ContentslistsavailableatScienceDirect
Journal
of
Hydrology:
Regional
Studies
j ou rn a l h o m e pa g e :w w w . e l s e v i e r . c o m / l o c a t e / e j r h
Impacts
of
recent
climate
change
on
the
hydrology
in
the
source
region
of
the
Yellow
River
basin
Fanchong
Meng
a,b,
Fengge
Su
a,c,∗,
Daqing
Yang
d,
Kai
Tong
a,
Zhenchun
Hao
eaKeyLaboratoryofTibetanEnvironmentChangesandLandSurfaceProcesses,InstituteofTibetanPlateauResearch,ChineseAcademyof
Sciences,Beijing,China
bUniversityofChineseAcademyofSciences,Beijing,China
cCASCenterforExcellenceinTibetanPlateauEarthSciences,Beijing,China
dNationalHydrologyResearchCenter,EnvironmentCanada,Saskatoon,Saskatchewan,Canada
eStateKeyLaboratoryofHydrology—WaterResourcesandHydraulicEngineering,HohaiUniversity,Nanjing,China
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received5July2015
Receivedinrevisedform25March2016 Accepted28March2016
Availableonline11April2016
Keywords:
Climatechange Hydrology Snow
SourceregionoftheYellowRiver
a
b
s
t
r
a
c
t
Studyregion:ThesourceregionoftheYellowRiver(SRYE)inthenortheasternTibetan Plateau.
Studyfocus:Thespatial-temporalchangesofhydrologicalandmeteorologicalvariablesand theirlinkagesovertheSRYEwereinvestigatedfor1961–2013.Meanwhile,wequantified theimpactsofprecipitationandevapotranspirationonhydrologicalchangesthrough cli-mateelasticitybyapplyingalandsurfacehydrologicalmodel.Furthermore,theimpactsof warmingclimateontheseasonalsnowcoverandspringflowovertheSRYEwereexamined. Newhydrological insightsfortheregion: Decreasedprecipitationand lightly increased evapotranspirationbothcontributedtoreducedrunoffinthe1990s,withthedecreased pre-cipitationplayingamoreimportantrole(70%)thantheincreasedevapotranspiration(30%). Inthe2000s,precipitationcontributed3%totherunoffreduction,whiletheincreased evap-otranspirationaccountedfor97%.Alongwithrapidwarming,evapotranspirationisplaying anincreasinglyimportantroleinaffectingrunoffchangesintheSRYE.During2001–2012, snowcoverinMaydecreasedovertheregion.Springpeakflowmainlycausedbysnowmelt occurredearlierforabout15daysattheJimaihydrologicalstationduetoanearliersnow meltassociatedwiththeclimatewarminginthepast3decades.
©2016TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCC BY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
TheYellowRiveroriginatesfromtheTibetanPlateau(TP),andflowsacrosseightprovincesfromwesttoeastacrossChina (Fig.1).Itis5464kmlongwithabasinareaof752,443km2,thesixthlongestriverintheworldandthesecondinChina
(Fuetal.,2004).TheYellowRiverplaysanimportantrolenotonlyinthewatersupplyfor107millionpeople(Wangetal., 2006)butalsointheagriculturalproductioninChinabecause13%ofthecountries’totalcultivatedareadependsonthe waterresourcesfromthisbasin(CaiandRosegrant,2004).ThedrainageareaupstreamoftheTangnaihai(TNH)hydrological station(Fig.1),locatedinthenortheastoftheTP,isgenerallyconsideredasthesourceregionoftheYellowRiver(SRYE) basin.TheSRYEisthe“watertower”oftheYellowRiverbasinsinceitcontributesabout35%oftotalannualrunofffrom
∗ Correspondingauthorat:KeyLaboratoryofTibetanEnvironmentChangesandLandSurfaceProcesses,InstituteofTibetanPlateauResearch,Chinese
AcademyofSciences,Beijing,China.
E-mailaddress:fgsu@itpcas.ac.cn(F.Su). http://dx.doi.org/10.1016/j.ejrh.2016.03.003
Fig.1.LocationandtopographyofthesourceregionoftheYellowriver(SRYE).Redtrianglesdenotedischargestations.Fromuptodownstream,these areHuangheyan(HHY),Jimai(JM),Maqu(MQ)andTangnaihai(TNH)stations,respectively.Blackpointsrepresentmeteorologicalstations.Meanannual precipitationcontours(mm)arealsoindicated.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversion ofthisarticle.)
about16%ofthebasinarea(Lanetal.,2010b).Therefore,itisofvitalimportanceinmeetingdownstreamwaterresources requirements(Zhengetal.,2007).
Similartootherregions,climatechangeistakingplaceintheYellowRiverbasin(Wangetal.,2014;Yangetal.,2004; Zhaoetal.,2007).Studiesoflongtermclimaticrecordssuggestedanoticeablewarmingtrendof0.31–0.35◦C/10yroverthe SRYEinthepast5decades(Cuoetal.,2013;Huetal.,2011;Lanetal.,2010b).Nosignificantlong-termtrendshavebeen observedinthebasin-wideprecipitation(Huetal.,2012),althoughlargedecadalandspatialvariationsinprecipitationexist inthisregion(Lanetal.,2010b;ZhouandHuang,2012).Alongwiththechangingclimate,meanannualflowattheTNH hasdecreasedinthepast50years(Cuoetal.,2013;Huetal.,2011;Lanetal.,2010b;Lietal.,2012).Ithasbeennotedthat theflowinthe1990ssufferedaseriousdecreaseinthisregion(Chenetal.,2007),accompaniedwithanincreaseinthe numberofzero-flowdaysatthemostupstreamgaugingsite—theHuangheyan(HHY,Fig.1)station(Zhangetal.,2004a). AttemptshavebeenmadetounderstandthecausesofthechangesinstreamflowovertheSRYE(Cuoetal.,2013;Huetal., 2011;Lanetal.,2010a;Zhaoetal.,2009;ZhouandHuang,2012).Itisgenerallyrecognizedthatthehydrologicalchangesare mostlyattributedtoclimatechangeandclimatevariability.Studiesshowthatchangesinseasonalandspatialdistribution ofprecipitationplayedanimportantroleinregionalhydrology(Huetal.,2011;Lanetal.,2010a;ZhouandHuang,2012). However,itisstillnotcleartowhatextentprecipitationandtheclimatewarmingaffectedthestreamflowregimesoverthe region.
