Extreme hydrological events and the influence of reservoirs in a highly regulated river basin of northeastern Spain

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ContentslistsavailableatScienceDirect

Journal of Hydrology: Regional Studies

jou rn a l h om ep a ge :w w w . e l s e v i e r . c o m / l o c a t e / e j r h

Extreme hydrological events and the inﬂuence of reservoirs in a highly regulated river basin of northeastern Spain

S.M. Vicente-Serrano

^a,∗

, J. Zabalza-Martínez

^a

, G. Borràs

^b

, J.I. López-Moreno

^a

, E. Pla

^c

, D. Pascual

^c

, R. Savé

^d

, C. Biel

^d

, I. Funes

^d

, C. Azorin-Molina

^a

,

A. Sanchez-Lorenzo

^a

, N. Martín-Hernández

^a

, M. Pe ˜ na-Gallardo

^a

, E. Alonso-González

^a

, M. Tomas-Burguera

^e

, A. El Kenawy

^f

aInstitutoPirenaicodeEcología,ConsejoSuperiordeInvestigacionesCientíﬁcas(IPE-CSIC),Zaragoza,Spain

bOﬁcinaCatalanadelCanviClimàtic,GeneralitatdeCatalunya,Barcelona,Spain

cCREAF,CerdanyoladelVallèsBarcelona,Spain

dIRTA,EnvironmentalHorticulture,TorreMarimon,CaldesdeMontbui,Barcelona,Spain

eEstaciónExperimentaldeAulaDei(EEAD-CSIC),Zaragoza,Spain

fDepartmentofGeography,MansouraUniversity,Mansoura,Egypt

a rt i c l e i n f o

Articlehistory:

Received23June2016

Receivedinrevisedform16January2017 Accepted29January2017

Availableonline13April2017

Keywords:

Droughts Floods Segrebasin Hydrologicaltrends Dams

Reservoireffects

a b s t ra c t

Studyregion:TheSegrebasin(northeasternSpain).

Studyfocus:TheSegrebasinisextensivelyregulated,throughadensenetworkofdams, duringthesecondhalfofthe20thcentury.Thisstudyassessedtheimpactofriverregu- lationontheevolutionofhydroclimatologicalextremeeventsacrossthebasinduringthe pastsixdecades(1950–2013).Weassessedwhethertheoccurrenceoffloodsandhydrolog- icaldroughtshaschanged,andwhetherthesechangeshavedifferedspatiallybetweenthe headwatersandlowerareasofthebasin.Forthispurpose,weemployedasetofhydroclima- tologicalindicesinordertoquantifytheevolutionoftheamountaswellasthefrequencyof quantilesofhighprecipitationandfloodevents.Changesinthesevariableswereassessed bymeansofthenonparametricMann–KendallTaucoefficient.

Newhydrologicalinsights:Resultsrevealageneralreductionintheoccurrenceofextreme precipitationeventsintheSegrebasinfrom1950to2013,whichcorrespondedtoagen- eralreductioninhighflowsmeasuredatvariousgaugedstationsacrossthebasin.While thisstudydemonstratesspatialdifferencesinthedecreaseofstreamflowbetweenthe headwatersandthelowerpartsofthebasin,mainlyassociatedwithchangesinriver regulation,therewasnoreductioninthefrequencyoftheextraordinaryfloods.Changes inwatermanagementpracticesinthebasinhavesignificantlyimpactedthefrequency, duration,andseverityofhydrologicaldroughtsdownstreamofthemaindams,asaconse- quenceoftheintensewaterregulationtomeetwaterdemandsforirrigationandlivestock farms.Nonetheless,thehydrologicalresponseoftheheadwaterstothesedroughtsdiffered markedlyfromthatofthelowerareasofthebasin.

∗ Correspondingauthor.

E-mailaddress:[email protected](S.M.Vicente-Serrano).

http://dx.doi.org/10.1016/j.ejrh.2017.01.004

licenses/by-nc-nd/4.0/).

