswaterexchangesthatincludeplants
transpiration,evaporationofwaterinterceptedbythefoliage,evaporationfromtheground,aswellasseveral
hydrologicalprocesses(drainage,infiltrationandcapillaryattractioninthesoil).Byaccountingforthese
water exchanges and the hydrological properties of the ground, and based on a three-layer vertical
discretizationofthesoilcolumn,themodelcomputestheevolutionwithtimeofthesoilwatercontents
(Booneetal.,1999).ISBAisalsoabletosimulatedifferenttypesofvegetation,especiallybyvaryingtheveg-
etationfraction,theleafareaindex,orthestomatalresistance.Theseprocessesareincludedintheurbancli-
matemodeling,sothattheresultingmodelisabletoquantifytheircoolingeffectsonthecity.Itishowever
importanttoemphasizethatthecurrentversionofTEB-ISBAdoesnotconsiderintheradiativebudgetthe
shadoweffectsoftreesonwallsandground-basedsurfaces(Lemonsuetal.,2012).
Onthebasisof1-kmspatialresolutionatmosphericsimulationsofthe2003heatwaveperformedbyde
Muncketal.(2013)andKounkou-Arnaudetal.(2014)forParisregion,wehavebuiltasetofsynthetic
heatwavesofvariableintensitiesbymodulatingtherealmeteorologicalconditions(maximumdailytemper-
ature,humidity,andincominglongwaveradiation).Thedurationoftheeventcanalsobeadjustedbyrepli-
cating the daily meteorological conditions day after day. Four different intensities are considered,
correspondingtoadailymaximumtemperatureof34,38,42,and46°C(referredtoasHW34,HW38,
HW42,andHW46,respectively).Theseintensitieshavebeendefinedaccordingtoastatisticalanalysisoffu-
tureheatwavesoverParisbasinsimulatedbyanensembleofregionalclimatemodels(Lemonsuetal.,2014).
NotethatthesyntheticheatwaveHW38foradurationofsevendaysisquitecomparabletothereal2003heat
wave.ThesemeteorologicalforcingsareusedtoruntheTEB-ISBAsurfacemodelinanofflinemode,toassess
thesensitivityoftheurbanclimatetoirrigationpracticesforvariableheat-waveconditions.Inthispaper,we
willmostlyfocusontheseven-dayHW38.However,asensitivitytoheat-wavesintensitiesanddurationsis
carriedoutinSection7.
Initialconditions,especiallythesoilwatercontenttowhichresultscanbequitesensitive,aredefinedthe
samewayforeachirrigationscenarios,startingfromthecompletesimulationofthe2003heatwaveper-
formedbydeMuncketal.(2013).NotethatthesimulationofdeMuncketal.(2013),conductedwiththe
Meso-NHnon-hydrostaticmodel(Laforeetal.,1998)coupledtotheTEBmodel,hadbeenevaluatedusing
localobservationsinParisregion.Inaddition,themethodologyofurbanclimatemodelingadoptedin
VURCAproject,forwhichtheTEBurbancanopymodelisinitializedusingacitydescriptionprovidedbythe
urbanexpansionsocio-economicmodel(seenextSection),hasbeenalsoevaluatedforonedayofthe2003
heatwave.DetailedresultsareavailableinLemonsuetal.(2015).
3.1.2.Urbanexpansionmodelingandcityevolution
TEB-ISBAmodelisrunoveradomainof100km×100kmcenteredonPariscitycenterwithahorizontal
resolutionof1km(seethedomaininFig.1).TofulfilltherequirementoftheVURCAproject,anurbanexpan-
sionscenarioofParisurbanareaissimulateduntil2100usingtheNEDUM-2Dmodel(Viguiéetal.,2014).This
socio-economicmodelsimulatesthespatialdistributionoflandandrealestatevaluesaswellasthemain
characteristicsofthecitysuchaspopulationandbuildingdensity,greenandhousingareas.Itisbasedona
dynamicextensionoftheurbaneconomictheory(Fujita,1989).Simulatedcityshapeevolutionandcharac-
teristicshavebeencalibratedovertheperiod1900–2010,anddetailsofitscalibrationarepresentedin
Viguiéetal.(2014).
M.Danieletal./UrbanClimate23(2018)287–308 291
Fig.1.SpatialextentoftheParisconurbationsimulatedin2100bytheNEDUMmodel.Urbanareasareclassifiedinfourtypologies:his-
toricalcitycenter(HCC),collectivehousing(COL)andresidentialhousing(RES),andofficebuildings(OFF).