Throughstatisticalanalysis,Huetal.(2011)suggestedthatdecreasedprecipitationinthewetseasonandrising tempera-tureovertheperiod1959–2008mayberesponsibleforthegeneralflowreductionovertheSRYE.Satoetal.(2008)developed anewhydrologicalmodeltoinvestigatethewaterbalanceoftheSRYEbasinduring1960–2000.Althoughanincreasein evapotranspirationwasdetected,theyconcludedthatthedecreaseinprecipitationwasthemainfactorforthedecrease inriverdischarge.ZhouandHuang(2012),usingapointscalelandsurfacemodelandsurfacemeteorologicalobservations for1960–2006,investigatedtheinfluencesofclimaticchangesonthewaterbudgetovertheSRYE.Theirresultssuggested thatthechangesinspatialprecipitationpatternwasanimportantfactorforstreamflowchanges.Inaddition,increasein evapotranspirationduetorisingtemperaturewasanothercauseforrunoffdecrease.
isthereforeimportanttounderstandhowsnowcoverandsnowmeltrunoffrespondedtoclimatewarminginthisregion. StudiesonsnowcoverintheTPexist(Qinetal.,2006;Zhangetal.,2004b),butonlyfewfocusedonthesourceregionofthe YellowRiver(Luetal.,2009;Yangetal.,2007).
Inthiswork,weprovidedanupdateontheimpactsofclimatechangetothehydrologyoftheSRYEduring1961–2013.We appliedalargescalelandsurfacehydrologymodeltoquantifyevapotranspirationchanges.Thespecificobjectivesare:1)to investigatethespatial-temporalchangesofrunoff,precipitationandtemperature,aswellasthelinkagebetweenrunoffand climatevariables;2)toquantifytheimpactsofprecipitationandtemperaturevariationsonthehydrologicalchangesthrough climateelasticitybyapplyingthelandsurfacehydrologicalmodel;and3)toexaminetheimpactsofclimatewarmingon theseasonalsnowcoverandspringflowovertheSRYE.
2. Studyarea
TheSRYE,locatedintheregionbetween95◦50′E–103◦30′Eand32◦N–35◦40′N(Fig.1),hasanareaof121,972km2,and
annualrunoffof2.04×1010m3,accountingfor34.5%oftotalannualrunoffoftheYellowRiverbasin.Itoriginatesfromthe BayanHarmountains,withthealtituderangingbetween2680mand6248mabovesealevelanddecreasingtowardsthe east.Thelandsurfaceinthisregionischaracterizedbyglaciers,snow,lakesandfrozensoils.Thevegetationtypeismostly grassland,covering80%ofthisregion(Zhengetal.,2009).ThehighestelevationisfoundattheAnyemaqenMountains,with permanentsnowcoverand58glaciers,accountingfor95.8%oftotalglacierareas(134km2)overtheSRYE.Sinceglacier
occupiesonlyabout0.11%ofthebasin(Zhangetal.,2013),itisnotconsideredinthiswork.Therearefourhydrological stationsinthemainstreamoftheSRYE:Huangheyan(HHY),Jimai(JM),Maqu(MQ),Tangnaihai(TNH)(Fig.1).TheHHY, withmeanannualrunoffof4.41×108m3(Zhangetal.,2012),contributeslessthan5%ofthetotalflowatTNH.Duetosmall flowcontributionandthemissingstreamflowdataduring1969–1975,datafromHHYwerenotusedintheanalysis.We dividedtheSRYEintothreeregions,theregionupstreamofJimai(JM)hydrologicalstation,betweenJMandMQ(JM-MQ), andbetweenMQandTNH(MQ-TNH).TheregionJM-MQ(Fig.1)isthemajorrunoffgenerationarea,witharunoffratioof 0.38andacontributionof51%tothetotalflowattheTNH.TheregionsupstreamofJMandMQ-TNHarerelativelydrywith runoffratiosof0.21–0.34andcontributing21%and28%tothetotalflow,respectively.Thereareabout5300lakeswitha totalareaof2000km2overtheSRYE(Huetal.,2011),ofwhichmorethan4000lakesarelocatedabovetheHHY(Lietal.,
2013).TheZalingandElinglakesarethetwolargestones,withareasof550km2and610km2,respectively(Fig.1)(Huetal.,
2011).Becausetherearenolargedamsintheregionandthepopulationdensityislow,humanactivitiesandtheirimpacts tobasinhydrologywerenotconsideredinthiswork.
TheaverageannualprecipitationovertheSRYEisabout522mm,rangingfrom350mminthenorthwestto750mmin thesoutheast(Fig.1).About75–90%ofprecipitationfallsinthewetseason(June–September)duetothesoutheastmonsoon fromtheBayofBengal(Zhengetal.,2009).Themeanannualtemperaturevariesbetween−4◦Cand−2◦Cfromthenorthwest tothesoutheast(Huetal.,2011).Januaryisthecoldestmonth,andthetemperaturestaysbelow0◦CfromOctobertoApril; thewarmestmonthisJuly,withameantemperatureof8.0◦C.