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1. Introduction

Determiningtheoccurrenceofextremeeventsinthehydrologicalcycleisoneofthemainprioritiesofhydrologistsand watermanagers,astheseeventscommonlyhavemajoreconomic,environmental,andsocialimpacts(e.g.Kunkeletal.,1999;

VanDijketal.,2013).Underglobalwarmingconditions,thefrequencyandmagnitudeofextremeprecipitationeventsare likelytoincrease(Trenberth,2012),duetothehigherspeciﬁcatmospherichumidityassociatedwiththeClausius–Clapeyron relationship(Santeretal.,2007;Trenberthetal.,2005;Allan,2012;Westraet al.,2014).ThemostrecentIPCCreport (Hartmannetal.,2013)showsthatchangesinprecipitationextremesareconsistentwithawarmerclimate.Nevertheless, thereportalsoemphasizesthatchangesinextremeprecipitationeventsshowlowspatialcoherence(Alexanderetal.,2006;

Westraetal.,2013;Dittusetal.,2015).

Droughtpatternsareevenmuchmoredifficulttodetermine(Vicente-Serrano,2016).Seneviratneetal.(2012)highlighted majoruncertaintiesintheevolutionofclimatedroughtsworldwide.Thesedifficultiesareconfirmedinarangeofstudies thatassesseddroughttrendsattheglobalscale(e.g.Sheffieldetal.,2012;Dai,2013;Trenberthetal.,2014),highlightingthe needtoanalyzetheevolutionofextremehydroclimaticeventsatregionalscales.

Anotherimportantuncertaintyishowextremeeventspropagatethroughoutthehydrologicalcycle,asclimaticand hydrologicalextremeeventsdonottypicallycoincideinmagnitude,spatialextent,andtime.Thisfeaturecanbelinked totopography(Lorenzo-Lacruzetal.,2013;Barkeretal.,2015),previousclimateconditions(Medieroetal.,2014),and vegetationcover(Lana-Renaultetal.,2012; Serrano-Muelaetal.,2015).Othervariables (e.g.landscapechanges,water regulationandmanagement,etc.)canalsocomplicatetheresponseofextremehydrologicaleventstoextremeclimate events,asreportedinearlierstudies(e.g.López-Morenoetal.,2006;Llasatetal.,2014;Medieroetal.,2014,2015;Machado etal.,2015;CrooksandKay,2015).Furthermore,whileextremerainfalltypicallyoccursatdailyorevensub-dailyscales,with notableregionalorlocaleffects,droughtsareusuallystudiedatmonthlyscalesandtendtoimpactlargerareas.Accordingly, itisimportanttoconsiderthedistincttimeandspatialscaleofthesetwotypesofextremeevents.

InthewesternMediterraneanregion,thereisanevidenceofadecreaseinthefrequencyandmagnitudeofextreme precipitationeventsoverrecent decades(López-Moreno etal.,2010;Gallegoetal.,2011;Valenciaet al.,2012), while thereisanincreaseinthedurationandseverityofclimatedroughts(Vicente-Serranoetal.,2014;Spinonietal.,2015;

Lorenzo-LacruzandMorán-Tejeda,2016;Colletal.,2016).Riverﬂoodshavealsodecreased,asaconsequenceofchangesin precipitation,combinedwithhigheratmosphericevaporativedemand(AED)(Medieroetal.,2014)andincreasedvegetation coverintheheadwaters(López-Morenoetal.,2006).Hydrologicaldroughtsalsoshowedhigherincreasesinseverityand durationcomparedtometeorologicaldroughts.ThisfeaturecanbeexplainedbythehigherAED(Vicente-Serranoetal., 2014),increasedtourism,urbanwaterdemands,andtheexpansionofirrigatedareas(Lorenzo-Lacruzetal.,2013).

IntheMediterraneanregion,theavailabilityofwaterresourcesiscritical(García-Ruizetal.,2011).Managingwater resourcesinanyMediterraneanreservoirmustmakebalancebetweentheneedtostorewaterfordifferentwatersupplies anduses,andtheneedtomanagefloodsandtheircatastrophiceffects(López-Morenoetal.,2002).Thisbalanceiscritical, especiallyduringspringandsummer,duetothehighwaterdemandandthehighprobabilityofextremeprecipitation eventsduringtheseseasons.Inthiscontext,albeitwithnumerousstudiesinvestigatingtheeffectsofreservoirsonriver regimesandstreamflowsinthewesternMediterranean(e.g.Batallaetal.,2004;Piquéetal.,2016;Vicente-Serranoetal., 2016),onlyfewstudieshaveconsideredthejointeffectofdammingandreservoirmanagementontheseverityoffloods andhydrologicaldroughtsdownstream.