Beyond2010,theurbanexpansiondynamicsfollowsascenariobasedoncurrenttrendsofpopulationden-
sitydecrease,andonamoderatetotalpopulationincreasescenario(+9%in2100)(seeAppendixA):wesup-
posethatnospecificurbanpolicyorregulationisimplementedinordertolimiturbansprawl,sothatthecity
stillgrowsfollowingthecurrenttrends.Thesimulatedcityin2100covers3090km2including1420km2of
greenareas(gardensandpublicparks)forademographicprojectionof13,400,000inhabitants.
Thebuildingsareclassifiedinfourarchitecturaltypologies(seemapinFig.1)historicalbuildingslocated
inthecitycenter(Haussmannianarchitecturebuildingsdatingfrom19thcenturyandreferredtoasHCC),col-
lectivehousing(COL),singlehousing(RES),andofficebuildings(OFF).Materialscharacteristicsofbuildings
andrenovationtechniquesarespecifieddifferentlyforeachtypologyonthebasisofaninventoryofthecur-
rentParisianbuildingsandtherenovationtrends.
Projectedbuildingscharacteristicsin2100arebasedontheassumptionthattheenergeticperformancesof
buildingsaregoingtoimprovewithtimeduetothetechnologicalprogressesinconstructionandrenovation
methods,followingcurrenttrends.Acompletedatabaseofbuildingscharacteristics,builtfortheVURCApro-
ject(Marchadieretal.,2012),isusedhere.Itisassumedthatallcollectivehousing(COL)andsinglehousing
(RES)buildingsfollowtheFrenchthermalregulationsof2012.Thelevelofrenovationishigherforoffice
buildingswhichareequippedwithhigh-performancevacuuminsulationsystemsandreflectivematerials.In-
versely,becauseofarchitecturalandaestheticalreasons,thehistoricalbuildings(HCC)aretheleastrenovated
ones(asimpleinteriorthermalinsulationisimplementedinthiscase).
3.2.Evaporationprocessesandwateruseforrefreshing
3.2.1.Vegetationwatering
Evapotranspiration-relatedthermal-regulationcapacityofvegetationinurbanenvironmentistightlyre-
latedtowateravailability.Twomechanismsareinvolvedwithdifferentdynamics.First,soilwater,aswell
asthewaterinterceptedbythefoliageafteraperiodofrainfallorwatering,canevaporate.Thisphysicalpro-
cesscantakeplaceeitherdayornight.Theresponsetoapunctualwatersupplyisquickforwaterintercepted
bythefoliageandslowerforsoilwater.Thisphenomenonisaffectedbyenvironmentalconditions,especially
atmosphericwaterpressure.Waterexchangesarealsoinducedbyplantstranspiration.Thisphysiological
mechanism,coupledtophotosynthesis,takesplaceduringtheday(formostofvegetalspecies)andis
basedonwaterextractionfromthesoilbytherootsandwaterevaporationbytheleavesthroughthestomata.
292 M.Danieletal./UrbanClimate23(2018)287–308
TheISBAmodel-whichispartofSurfaceVegetationAtmosphereTansfer(SVAT)typemodels-simulates
thephysiologicalprocessesthatmanageevapotranspirationofvegetation.Italsoincludesaphysically-based
modelingofhydrologicalprocesses,sothatitsimulatesthesoilwaterstatusevolutionandtheretroactionon
transpirationcapacityofplants.ThemodelingoftheseprocessesinISBAisdetailedinAppendixB.1.Usingthis
model,thevegetationwateringpracticescanberepresented.Itcandealwithdifferentformsofwaterinputs
eitheronvegetation,attheground,orinthesoil,andthewaythiswaterisusedbythesoil-plantsystem.
3.2.2.Pavementwatering
Indensely-builturbanenvironment,pavementwateringcanbeasolutiontomitigateUHI.Thewater
sprinkledonthepavementisrapidlyevaporatedduringhotconditions,whichcoolstheambientairwhile
limitingsurfacewarming.Thisrefreshingstrategyofferstheadvantageofrespondingquicklytoapunctual
watering.Theevaporationpotentialishoweverstronglyconstrainedbythemaximumcapacityofwaterstor-
ageonroads.Beyondacertainwaterquantity,thesurplusislostbysurfacerunofftowardsewers.