3. Datasetsandmethodology
3.1. Data
Thedailystreamflowdatacollectedatthethreehydrologicalstations(JM,MQandTNH;Fig.1)wereobtainedfromthe YellowRiverConservancyCommission(YRCC).Theflowdataareavailableduring1961–2009fortheJMandMQ,andduring 1961–2013fortheTNH.Thedailymeteorologicaldata(1961–2013)includingthemaximumtemperature(Tmax),the min-imum(Tmin),temperature,precipitationandwindspeedfrom20climatestationsovertheSRYEandthesurroundingareas (Fig.1)wereobtainedfromtheChinaMeteorologicalAdministration(CMA).ThedailyTmax,Tmin,precipitationandwind speedforthesestationswereinterpolatedtoobtain1/12◦×1/12◦gridsdatathroughtheinversedistanceweightingmethod. Thetemperaturewasadjustedforelevationbyapplyingacommontemperaturelapserate(0.6◦C/100m)forinterpolation frompointstogrids.
The global 8-day and 0.05◦ Moderate Resolution ImagingSpectroradiometer (MODIS) snow products(MOD10C2) (http://nsidc.org/data/modis/index.html)during2001–2012wereusedforsnowcoveranalysis.
Theterrestrialwaterstorage(TWS)wasestimatedfromtheGravityRecoveryandClimateExperiment(GRACE)satellite launchedinMarch2002(Tapleyetal.,2004).GRACEproductshaveshownaremarkableprospectinwatermasschange (Wahretal.,2004).TherearethreeinstituteswhichofficiallyprovideGRACEproducts:theCenterforSpaceResearch(CSR)at theUniversityofTexasatAustin,GeoForschungsZentrum(GFZ)andJetPropulsionLaboratory(JPL).TheGRACEdatacanbe accessedfromhttp://www.csr.utexas.edu/grace/.Inthisstudy,theGRACERL5.0monthlysolutionsfromCSRfor2004–2013 wereusedtoderivetheevapotranspirationovertheSRYE.TheCSRproductsprovidemonthlyanomaliesoftotalTWSat 1◦
etal.,2004).WiththeP,RandGRACEdata,itispossibletocalculatetheevapotranspiration(ET)throughthewaterbalance equation:
ET =P−R−W; (1)
wherePisprecipitation(mm),Risrunoff(mm),andW(mm)istheTerrestrialwaterstoragechange(TWSC),whichisthe differenceoftwosequentialGRACEsolutions.
3.2. Hydrologicalmodel
Alarge-scalelandsurfacehydrologicalmodelnamedastheVariableInfiltrationCapacity(VIC)(Liangetal.,1994,1996; Lohmannetal.,1998)hasbeenusedinthiswork.TheVICmodel,agrid-basedlandsurfacemodel,parameterizesthedominant hydrometeorologicalprocessestakingplaceatthelandsurface-atmosphereinterface.Themodelsolvesbothwaterand energybalanceforindividualgridcells.Amosaicrepresentationoflandcoverandthevariableinfiltrationcapacitycurve accountingforsubgridheterogeneityinsaturatedextentareusedintheVICmodel.Throughachannelnetwork,surface runoffandbaseflowforeachgridcellareroutedtothebasinoutlet(Lohmannetal.,1998).TheVICmodelhasthecapacityto simulatecoldregionhydrologybecauseitadoptsatwolayerenergybalancesnowmodel(CherkauerandLettenmaier,1999; StorckandLettenmaier,1999)whichrepresentssnowaccumulationandablationandafrozensoil/permafrostalgorithm (CherkauerandLettenmaier,1999,2003)thatsolvesforsoilicecontents.FortheVICmodel,aroutingschemeisusedto obtainthedailysimulatedhydrographattheoutlets.TopographydatawereobtainedfromSRTM(resolution:90m×90m) (http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp).TheDEM(digitalelevationmodel)datawereusedtocreatetheflow directionfilewhichisneededintheroutingschemeatthe1/12◦
×1/12◦grids.
Theevapotranspiration(ET) intheVICmodel consistsof threecomponents:canopy evaporation,transpirationand evaporationfrombaresoils(Liangetal.,1994).TheETwascalculatedasfollows:
ET=
N
n=1
Cv[n]×(Ec[n]+Et[n])+Cv[N+1]×E1 (2)
whereNisthelandcoverclasses;Cv[n]isthefractionofthenthvegetationtypewithinagridcell,andCv[N+1] isthe
fractionofbaresoilwith
N
n=1
Cv[n]=1.Ec[n] andEt[n] arethecanopyevaporationandtranspirationforthenthlandcover
type,respectively.E1istheevaporationfrombaresoils.Ec,EtandE1areestimatedasafunctionofpotentialevaporation(EP)
basedonthePenman-Monteithequationandotherparametersrelatedtovegetationtypeandsoilmoisture.Inthiswork, evapotranspirationfromthewaterbalanceEq.(1)wasusedtocomparewiththeevapotranspirationfromtheVICmodel.
Zhangetal.(2013)setupamodelingframeworkata1/12◦
×1/12◦ spatialresolutionovertheentireTibetplateau.In thisstudy,theVICmodelsetupfortheSRYEwasadoptedfromZhangetal.(2013)includingsoilandvegetationparameters. VegetationtypeswereobtainedfromtheUniversityofMaryland’s(UMD)1kmGlobalLandCoverproduct(Hansenetal., 2000).Thelandcovertypewasconsideredtobefixedduringthemodelsimulationperiod1961–2013.Therefore,theannual variationofsimulatedETismostlycontrolledbymeteorologicalvariables.
3.3. Statisticalanalysis
ThedischargedatawereusedtoevaluatetheVICmodelsimulations,thecalibrationhastwocriteria:relativeerror(Er)and Nash-Sutcliffeefficiency(ENS)(NashandSutcliffe,1970),whichdescribesthepredictionskillofthesimulatedstreamflow
relativetotheobservations.TheENSandErwascomputedfromthefollowingequations:
ENS=1−
M
m=1
Qobsm −Qsimm
2M
m=1
Qobsm −Q¯
2(3)
Er=
M
m=1
Qsimm −Qobsm
M
m=1
Qobsm
Table1
Recommendedstatisticsforsimulationperformanceratings(Moriasietal.,2007).