Inthisstudy,weinvestigatedtheevolutionofextremeclimateandhydrologicaleventsinthepastsixdecadesacrossthe Segrebasin(northeasternSpain).Thisbasin,whoseheadwatersarelocatedinthePyrenees,hasbeenhighlyregulatedby numerousdamsduringthesecondhalfofthe20thcentury(Vicente-Serranoetal.,2016).Themainobjectiveofthisstudy wastodeterminewhethertheoccurrenceandseverityofﬂoodsandhydrologicaldroughtshavechangedinrecentdecades, andwhetherthesechangeshavedifferedbetweentheheadwatersandlowerareasofthebasin.

2. Studyarea

TheSegrebasinislocatedinnortheasternSpain,anditsdrainageareacoversapproximately13,000km².Thebasinhas threemainrivers:theSegreRiver(8167km²;themaintributaryoftheEbroRiver),theNogueraPallaresaRiver(2807km²), andtheNogueraRibagorzanaRiver(2061km²)(Fig.1).Theelevationrangesfrom175m,wheretheSegreRiverentersthe EbroRiver,tomorethan3200minthePyrenees.Thereliefcausesmarkedclimaticandlandscapecontrastsinthebasin.

InthePyreneanheadwaters,theprecipitationexceeds1100mmyear⁻¹,butinthesouthernlowlandstheaverageannual precipitationis<300mmyear⁻¹.Annualreferenceevapotranspirationintheheadwatersis<600mmyear⁻¹,butitexceeds 1100mmyear⁻¹inthesouth.Climateandtopographicfactorsareresponsiblefortheremarkablelandscapecontrastsinthe basin.Inthenorth,thedominantlandscapeunitsarealpinepasturesandsubalpineandsub-Mediterraneanforests,including Pinusuncinata,Pinussylvestris,Fagussylvatica,andQuercussp.Inthecenterofthebasin(elevationsof800–1000m),shrubs andforestsdominateinsomeareas,reﬂectingsuccessionalchangesassociatedwiththeabandonmentofthecultivated slopesduringthe20thcentury(García-RuizandLana-Renault,2011;Buendiaetal.,2015).Irrigatedagriculturaloccursin thelowerpartofthebasin,facilitatedbytheconstructionofdams.Thebasinhas144,000irrigatedhectares,servedby thecanalsofUrgell,Pinyana,AragónandCatalunya,andSegarra-Garrigues.Thus,agro-industriesandintensivelivestock

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Fig.1. Spatialdistributionofgaugestations(blacksquares),precipitationstations(bluecircles),andreservoirs.Backgroundcolorsrepresentelevation.

Dashedlinesrepresenttheboundariesofthedifferentsub-basins.

productionarethemaineconomicactivitiesinthebasin.Thefewremainingnaturalareasinthearidlandsofthebasin correspondtothenorthernmoststeppesofEurope(Braun-BlanquetandBolòs,1958).

Innormalconditions,theSegreRivershowsaclearseasonalregime,withthemainflowsoccurringduringMayand June,duetosnowmeltingandhighspringtimeprecipitation.However,inthelowerpartofthebasin,theriverregimeis highlymodified,asaconsequenceofimpoundmentoftheriveraswellaswatermanagementforirrigationusesandurban supplies(Batallaetal.,2004;Piquéetal.,2016).Currently,thereare35reservoirsintheSegreRiverbasin,providinga totalstoragecapacityof2084hm³(1hm³=1,000,000m³):avaluewhichisveryclosetotheaverageannualstreamflow recordedclosetothemouthoftheriver(2130hm³).Mostwaterregulationoccursintheheadwatersandthemiddlereaches oftheNogueraRibagorzanaRiverand theNogueraPallaresaRiver,wheretheEscales(163hm³),Canelles(687.5hm³), SantaAnna(236.6hm³),Talarn(205.1hm³),andCamarasa(163hm³)reservoirsarelocated.TheSegreRiverisregulated bytheOliana(101hm³)andRialb(402.8hm³)reservoirs;thelatterwasestablishedin2000.Thesereservoirscauseda markeddecrease(>60%)intheannualstreamflowoftheSegreRiverduringthepastsixdecades(Vicente-Serranoetal., 2016).