Basedonexperimentalworks,Takahashietal.(2010)studiedandquantifiedtheimpactofthispracticeon
localairtemperature.Theyfoundthatincaseofhighairtemperaturesgreaterthan30°Cinthecity,airtem-
peraturecandecreasebyabout2°Cinthemorningand4°Cintheafternoon.Variousexperimentalprotocols
weretestedandthemostimportantcoolingeffectsweremeasuredwhensprinklingforthreeminutesevery
30minwitharateof0.33Lm−2min−1.Anotherexperimentalsensibilityanalysiswasperformedduring
summer2013byHendeletal.(2015).Itwasfoundthatforsunnydayspavementwateringshouldbeactivat-
edevery30minwitharateof0.31to0.41Lm−2h−1.Usingthisset-up,hefoundthatsurfacetemperature
couldbereducedby4°Cinthemorningandupto13°Cintheafternoon.Healsofoundagainof2.3°Cto
5.9°Coftemperature5cmbelowthesurface,provingthatpavementwateringisefficientwhenaimingatde-
creasingheatstorageinimpervioussurfaces.
TheTEBmodelsareoneofthefewurbanclimatemodelsthatincludesurfacewaterexchanges.Thecurrent
parameterizationisrathersimple,butenablestorepresentrainwaterinterceptiononroadsandroofs,evap-
orationofavailablewater,andsurfacerunoff(seeAppendixB.2fordetails).Theimplementationofpavement
wateringinTEBisbasedonthisparameterization,simplybytreatingthiswatersupplythesamewaythan
rainfallonroads.
3.2.3.Irrigationscenarios
Theconfigurationusedhereasreferencescenarioisthesimulationperformedwithoutanthropicwater
supplyforvegetationduringthewholeheatwave(simulationreferredtoasREF).Itisassumedthatno
waterisavailableforotheractionsthandailylifeusages,sothatprivategardensandurbanparksarenot
watered.
Twoalternativesforvegetation(publicparksandprivategardens)watering,whichrepresent1419km2
(seeTable1),arethentestedandsimulated,inordertoassesstheirimpactsonsoilwaterstatus,surfaceen-
ergy balance, localmicroclimate, andcomfort conditions. Watering aimsatmaintainingsufficientsoil
Table1
Assessmentofwaterconsumptionsthedifferentirrigationscenarios:Unrestricted(R),Pavementwatering(P),Realistic(R)andCom-
bined(C).Theseconsumptionsarecomparedwithwaterconsumptionin2009,themeanandlowflowsoftheSeineRiverandtheexpect-
edSeinemeanflowin2100.
(U) (P) (R) (C)
Wateredsurfaces(km2) Gardens/parks 1419 0 1419 1419
Pavements 0 603 0 24
Waterdemand 5.5 1.5 4.9 5.0
(106m3.day−1)
%2009waterconsumption 232.1 63.3 209.7 211.0
(2.37106m3.day−1)
%Seinemeanflow 18.7 5.1 17.3 17.5
(328m3.s−1)
%Seinelowflow 93.0 25.4 86.0 87.1
(66m3.s−1)
%2100Seinemeanflow 26.7 7.3 24.6 25.0
(230m3.s−1)
M.Danieletal./UrbanClimate23(2018)287–308 293
moistureforplantsbutitisdifficulttocomputethequantityofwaterneeded,asthetranspirationintensity
andthewaterquantityexchangedwithatmospherearecontrolledbytheenvironmentalconditions(sunlight,
temperature,wind…),thesoilwaterstatus,andtheplantintrinsicpropertiessuchasstomatalconductance
andleafareaindex.
Thefirstoneisarealisticirrigationalternative(referredtoasR),basedonthemostcommonwatering
practicesinParisaccordingtodeMunck(2013).Anirrigationbysprinklingisactivatedatnightfrom11
pmto7amwithawatersupplyof1.210−7m3s−1(i.e.about3.46Lperdayandperm2ofgarden).Notes
thatitisgenerallyrecommendedtowaterplantsatnightbecause,duringtheday,waterlossbyevaporation
significantlyreducesthewatersupplyforthesoil,andthewaterdropletsontheleavesmagnifythesunlight
andthreatentoburnthefoliage(seedeMunck,2013).Fromatechnicalpointofview,thisirrigationprocessis
modeledinTEB-ISBAasaninputwaterfluxthatisreceivedbysoilandvegetation(asitisalreadydonefor
rainfall,forinstance).Bymodelingwatersupplythisway,apartofwater(dependingonleafareaindexofveg-
etation)isinterceptedbythefoliage,andconsequentlycanbedirectlyevaporated(seeAppendixB.1).