Rating ENS Er(%)
Excellent 0.75<ENS≤1.00 |Er|<10 Good 0.65<ENS≤0.75 10≤|Er|<15
Satisfactory 0.50<ENS≤0.65 15≤|Er|<25
Unsatisfactory ENS≤0.50 |Er|≥25
wheretheQobsm meanstheobservedmonthlystreamflow,andtheQsimm isthesimulateddischarge; ¯Q istheobservedmean monthlystreamflow;Misthenumberofmonths.AnENSvaluecloserto1andErcloserto0implybettersimulationresults; seeTable1forthesimulationperformanceratingsappliedinthisstudy(Moriasietal.,2007).
Theprecipitation,temperatureandrunoffvalueswerenormalizedbysubtractingtheirtimeseriesmeanvaluesand dividingbytheirstandarddeviations.Lineartrendanalysisthroughsimpleregressionallowedustoinvestigatelong-term changesofthehistoricaldata.Inthismethod,thesumofsquaredresidualsasthedifferencebetweentheobservedvalues andthefittedvaluesisminimized.Thestatisticalsignificanceofthetrendsinthisstudywassetatthe10%level.
Correlationanalysiswasusedtoexaminethestrengthanddirectionoftherelationshipbetweenthehydrologicaland meteorologicalvariables.Thecorrelationcoefficient(r)iscalculatedusingthePearsonmethod.Thestatisticalsignificance ofthecorrelationswasagainsetatthe10%level.
Climateelasticity,proposedbySchaake(1990),wasappliedtoevaluatethesensitivityofstreamflowtoclimatechange (Fuetal.,2007;Sankarasubramanianetal.,2001;YangandYang,2011).Morespecifically,therelativecontributionof precip-itationandevapotranspirationchangestorunoffchangeswasquantifiedwhereevapotranspirationinsteadoftemperature wasconsideredsinceevapotranspirationbetterrepresentstheeffectsofclimatechangeonbasinwaterbalance(Zhengetal., 2009).Onthelongterm,thebasinwaterstoragechangescanbeneglected,thusthebasinwaterbalancecanberepresented as:
R= P−ET; (5)
Withoutconsideringtheimpactsofhumanactivity,thechangesofrunoffbetweentwoperiods(R)canbeestimated as:
R= RP+RET; (6)
whereRPandRETarechangesinrunoffduetoprecipitationandevapotranspirationchanges,respectively.Rcanbe
estimatedasfollows(Doogeetal.,1999;Liuetal.,2012):
R= RP+RET= (pP/P+ETET/ET)R; (7)
wherePandETarethechangesofPandETbetweentwoperiods.PandETaretheclimateelasticityofPandETto runoff,whichimpliesthat1%changeinPorETinduces%changeinR(Tangetal.,2013).Inthisstudy,twoperiods1990s and2000srelativeto1960–1990wereconsidered.SincestreamflowdataattheJMandMQwereonlyavailableto2009,we definetheperiod2001–2009asthe2000s.Eq.(7)wassetupforeachperiodandthevaluesforPandETcomputedfrom thetwoequation,toobtaintheimpactofPandETchangesonrunoff.
4. Precipitation,temperature,andrunoffchanges
4.1. Long-termchanges
Fig.2displaysannualtimeseriesofnormalizedrunoff,regionalmeanprecipitationand temperatureforthebasins upstreamoftheJM,MQandTNHstationsduring1961–2013.Precipitationshowspositivetrendsforallthebasins(Fig.2a–c), withincreasingratesof8.3mm/10yr,1.1mm/10yr,and2.1mm/10yrfortheregionsupstreamofJM,MQ,andTNH, respec-tively.However,thetrendsarenotstatisticallysignificantexceptforthebasinupstreamofJM.Insignificantprecipitation changesduring1960–2006werealsosuggestedbyHuetal.(2012),whopointedoutthatannualprecipitationchangesover theSRYEwerenotnoticeableexceptintheupperpartoftheregion.Theentireregionshowsasignificantwarmingtrend dur-ing1961–2013(Fig.2d–f),withameanwarmingrateofabout0.35◦C/10yr.Particularly,anacceleratedwarmingisnoticed fortherecent30yearsacrosstheSRYE.Differentlyfromthechangesinprecipitation,yearlyrunoffshowsadecreasingtrend forallthethreebasins,althoughthetrendsarenotstatisticallysignificantexceptforthatatMQ(decreaseby9.2mm/10yr). AtJMandTNH,therunoffdecreasesby3.2mm/10yrand6.0mm/10yr,respectively.
4.2. Decadalvariation
Fig.2. Annualtimeseriesofnormalizedbasin-averagedrunoff,precipitationandtemperatureforthebasinsupstreamofJM,MQandTNHstationsforthe period1961–2013.Dashedlinesarethelineartrends.
Fig.4. Correlationcoefficientsbetweenmonthlyrunoffandprecipitation(a,b)ortemperature(c,d)forcurrentmonth(a,c)andtherelationsbetween currentmonthandpreviousmonth(b,d)overthethreebasinsfor1961–2009.
themeanvaluesfortheperiods1961–1990,1990sand2000s.Amongthesethreeperiods,theperiod1961–1990showsthe highestrunoffinallthreebasins;whiletheflowdramaticallydecreasesinthe1990s(Fig.3a–c)accompaniedbyalower precipitationforallthebasins(Fig.3d–f).Precipitationreboundedinthe2000sandreturnedtoasimilarlevelasduringthe period1961–1990.However,therunoffinthe2000sdidnotreboundtothelevelasinthereferenceperiod.Atthesame time,althoughprecipitationinthe2000swashigherthanthatinthe1990sinallthreebasins,runoffwasalmostthesame inthetwoperiodsexceptatJMwithhigherflowsinthe2000s.Acontinuouswarmingwasobservedforallbasinsduring 1961–2013(Fig.3g–i).ThisresultisconsistentwiththeIntergovernmentalPanelonClimateChange(IPCC)fifthreport, whichrevealsthateachofthepastthreedecadeshasahighertemperaturethanallthepreviousdecadesintheinstrumental records(IPCC,2013).Whyrunoffdidnotrecoverduringthe2000sattheMQandTNHstationsalongwiththerecovered precipitationrelativetotheperiod1961–1990,ZhouandHuang(2012)explainedthattheincreaseinprecipitationmostly occurredinthedryregionoftheSRYEwhereprecipitationismostlyevaporated.Wewillfurtherdiscussandquantifythe impactsofprecipitationandtemperaturechangesonrunoffovertheSRYEinSection5.2.