Theoccurrenceofextremeprecipitationeventsiscommoninthebasin(LlasatandPuigcerver,1997;Begueríaetal.,2009).

Theseeventsareassociatedwithshort-lived,isolated,andhigh-intensitythunderstormsinsummer.Inautumn,theseevents arelinkedtothebackwardtrajectoriesoftheMediterraneanperturbations,whichinvolvetheoccurrenceoflowlevelwarm andhumidnortheasterlyandeasterlyflows,withcoldcontinentalairabovecut-offlowsovertheMediterraneanSea(Ramis etal.,1997).Inadditiontotheseatmosphericconfigurations,extremeprecipitationeventsareenhancedbythecomplex terrain(Pastoretal.,2001),makingsomehistoricalfloodsinthebascatastrophic,withseriousenvironmental,economic, andsocietalimpacts(Arbioletal.,1984;Barriendosetal.,2003;Thorndycraftetal.,2006;Llasatetal.,2009,2010,2013).

InnortheasternSpain,climatedroughtperiodsoccurfrequently,duetotheoccurrenceoflowﬂowsandhydrological droughts(Vicente-Serrano,2006;Lorenzo-Lacruzetal.,2013).Inparticular,somehydrologicalbasinssufferedfromlong, intenseandseveredroughtsoverthelastthreedecades,inresponsetothedecreasedprecipitationandincreasedAED (Lopez-Bustinsetal.,2013;Vicente-Serranoetal.,2014).

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3. Dataandmethods 3.1. Data

TheEbroRiverBasinManagementAuthority(ConfederaciónHidrográficadelEbro)providedthedailystreamflowdata fortheSegrebasin.Thedailystreamflowdatafor11gaugestationswerechosenforthisstudybecausethesestationshad lessthan15%ofmissingdatafortheperiod1950–2013(Fig.1).Gapfillingwasperformedusingalinearregressionanalysis, inwhichtheindependentserieswerederivedfromthesameriverswhosedataweremissing,orfromnearbytributaries.The minimumPearson’scorrelationcoefficientbetweentheseriesinthemodelwassetto0.6,followingLorenzo-Lacruzetal.

(1945–2005).AsdepictedinFig.1,sevengaugestationsarelocatedupstreamofthemainreservoirs(Puigcerdà,Organyà, Arabó,laSeud’Urgell,Valira,laPobladeSegurandPontdeSuert),whilefourarelocateddownstream(Oliana,Pinyana, BalaguerandSeròs).Todeterminethedrainagebasincorrespondingtoeachgaugestation,weusedadigitalelevation model(DEM)ataspatialresolutionof100m,andther.watershedtoolinGRASS(v.6.4).Thedrainageareaofeachgauge stationwasdeﬁnedastheupstreamareafromtheheadwaterstothegaugestation.Wealsousedmonthlyaveragereservoir storagesinthebasinfrom1950aswellasthereservoircapacityupstreamthegaugestationslocatedinthelowerboundof thebasin.