Thesecondscenario(referredtoasU)representsan“unrestricted”watersupplyforurbanvegetation.Itis
amaximalistscenario.Inthiscase,thesoilwatercontentw oftheISBAmodeliscontrolledandadjustedat
g
eachtimestepofthesimulation,sothatthesoilwaterreservoirisalwaysatleasthalf-filled.Thisisdoneby
verifyingthatthesoilwetnessindex(SWI)isalwaysequalorgreaterto0.5.Thisindexiscomputedasfollows:
w −w
SWI¼ g wilt ≥0:5 ð1Þ
w −w
fc wilt
withw thesoilwatercontentcomputedateachtimestep,andw andw ,thewatercontentsatthewilting
g wilt fc
pointandthefieldcapacity,respectively.Thisapproachisnotrealisticinaphysicalsensebutaimsatinsuring
amaximalevapotranspirationofplantsandatassessingtheassociatedcoolingpotential.Notethatinthiscon-
figuration,waterisdirectlyinjectedinthegroundsothatthereisnosurfaceevaporationasintheprevious
case.Theissueisthatforthissimulation,nodiagnosticinthemodelallowedustocomputethewaterneeded
tofillthesoillayer.Weassumedthenafterwardsthatthewatersupplyisatleastequaltotheevapotranspi-
rationthatoccurred.
Inaddition,ascenarioofpavementwatering(referredtoasP)issimulatedinthepresentstudy.Watering
isset-upforsidewalksalone(603km2,seeTable1),assumingtheycovers50%ofthepavementsurfaces
(1206km2,seeTable1).Thewatersupplyisbasedonasensitivitystudyconductedwithintheframeworks
oftheEPICEAproject(Kounkou-Arnaudetal.,2014).Wesupposethatthepavementsiswateredatarate
of1.110−6m3s−1duringthreeminutesatthebeginningofeveryhourbetween8amand8pm(i.e.about
2.77Lday−1andperm2ofroads).Forhighersprinklingrates,thewaterquantityisgreaterthanthemaxi-
mumstoragecapacityoftheroadreservoirsimulatedinthemodel(Masson,2000,Lemonsuetal.,2007),
sothewaterexcesswouldbelostbysurfacerunofftothesewer(seeAppendixB.2).Notethatinthisscenario,
gardensandparksarenotirrigatedatallinordertobeabletoassessseparatelythedifferentpractices.
4.Impactsofwateringonevaporation
Forclarityandsimplicity,theanalysisofprocessesisfirstlycarriedoutanddiscussedwithintheframe-
workoftheheatwaveHW38withadurationofsevendays,quitecomparabletothereal2003heatwave
asindicatedpreviously.
4.1.Referencecase
Fig.2(top)shows,forscenario(REF)andfordayseven,thedailycycleoflatentheatflux(Q )inWm−2in
E
bothhistoricalcitycenter(HCC)andresidentialareas(RES)(cf.themapinFig.1).Withoutanywatersupply
onimpervioussurfaces,thisfluxincludesonlycontributionsfromnaturalcoversthroughsoilevaporationand
vegetationtranspiration.
InRES,Q reachesamaximumof73Wm−2atmiddayaftersevendaysofheatwave,whereasitisnegli-
E
gibleatnightwhenvegetationisphotosyntheticallyinactive.Theabsenceofirrigationresultstheninanim-
portantwaterstressforvegetationwhichsignificantlylimitstherelevanceofgreeningstrategies.InHCC,Q
E
doesnotexceed11Wm−2inthemiddleofthedayduetothepredominanceofimperviouscoversinthis
area.
2
9
4
M
.D
a
n
iel
e
t
a
l.
/
U
rb
a
n
C
lim
a
te
2
3
(2
0
1
8
)
2
8
7
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3
0
8
Fig.2.Top:dailycycleoflatentheatfluxforthereferencecase.Bottom:differencebetweendailycyclesoflatentheatfluxofeachwateringscenarioandreferencecase.Inbothcases,fluxesarespatially
aggregatedoverRESarea(left)andHCCarea(right).Thedashedlinesdelimitthetimeperiodsofvegetationandpavementwatering.
M.Danieletal./UrbanClimate23(2018)287–308 295
4.2.Benefitsofwateringstrategies
Fig.2(bottom)showsthedifferenceinlatentheatfluxbetweeneveryscenarioandthereference(REF).