4.3. Linkagebetweenrunoffandclimatevariables
Throughacorrelationanalysis,thenormalizedannualtimeseriesofprecipitation,temperature,andrunoff(Fig.2)reveal thattheinter-annualrunoffvariationsarehighlyconsistentwiththeprecipitationfluctuations(Fig.2a–c),withcorrelation coefficientsrof0.75,0.86and0.85attheJM,MQandTNH,respectively(significantat10%level).Therelationshipbetween runoffandtemperatureisnegative,lessstrongandinsignificant(Fig.2d–f).Thegoodcorrespondencebetweenrunoffand precipitationvariationssuggeststhatprecipitationplaysadominantroleintherunoffgenerationovertheSRYE.
Fig.4showscorrelationsbetweenmonthlyrunoffandprecipitation/temperaturewithlagsfrom0to1month.There isasignificantpositiverelationshipbetweenprecipitationandrunoffduringJune–Octoberforallthebasins(rvaluesof 0.32–0.70),withthehighestcorrelationsinJune,JulyandSeptember,andthelowestoneinAugust(Fig.4a).Thestrong correlationslastfromsummertoNovemberatMQandTNH(rvaluesof0.27and0.36).Fig.4bexhibitsthecorrelations betweenrunoffandprecipitationinthepreviousmonth.RandPmon-1arepositivelycorrelatedsignificantlyfromMayto
NovemberatJM,whileitlastsfromApriltoDecemberatMQandTNH.ThervaluesarehigherinAugust,Octoberand Novemberthanthosewithazerolag,suggestingthatrunoffisinfluencedbyprecipitationnotonlyincurrentmonthbut alsopreviousmonthduetothedelayofflowtravelingdownstream.ItwasalsonoticedthatthervaluesbetweenRandPmon-1
atMQandTNHaregenerallylargerthanthoseforJMduringApril–November(Fig.4b),indicatingthattheconcentration timeofstreamflowclosetothemonthlyscaleconsideredforprecipitationishigherdownstreamthanupstream.
Fig.5.MonthlytimeseriesofsimulatedandobservedstreamflowfortheSRYEatTNHstationduringthecalibrationperiod1961–1990(a)andthevalidation period1991–2013(b).
StrongnegativerelationshipalsoexistsbetweenRandTmon-1duringApril–JunforMQandTNH(negativervaluesinthe
range0.31–0.62)(Fig.4d).Thereasonwhyrunoffisnegativelycorrelatedwithtemperatureduringspringandearlysummer isstillunknown.AsignificantpositiverelationshipbetweenRandTmon-1inNovemberisobserved,withrvaluesintherange
0.33–0.41(Fig.4d)forallbasins.Thestrongpositiverelationsbetweenrunoffandtemperatureintheautumnmaysuggest afastmeltofsnowfalltoproducerunoffinthisperiod.ThemodelingresultsinZhangetal.(2013)alsosuggestedasnowfall runoffpeakinOctoberovertheSRYE.
5. Hydrologicimpactsofmeteorologicalchanges
ThecorrelationanalysisinSection4.3hassuggestedadominantroleofprecipitationinrunoffgenerationovertheSRYE. However,theoveralldecreasingrunofftrendoverthestudyperiodwasaccompaniedbyanoveralllong-termincreasing trendinprecipitation(Fig.2).Thisresultmayimplythatotherfactorsalsoaffectflowvariations.Onthedecadaltimescales (Fig.3),precipitationinthe2000swas3.1%and5.3%higherthanthatinthe1990sforMQandTNH,whereasrunoffwas almostthesameinthesetwotimeperiods(Fig.3b–c).Thismaysuggestareductioninrunoffgeneration.Theincrease inevapotranspirationinarapidlywarmingclimatemaybethereason.Sinceactualevapotranspirationobservationswith long-termrecordsatlargescalesarenotavailableintheSRYE,wequantifiedtheevapotranspirationchangesinbothtime andspacewiththeVIClandsurfacemodel.TheVICmodelwasevaluatedthroughcomparisonsbetweensimulatedand observedstreamflowattheTNHstation.
5.1. Calibrationandvalidationofhydrologicalmodel
Fig.5presentsmonthlytimeseriesofsimulatedandobservedstreamflowattheTNHstationduring1961–2013.Regarding theuseofthehydrologicalmodelforsimulatingrunoffchangesduetochangesinmeteorologicalconditions,wesplitthe studyperiodintocalibrationperiod(1961–1990)andvalidationperiod(1991–2013).TheVICmodelsimulationscaptured thevariationsandmagnitudeofstreamflowwellduringboththecalibrationandvalidationperiods(Fig.5),withthe Nash-Sutcliffeefficiency(ENS)andrelativeerror(Er)valuesof0.90,and−1.4%,respectivelyinthecalibrationperiodand0.84and 2.8%inthevalidationperiod.Thisperformancecanbeclassifiedasgoodforthecalibrationperiodandsatisfactoryforthe validationperiodfollowingtheratingsinTable1.
Fig.6.EstimatesofmonthlyactualevapotranspirationfromtheVICmodelandGRACEdatafor2004–2013.