Dailystreamflowrecordswereusedtoanalyzetheevolutionoffloodevents,andmonthlystreamflowserieswerecom- putedfromthedailyseriestocalculatehydrologicaldroughtindices.Dailyprecipitationserieswereobtainedfromthe SpanishandCatalanmeteorologicalagencies(AEMETandSMC).Atotalof432stationswithdailyprecipitationdatawere availablefortheentirebasin.Thedevelopmentofcomplete,qualitycontrolledandhomogeneousdailyprecipitationseriesis essentialforrobustclimateassessments.Therearemultipleapproachesforgapfillingandhomogeneityassessment,which werecomprehensivelyrevisedandcomparedinVicente-Serranoetal.(2010)fortheentireEbrobasin(NESpain).Herewe followtherecommendationsofthisresearchtodevelopaqualitycontrolledandhomogeneousdailyprecipitationdataset intheSegrebasin.Fromtheentiredataset(N=432),only52candidatestations,withmorethan30yearsofdata,werefirst selected.Theotherseries(referenceseries)wereusedtoreconstructandfillgapsinthecandidateseries.Dailyprecipita- tionseriesweretransformedtoquantiles,basedontheirempiricalcumulativedistributionfunction.Foreachcandidate series,weselectedthenearestreferenceseries(withinamaximumdistanceof10km),withatleast3yearsofcommon data.Thegapsinthecandidateserieswerefilledusingthequantilevaluesofthenearestavailablereferencestation.Infew caseswherethecandidateseriescouldnotbecompletedusingthenearestneighborseries,theexistinggapswerefilled usingbothreferenceandothercandidateseriesuptoamaximumdistanceof25km.Thisprocedureprovidedcomplete datasetsforeachofthe52candidateseries.Fromthedailyprecipitationseries,wecalculatedthemonthlysumtotestthe temporalhomogeneityoftheseries.Forthispurpose,weusedHOMER(HomogenizationinR)(Mestreetal.,2013),which isbasedonthepairwisealgorithmdescribedbyCaussinusandMestre(2004)andatwofactorANOVAmodelforcorrection.

HOMERfacilitatescomparisonofsetsofstations,andestimationofthenumberandpositionsoftheirbreakpoints.Few temporalinhomogeneitieswereidentiﬁedintheseries(68),andthecoefﬁcientsobtainedwereappliedtothosedayshaving precipitationinthemonth,accordingtoVincentetal.(2002).

Wealsousedgriddedmonthlyprecipitationandmonthlyreferenceevapotranspiration(ETo),ata500mgridinterval, obtainedusingtheequationofHargreavesandSamani(1985).ThesegriddeddatasetswerebasedontheMOPREDASand MOTEDASdatasets(González-Hidalgoetal.,2011,2015),whicharethemostcompletequalitycontrolledandhomogeneous monthlyclimatedatasetsforSpain.Detailsoftheproceduresusedtoobtainandvalidatethesegriddeddatahavebeen describedbyVicente-Serranoetal.(2016).Usingthedrainagebasincorrespondingtoeachgaugestation,wedetermined thetotalmonthlyprecipitationandETofortheentirebasin.Thisprocedureenabledcomparisonoftheaverageclimateseries (precipitationandETo)correspondingtothedrainageareaateachgaugestationwiththemonthlystreamﬂowdata.

3.2. Analysis

3.2.1. Floodsandextremeprecipitationevents

Wequantifiedthetrendsinthepercentageofannualstreamflowcorrespondingtodailyriverflowsofdifferentmagni- tudes.Forthispurpose,weusedthemethodproposedbyOsbornetal.(2000),wherebystreamflowvaluescorresponding toeach5thquantileunitwereextractedfromallthedailystreamflowdataforeachgaugestation.Usingthisprocedure,we classifiedthedailystreamflowrecordsinto20categories.Then,wedeterminedthecontributionofthedailystreamflowin eachcategorytothetotalannualstreamflow,andanalyzedthetemporaltrendsinthecontributionofeachcategoryusingthe nonparametricMann–KendallTaucoefficient.Statisticallysignificanttrendsweredefinedasthosehavingp-values<0.05.

Wealsousedthisapproachtodeterminechangesinthepercentageofannualprecipitationcorrespondingtoeventsabove the95thquantile.Inaddition,weanalyzedtrends(Mann–Kendalltaucoefficient;significancelevel:p<0.05)intheannual frequencyofhighprecipitation(>95th,>99th,and>99.9thquantiles)andstreamflow(>95th,>98th,>99th,>99.5th,>99.9th, and>99.95thquantiles)events.Finally,toaccountforthepossibleinfluenceofthereservoircapacityandthereservoir storageupstreamthegaugestationslocatedinthelowerboundofthebasin,werelatedtheannualfrequencyofdaysabove the95th,99thand99.9thpercentileswiththeaverageannualreservoirstorage,reservoircapacityandtheratiobetween thereservoirstorageandcapacity.