InRES(Fig.2),thevegetationwateringbooststranspirationofplantssothatQ increasesby+274Wm−2
E
and+138Wm−2for(U)and(R),respectively.Onenotesthatduringtheday,(U)maintainstheevaporation
cycleuntil2100UTC,whereasitstopsmuchearlierin(R).Thishighlightsthentheneedforthevegetationto
beirrigatedinordertoconserveitstranspirationpotential.Inversely,theevaporationofwaterinterceptedby
leavesduringwatering(seeAppendixB.1)inducesanadditionalevaporationphasein(R)overnight,with
fluxesupto60Wm−2.(P)doesnotseemtoprovideanyimprovementsexceptforthethreefirsthoursof
wateringwithadditionalfluxesupto50Wm−2.Lookingatthedifferencesbetweenthescenarios,onecan
concludethatirrigationismandatorytokeepasignificantvegetationevaporation.
InHCC,theadditionalevapotranspirationislimitedinthetwoscenariosbasedonvegetationwatering(U)
and(R)sincethevegetationcoverfractionislow.For(U),Q ishoweverdoubledintheafternoon(+10Wm
E
−2)comparedtothereference.Thepavementwatering(P)isheremoreefficient.ItleadstoanadditionalQ
E
upto+33Wm−2.Inthispartofthecity,theroadsprovideanimportantsurfaceareaforsprinkling,andcon-
sequentlyofferahigherevaporationpotentialthanvegetation.
5.Impactsofwateringonurbanheatisland
5.1.Referencecase
Forthereferencescenario,thedailymaximumdailytemperatureaftersevendaysofheatwaveisabout
40°Callovertheurbanarea.Atnight,theminimumdailytemperaturevariesbetween25.8°Cinresidential
areasand28.4°Cinthecitycenter.Bywayofcomparison,onecanremindthatthetemperaturethresholds
thatareprescribedforheatwavewarningoverParisregionare18°Cand34°Cforminimumandmaximum
dailytemperatures,respectively(Lemonsuetal.,2014).Thishighlightstheextremeconditionsofthisheat
wavewhichiscomparabletothe2003heatwave.MapsofUHIarecalculatedasthedifferencebetweenthe
localairtemperature(temperaturesimulatedbyTEB-ISBAmodel2mabovetheground)andareferencetem-
peraturecorrespondingtothetemperatureofruralareas,i.e.themeantemperatureofperipheralandunin-
habitedgridcells.MoredetailsonthemethodcanbefoundinLemonsuetal.(2015).Mapsofdaytimeand
nighttimeUHIareproducedbyaveragingthemodeloutputsbetween1400and1600UTC,andbetween
0200and0400UTC,respectively.
Duringtheday(Fig.3,left),theUHIislarge(3040km2areimpactedbyanUHIgreaterthan1.5°C,i.e.
about98%ofthecityarea)withpeaksofintensitythatoccurinthesuburbs.Indeed,shadingeffectofbuildings
inthecitycenterreducestheintakeofsolarradiation.ThatgivesamaximalUHIinthecitycenterof1.3°C
comparedto2.5°Cinthesuburbs.AsalreadymentionedbyLemonsuetal.(2015),atnight(Fig.3,right)
theUHIhasadifferentpattern:itislessextendedthanduringthedaybutreacheshighervaluesinthe
Fig.3.SpatialrepresentationofUHIfortheHW38aftersevendaysofheatwavebothfordaytime(left)andnighttime(right)hours.
296 M.Danieletal./UrbanClimate23(2018)287–308
mosturbanizedareas.ThemaximumUHIintensityof3.2°Cistobefoundinthecitycenterandthearea
coversbyanUHIgreaterthan1.5°Cdiminishesto817km2,i.e.26%ofthecityarea.
5.2.Benefitsofwateringstrategies
ThebenefitsofwateringstrategiesonUHIarecomputedasthedifferencebetweenthereferencescenario
(REF)andeveryothersscenarios.ThemapsofUHImitigationarepresentedinFig.4fordaytimehoursandin
Fig.5fornighttimehours.