Fig.7.Spatialdistributionofmeanannualprecipitation,evapotranspirationandrunoffinthereferenceperiod1961–1990(a–c),andtheirchangesinthe 1990s(d–f)and2000s(g–i)relativetothereferenceperiod.
actualevapotranspirationestimatessuggeststhattheVICsimulatedETisreasonableandcanbeusedtoquantifytheET changesovertheSRYE.
5.2. Meteorologicalchangeimpactsonannualrunoff
Table2
Precipitation(P),runoff(R)andevapotranspiration(ET)changesinthe1990sand2000scomparedwiththereferenceperiod1961–1990forJM,JM-MQ, MQ-TNHandtheentireSRYE.
JM JM-MQ MQ-TNH TNH
1961–1990 427.1 682.5 482.3 525.3
P 1991–2000 415.2(−2.8%) 635.4(−6.9%) 454.4(−5.8%) 497.9(−5.2%) 2001–2009 442.8(3.7%) 637.6(−6.6%) 506.6(5.1%) 524.4(−0.2%)
1961–1990 99.3 274.3 178.2 180.8
R 1991–2000 75.7(−23.8%) 225.4(−17.8%) 133.8(−24.9%) 143.1(−20.8%) 2001–2009 88.1(−11.3%) 207.2(−24.5%) 144.8(−18.7% 144.9(−19.9%)
1961–1990 294.8 417.9 328.1 344.3
ET 1991–2000 307.1(4.2%) 425.6(1.8%) 333.8(1.7%) 352.2(2.3%)
2001–2009 320.4(8.7%) 441.6(5.7%) 352.6(7.5%) 369(7.1%)
Table3
Contributionofprecipitationandevapotranspirationtorunoffchangesinthe1990sand2000s.
Time Formula Contribution
P E
JM 1991–2000 −23.8=4×(−2.8)+(−3)×4.2 47% 53%
2001–2009 −11.3=4×3.7+(−3)×8.7 100%
JM-MQ 1991–2000 −17.8=2.09×(−6.9)+(−1.88)×1.8 81% 19%
2001–2009 −24.5=2.09×(−6.6)+(−1.88)×5.7 56% 44%
MQ-TNH 1991–2000 −24.9=2.97×(−5.8)+(−4.51)×1.7 69% 31% 2001–2009 −18.7=2.97×5.1+(−4.51)×7.5 100%
TNH 1991–2000 −20.8=2.8×(−5.2)+(−2.72)×2.3 70% 30% 2001–2009 −19.9=2.8×(−0.2)+(−2.72)×7.1 3% 97%
the1990sand2000srelativetothereferenceperiod.Thedatashowthehighestannualprecipitationinthesoutheastofthe SRYE,withameanannualvalueofabout700–800mm,anddecreasingtowardsthenorthwest,withannualprecipitationas lowas150–250mmintheveryupstreampartsoftheSRYE(Fig.7a).Thespatialpatternsofactualevapotranspirationand runoff(Fig.7bandc)generallyfollowthatofprecipitation,withthehighestevapotranspirationandrunoffinthesoutheastof thebasinwherethereissufficientwaterforevaporation.TheregionJM-MQ(Fig.7c)inthesoutheastisalsothemajorrunoff generationarea(annualRof300–500mm)intheSRYE.ThedriestareaisintheupstreamregionofJM(annualR<150mm) andtheverydownstreampartsoftheSRYE(Fig.7c).
Precipitationconsistentlydecreasedalmostovertheentirebasininthe1990s(Fig.7d)by2.8–6.9%relativetothereference periodoverthethreesub-regions(Table2).Thespatialpatternofrunoffchangesinthe1990sissimilartothatofprecipitation (Fig.7f),butwithastrongerdecreaseof20.8%(Table2).However,actualevapotranspirationshowspositivechangesinthe 1990s(Fig.7e),withameanincreaseof2.3%(Table2),whichisconsistentwiththecontinuouswarminginthe1990s (Fig.3).Therefore,thedecreaseinprecipitationandincreaseinevapotranspirationbothcontributedtotherunoffdecrease inthe1990s.Inthe2000s,precipitation(Fig.7g)exhibitedinhomogeneouschangeoverthebasin,withanincreaseinthe upstreamregionsofJM(3.7%)andMQ-TNH(5.1%),opposedtoadecreaseinthemajorrunoffgenerationareaforJM-MQ (6.6%).Despitethesechanges,precipitationalmoststayedunchangedforthebasinasawhole(Table2).Inthe2000s,the patternofrunoffchangesdifferedfromthatofprecipitation(Fig.7gandi).RunoffdecreasedintheupstreampartsofJM andMQ-TNH(11.3–18.9%)whileprecipitationincreased,andtherunoffdecreasedevenmoreintheregionJM-MQ(24.5%), leadingtoameandecreaseinrunoffbynearly20%overtheentireSRYE.Alongwiththerapidwarminginthe2000s(Fig.3), evapotranspirationlargelyincreasedinthe2000s(Fig.7h)relativetothereferenceperiod(meanincreaseof7.1%;Table2). ResultsinFig.7andTable2alsosuggestthattherunoffchangesinthe1990sand2000sovertheSRYEaretheresultofboth precipitationandevapotranspirationchangesassociatedwiththewarmingclimate.However,whatisthecontributionof eachmeteorologicalvariabletotherunoffchangesovertheSRYE?
Fig.8. Spatialdistributionofmeanmonthlysnowcoverage(%)intheSRYEfor2001–2012.
ofevapotranspiration(44%;Table3).Forthebasinaverage,evapotranspirationcontributed97%oftherunoffchangesin the2000s(Table3),suggestingthattheinfluenceofevapotranspirationontherunoffisincreasingalongwiththeincreased warmingovertheSRYE.Thismaypartlyexplainthereasonwhyprecipitationrecoveredtothelevelof1960–1990inthe 2000sbutrunoffwasstilllow(Fig.3).TheincreasedprecipitationinJMandMQ-TNHmostlyevaporatedduetotherapid warminginthe2000s.AnotherreasonwasthatprecipitationdecreasedinthemajorrunoffgenerationareaJM-MQinthe 2000s,resultinginalargerrunoffdecreaseaccompaniedwiththeincreasedevapotranspiration(5.7%;Table2).