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3.2.2. Droughtquantiﬁcationandanalysis

HydrologicaldroughtswerequantifiedusingtheStandardizedStreamflowIndex(SSI;Vicente-Serranoetal.,2012),and climaticdroughtswerequantifiedusingtheStandardizedPrecipitationEvapotranspirationIndex(SPEI;Vicente-Serrano etal.,2010)attimescalesrangingfrom1to48months.TheSSIenablescomparisonofstreamflowdeficitsandsurplusesin timeandspace,regardlessofthemagnitudeoftheseriesandtheriverregimesinvolved.TheSSIisobtainedbytransforming themonthlystreamflowseriesintoadimensionalseriesofstandardizedanomalies.ToobtainareliableSSIthatencompasses largevariabilityinthestatisticalpropertiesofthemonthlystreamflowdata,theserieswerefittedtothemostsuitable probabilitydistribution,accordingtotheminimumorthogonaldistancebetweenthesampleL-momentsatsiteiandthe L-momentrelationshipforaspecificdistribution,selectedfromthegeneralextremevalue,thePearsonTypeIII,thelog- logistic,thelog-normal,thegeneralizedPareto,andtheWeibulldistributions.MoredetailsonthecalculationoftheSSIare providedbyVicente-Serranoetal.(2012).

TheSPEIisaclimaticdroughtindexthatcanbeobtainedatvarioustimescales,similartotheStandardizedPrecipitation Index(SPI)(McKeeetal.,1993);thisisessentialforidentifyingthecomplexresponseofhydrologicalsystemstoclimate variability(Vicente-Serranoetal.,2011;López-Moreno etal.,2013;Barkeretal.,2015).Hydrologicaldroughtsusually respondtodifferenttimescalesofclimatedrought,asafunctionofenvironmentalconditions(e.g.lithology,vegetation cover,andmanagement)(Lorenzo-Lacruzetal.,2013).TheSPEIisbasedonprecipitationandETo,andincorporatesthe sensitivityofdroughtseveritytochangesinAEDinthemulti-temporalnatureofdroughtsbasedonamonthlyclimatic waterbalance(P–ETo),whichisadjustedusingathree-parameterlog-logisticdistribution.Thevaluesareaccumulatedat varioustimescales,followingthesameapproachasisusedfortheSPI,andconvertedtostandarddeviationswithrespect toaveragevalues.ForthispurposeweusedthemonthlytotalprecipitationandETogriddedseriescorrespondingtoeach drainagebasin.

UsingtheSSIandSPEI,wedefinedindividualdroughtevents.Thisiscommonlydonebyselectingathresholdinthe series(Fleigetal.,2006;SharmaandPanu,2014).Todefinedroughtevents,athresholdlevelthatdidnotvaryintimeand spacewasappliedtotheSSIseriesforeachbasinandtheSPEIseries.Nevertheless,theresponseofhydrologicaldroughts totheoccurrenceofclimatedroughts canbestronglycomplex.Accordingtothetopographic/lithological/management characteristicsofthebasins,thetime-scaleoftheclimaticdroughtsatwhichthehydrologicaldroughtsarerespondingcan beverydifferent(seeforexample,López-Morenoetal.,2013;Lorenzo-Lacruzetal.,2013).Forthisreason,beforerelating theSSIandSPEI,weanalyzedthebestSPEItime-scaleatwhichtheSSIisresponding.TheselectedthresholdforSSIand SPEIwas0;consequentlyadroughteventwasrecordedwhenthemonthlySSIorSPEIfellbelowthislevel.Basedonthis threshold,eachidentifieddroughteventwascharacterizedaccordingtothedroughtdurationandmagnitude.Theduration ofagivendroughteventwasdefinedasaconsecutiveanduninterruptedtimeperiod(oneormoremonths),withaSSIorSPEI valuelowerthan0.Thedroughtmagnitudewastheaccumulateddeficitvolume(definedasthesumofthedeficitvolumes generatedduringanuninterruptednumberofmonths)delimitingadroughteventandexpressedastheaccumulateddeficits

Fig.2. Left:Evolutionofthepercentageofannualprecipitationcorrespondingtoeventsexceedingthe95thpercentile.Right:Evolutionofthenumber ofeventsexceedingthe95th,99th,and99.9thpercentiles.Red:Significantnegativetrends,Blue:Significantpositivetrends,Graycolor:non-significant trends.