Duringtheday(Fig.4,left),intheunrestrictedirrigationscenario(U),thecityisalmostunaffectedbya
1.5°CUHI(only2%oftheurbanareaisaffected)thankstoanabundantevapotranspirationofgreenareas
thatefficientlydecreasesairtemperature.Thecooling,whencomparedtothereferencescenario,reaches
1.0°Cinthecitycenterandupto3.8°Cinsuburbanareas.Ithenceconfirmstheimportanceofwateringveg-
etationtomaintainapotentialcoolingeffect.Thepavementwatering(P)andrealisticirrigation(R)scenarios
leadtoin-betweenresults.Eachofthemhasadistinctivepattern.Pavementwateringalternativeinducesa
coolingthatgoesupto1.0°Cbutthatislocatedinthecitycenteronly.Thisisduetothefactthatthefraction
ofthegroundoccupiedbyroadsissmallinsuburbanresidentialdistrict(roadsrepresent31%ofgroundsur-
facesinresidentialareas).Resultsarealmostoppositeintherealisticirrigationscenario,withamitigationof
theUHIinsuburbanareasof0.8°Candalimitedimpactinthecenter.Thisisduetothefactthattheproportion
ofgreenspacesislowinthecitycenterandhighinsuburbanresidentialdistricts.Therefore,thesignificant
evapotranspirationcoolingthatoccursinresidentialneighborhoodisnotallowedindenselybuiltareas.
Atnight(Fig.5,right),thepavementwateringhasalimitedimpactsinceitisactivatedonlybetween8am
and8pmandwaterevaporatesrapidlyonimpervioussurfaces.Aslightbutnotsignificantcooling(lessthan
0.5°C)ishowevernotedinthecitycentercomparedto(REF)becauseduringthedaytheroadsconsumea
largepartoftheenergytheyreceivedbyevaporationandtheyconsequentlystorelessheat.Forbothscenarios
(U)and(R),aUHImitigationbetween0.5and1.5°Cisdisplayedinresidentialareas,andtherealisticirriga-
tionscenarioappearsevenmoreefficientthantheunrestrictedone.Actually,atthattimeoftheday,both
typesofirrigationareactivatedbutdonotapplyinthesameway.Forunrestrictedirrigation,waterisdirectly
injectedintheground(seeSection3.2.3)andtakenbytherootsfortranspirationofplantsmainlyduringthe
day.Forrealisticirrigation,thevegetationiswateredbysprinkling,sothatapartofthewaterisinterceptedby
thefoliageandcanbedirectlyevaporated,therebyinducinganocturnalcooling.Thereforethecooling(upto
2.2°C)observedfor(U)isduetoalowerdailyheatstoragewhilefor(R)coolingupto2.6°Cisexplainedby
thelowerdailyheatstoragebutalsobythesignificantevaporationatnight(seeSection4.2).
5.3.Combinationofwateringpractices
Thewateringofvegetationandpavementaretwosolutionsthatenablealocalcoolingeffect,eachbyop-
eratingonaspecificenvironment(naturalorimperviouscovers)throughdifferentphysicalprocesses.They
canconsequentlybecombinedtomaximizetheUHImitigation,byadaptingtheirimplementationaccording
totheareaswheretheyarethemostefficient.
Thecombinedscenario(referredtoasC),whichcombinesirrigationofurbangreenareas(accordingtothe
sameprotocolthanfortherealisticirrigationscenario(R),seeSection3.2.3)andpavementwateringonlyfor
roadsofthecitycenterandinnersuburbs,leadstothemaximalcoolingbothatdaytimeandnighttime,
whetherinresidentialareasandcitycenter.
AscanbeseenonFig.6,suchacombinedscenariocumulatesthebenefitsofthetwooptions.Duringthe
dayandatnight,thetemperaturedecreasesbothinthecitycenter(withamaximumdecreaseduringtheday
of1.1°C)andinsuburbanareas(withamaximumdecreaseatnightof2.6°C).Atnight,themaximumtem-
peratureanomalyisnoticeablyloweredwithagainof0.6°C,0.3°Cand0.4°Ccompareto(REF),(R)and(P),
respectively.Atthattimeoftheday,theareaofthecityaffectedbya1.5°CUHI(164km2)is79%smallerthan
for(REF)butishowevernotsignificantlysmallerthanfor(R)(173km2).Duringtheday,themaximumtem-
peratureanomalyisreducedby0.8°Ccompareto(REF).Thecomparisonof(C)with(R)(respectively(P))
leadstoareductionofUHIspatialextentof8%,i.e.79km2(respectively56%,i.e.1130km2)whilethemaxi-
mumtemperatureanomalyisreducedby0.4°C(0.2°C).
However,theseresultsmustbetempered.Intermsofrealairtemperature,thecombinedscenarioleadsto
minimumdailytemperaturesof24.93°Conaverage,andmaximumdailytemperaturesof39.48°C.Despitea