5.3. Meteorologicalchangeimpactsonseasonalsnowcoverandspringflow
TheSRYEisextensivelycoveredbysnowwithameanannualcoverageofabout18%basedontheMOD10C2dataduring 2001–2012(Fig.8).TheregionupstreamofJMhasthemostextensivesnowcover,withmeanannualcoverageofabout 21%,whiletheJM-MQ(16%)andMQ-TNH(14%)areashaverelativelysmallercoverage.Thesnowpackbeginstoaccumulate inOctober,withthehighestconcentrationinthesoutheastoftheupstreambasinsandtheAnyemaqenmountains(Fig.8). ThesnowcoverstartstomeltinAprilandMay,andmostlymeltsawayinJuneexceptfortheAnyemaqenMountains.We examinedthetrendsofsnowcoverforeachmonthduring2001–2012andfoundthatitdecreasedforallthebasinsinMayby 1.1–1.3%/yr(Fig.9a).Fig.9b–dpresentthenormalizedvariationsoftemperatureandsnowcoverinMayduring2001–2012 forthethreebasins.ThetemperatureinMayexhibitsawarmingtrendduring2001–2012andisnegativelycorrelatedwith thesnowcovervariations(Fig.9b–d),withhighcorrelationcoefficientsof−0.7atJM,−0.8atMQand−0.85atTNH.
Fig.9. TrendsofsnowcoverageinMayinthebasinsupstreamofJM,MQandTNH(a),andnormalizedsnowcoverageandtemperatureinMayduring 2001–2012inthethreebasins(b–d).
the1980s–2000s,consistentwiththeshiftofspringpeakflowsatJM.ForMQandTNH(Fig.10c–d),thespringflowrises moresmoothlythanatJMandseldomhasclearsharppeaksduringMarch–May.ThisisprobablybecausetheMQandTNH areashavelesssnowcoverageandtherainfallrunoffplaysamoreimportantroleinthespringandsummerrunoffflowsof downstreamareas.
6. Discussion
Ouranalysessuggestthat,inthe1990s,runoffchangesovertheSRYEwerecausedbybothdecreaseinprecipitationand increaseinevapotranspiration,withthedecreaseinprecipitationplayingadominaterole.Inthe2000s,runoffchangeswere mainlycausedbytheincreaseinevapotranspirationespeciallyinthedryregionsupstreamofJMandMQ-TNH(runoffratios of0.21–0.34).ThewidespreadoflakesandwetlandsatJMmayfavortheincreaseinevapotranspirationalongwithclimate warming.Inthiswork,weonlytookmeteorologicalchangesandvariationsintoconsideration,whiletheimpactsofland coverchangesonrunoffwerenotanalyzedbecausetheSRYElessaffectedbyhumanactivities(Cuoetal.,2013,2015).Cuo etal.(2013),throughamodelingapproach,reportedthattherunoffchangesupstreamofTNHduringthepastfewdecades weremostlycausedbymeteorologicalchanges,andtheimpactsoflandcoverchangesonrunoffchangeswereverysmall. However,Zhengetal.(2009),usingaclimateelasticityapproach,estimatedthatthelandcoverchangescontributedfor morethan70%totherunoffreductioninthe1990s.Theinconsistenciesbetweenourandtheirconclusionsmaybepartly causedbythedifferentapproachesused.StudiesinotherbasinsintheTibetanPlateausuggestedthatlandcoverchangeand humanactivitiessuchassurfacewaterandgroundwaterexploitation,newwater-relatedpolicyimplementation,agricultural productionactivitiesmayexertgreatinfluenceonrunoffchanges(Zhangetal.,2015;PervezandHenebry,2015;Huoetal., 2008).Infact,inawarmingclimate,theSRYEisundergoinggreatchanges,suchaswetlandandfrozensoilchanges,aswell aslakesexpansion,whichmayresultinlandcoverchanges.Thegrasslandareaafter1990decreasedbyabout10%relative tothepreviousyears,andthesandylandincreasedbyaround4%overtheSRYE(Zhengetal.,2009).Limitedbytheharsh livingconditions,thepopulationsizeissmallinthisregion.Theurbanizationandindustrializationareveryslowaswell,thus waterabstractionshavenotchangedtoomuch.Whilethenumberoflivestockincreasedthreefoldduring1970–2000inthe areaupstreamofHHY(Wangetal.,2000),waterabstractionsforlivestockcertainlyincreased.Meanwhile,ahydropower 17KmdownstreamofElinglakewasbuildin1998andinoperationsince2001.Towhatextentthesechangesandhuman activitiesareresponsibleforthedecreasingrunoffshouldbeconsideredinthefuturework.
ThereareextensivefrozensoilsintheSRYE,whichisessentialtopreservethewaterresources(Lietal.,2012).Along withthewarmingclimate,manystudieshavesuggestedthattheTPisexperiencingpermafrostdegradation(ChengandJin, 2012;ChengandWu,2007;Wuetal.,2007).Theincreaseinactivelayerthicknessofpermafrostduetowarmertemperature leadstomorewaterforevaporationandhencereducedrunoff.Atthesametime,thegraduallythickenactivelayermayhold morewaterinthesoillayersandleadtolesssurfacerunoff.Althoughitisacceptedthatclimatechangeisoneofthemajor driversforhydrologicalchanges,theeffectsofpermafrostdegradationonrunoffprocessesstillremaincontroversial.Thus, furtherresearchisneededtoexploretowhatextentpermafrostdegradationmightimpactrunoffchangesovertheSRYE.In thecomplexbackgroundofenvironmentchangesincludingclimatechange,landcoverchanges,frozensoildegradationand humanactivities,soilmoistureandgroundwatermayalsochange,thusleadingtodifferentconditionsofrunoffgeneration. Thelong-termlineartrendsofprecipitation,runoffandtemperature(Fig.2)presentageneraltemporalchangeofthese variablesinthepast50years.However,giventherelativelyshortrecords,thesealsomightbetheinfluenceofinter-annual anddecadalvariability(Fig.3)(Hannafordetal.,2013;Willems,2013b).Fig.2clearlyexhibiteddecadalvariationsinrunoff andprecipitation.AsshownbyWillems(2013a)andTayeandWillems(2013)andothersforotherregionsintheworld, thedecadalvariationsmightbeexplainedbyatmosphericoroceanographicoscillationssuchasElNi ˜nosouthernoscillation (ENSO),thePacificDecadalOscillation(PDO),theSouthernOscillationIndex(SOI),theNorthAtlanticOscillation(NAO),and theAtlanticMultidecadalOscillation(AMO).AstudyintheBlueNileriverbasininEastAfricashowedthatmulti-decadal oscillationsmodulatedthehighstreamflowsandthattheinfluenceofwatershedcharacteristicschangesisverysmall(Taye etal.,2015).IntheSRYE,thetemporalchangesinrunoffappearexplainedbybothlong-termtrendsanddecadalvariations ofclimate.
AnnualrunoffandannualmeantemperaturewerefoundnegativelycorrelatedovertheSRYEduring1961–2009,which isconsistentwiththefindingoftheclimatesensitivityanalysiswiththeVICmodel,whichshowedthattheincreasein evapotranspirationalongwiththewarmingclimatewasthemainfactorforrunoffdecreaseinthe2000s.Attheannual scale,thenegativecorrelationbetweenrunoffandtemperaturewasnotsignificant.Thisisdifferentatmonthlyscale,where significantnegativecorrelationwasfoundduringMarchtoJune(Fig.4candd).Inthesemonths,theevapotranspirationmay increaseduetothewarmingtemperature,explainsthedropinstreamflow.Inthewetseason(July–September),therewas nosignificantrelationshipbetweentemperatureandprecipitation,becauseprecipitationwasquitehigh,andtemperature hadarelativelysmalleffectontherunoffchanges.Inspringandlatesummer,precipitationisnotthathighasinthewet season,thusitseffectonrunoffisveryweak,andtheroleoftemperatureonrunoffisbiggerrelativetoprecipitation.This mayexplainwhyrunoffandtemperaturearenotsignificantlycorrelatedatannualscales.
simulatechangesinrunoffasaresultofmeteorologicalchangesbeyondtherangeofmeteorologicalconditions consid-eredduringthestandardmodelcalibrationandvalidationasconsideredinthisstudy;seeRefsgaardetal.(2014)andVan SteenbergenandWillems(2012)forpotentialmethods.
7. Conclusions
Inthiswork,wehaveinvestigatedthespatial-temporalchangesofhydrologicalandmeteorologicalvariablesandthe linkagebetweenrunoffandprecipitation/temperatureovertheSRYEduring1961–2013.Theimpactsofprecipitationand temperatureonthehydrologicalchangeswerequantifiedthroughclimateelasticitybyapplyingtheVIClandsurface hydro-logicalmodel.TheimpactsofthewarmingclimateontheseasonalsnowcoverandspringstreamflowovertheSRYEwere alsoexamined.Themainfindingsofthisstudyareasbelow:
(1) Duringtheperiod1961–2013,annualprecipitationovertheSRYEexhibitedweaklyincreasingtrends,whilethe pre-cipitationupstreamofJMincreasedsignificantlybyabout8.3mm/10yr.Temperatureshowedconsistentlywarming trendsinallthebasinsoftheSRYEwithameanwarmingrateof0.35◦C/10yr.Meanwhile,runoffdecreasedatthethree hydrologicalstationsbyabout3.2mm/10yr,9.2mm/10yrand6.0mm/10yrattheJM,MQandTNHstations,respectively. (2) Relativetothereferenceperiod1961–1990,runoffdecreasedbyabout21%inthe1990sovertheSRYE.Thedecrease inprecipitationandtheweakincreaseinevapotranspirationbothcontributedtotherunoffdropinthe1990s. How-ever,decreaseinprecipitationplayedamoreimportantrole(70%)thanincreaseinevapotranspiration(30%)inthis runoffreduction.Runoffdecreasedbyabout20%inthe2000s,duringwhichprecipitationcontributedfor3%tothe runoffreduction,whiletheincreaseinevapotranspirationaccountedfor97%.DuetostrongwarmingovertheSRYE, evapotranspirationisplayinganincreasinglyimportantroleinaffectingrunoffchangesinrecentdecades.
(3) AnalysisoftheMODISdatashowadecreasedtrendofsnowcoverinMayovertheSRYEduring2001–2012;thischange wascloselyrelatedtothestrongwarmingtemperatureinthepastdecade.Inthepast30years,thespringpeakflow mainlycausedbysnowmeltoccurredearlierforabout15daysattheJMstation.Thisshiftinpeakflowtimingisexpected tobeduetoanearliersnowmeltingassociatedwiththeclimatewarmingovertheSRYE.
Conflictofinterest
Duetotheconflictofinterest,wewouldsuggestavoidthepotentialreviewersfromthesameinstituteasthefirstand correspondingauthors-theInstituteofTibetanPlateauResearch,CAS.
Acknowledgments
WethankProf.ZhongboYuforhishelpfulcomments.ThisworkwassupportedbytheNationalNaturalScienceFoundation ofChina(41190081,41171051)and the“StrategicPriorityResearchProgram(B)”of theChineseAcademyofSciences (XDB03030209).
AppendixA. Supplementarydata
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ejrh.2016.03.003.
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