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oftheSSIorSPEI.Annualseriesoftheaveragedroughtdurationandmagnitudewerecreatedforeachbasinfollowingthis approach.Here,itnoteworthyindicatingthatahydrologicaldroughtisconsideredasthisperiodinwhichthestreamflow wasbelowagiventhresholdquantifiedinrelativeterms(i.e.consideringtheentirestreamflowseries),independentlyifit isonlydrivenbyclimateanomalies,bywaterregulationandabstractionorboth.

Changesin hydrological and climatedrought duration and magnitude were alsodetermined by the nonparamet- ricMann–Kendall Taucoefficient. Statisticallysignificant trendswere defined as those having p<0.05. Todetermine themagnitude (amount)of change,a linear regression model betweentime (independent variable) and thedrought

Fig.3. (A)Exampleoftheamountquantileanalysiscorrespondingtothepercentageoftheannualstreamflowbyeventsabove0.95thpercentileinPont deSuert.(B)Plotsshowingthemagnitudeofchangeintheamountquantilescorrespondingtothevolumeofannualstreamflow.Blackcolumnsrepresent significanttrends.Thefourplotsatthebottomcorrespondtothebasinslocateddownstreamofthemaindams.

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duration/magnitudewasﬁtted.Theslopeofeachmodel(m)indicatedthemagnitudeofchange.We alsoanalyzedthe relationshipbetweentheannualSSI(asameasureoftheannualdroughtseverity)andtheaverageannualreservoirstorage, reservoircapacityandtheratiobetweenthereservoirstorageandcapacityinthegaugestationslocatedinthelowerbound ofthebasin.

4. Results

4.1. Extremeprecipitationevents

Theevolutionofthemostextremeprecipitationeventsconcurswiththegeneralreductioninprecipitationintheregion, asreportedinpreviousstudies.Trendsinthepercentageoftheannualprecipitationcorrespondingtoeventsexceedingthe 95thpercentiledidnotshowaclearstructure,althoughstationsshowingnegativetrendsdominated(Fig.2,left).However, thenumberofeventsexceedingthe95thpercentileclearlydecreasedovermostofthebasinintheperiod1950–2013(Fig.2, right).Amongthe52meteorologicalstationsusedinthisstudy,only16showedapositivetrendinthenumberofevents exceedingthe95thpercentile,withonly5stationsshowingstatisticallysignificanttrends.Otherstationsexhibitednegative trends,withonly18ofthemshowingstatisticallysignificanttrends.Astheprecipitationthresholdincreases,thepattern ismuchlessclear.Resultsindicatethatatotalof34stationsshowedadecreaseinthenumberofeventsexceedingthe 99thpercentile,albeitwithonly9stationsexhibitingstatisticallysignificanttrends.Thesefindingssuggestthatthepattern changedmarkedlywhenthethresholdwassettothe99.9thpercentile.Thiscanbeexplainedbythenotionthattheannual frequencyofeventsexceedingthisthresholdgenerallyincreasesintheheadwaters.

4.2. Highriverﬂows

Resultsrevealageneraldecreaseinthepercentageofstreamflowsassociatedwithdailyeventsexceedingthe90th percentilethroughouttheentirebasin.Nonetheless,thispatternwasmuchmoreevidentinthelowerreaches,downstream ofthedams(Fig.3).Forthestationslocatedintheheadwaters,therewasnochangeinthepercentageofstreamflowrecorded fordailyflowswithmagnitudeslessthanthe90thpercentile.Exceptionally,only4stationsintheheadwaterssignificant changesinthedailystreamflowrecordedforeventsexceedingthe95thpercentile.Forgaugestationslocateddownstreamof themaindams,thepercentageofthestreamflowassociatedwithdailyflowswithmagnitudeslessthanthe50thpercentile increased,whilethoseexceedingthe50thpercentiledecreased.Thispatternindicatesthat,inthelowerreachesofthe basin,therewasageneralincreaseinthefrequencyoflowstreamflows.Correspondingly,therewasadecreaseinthetotal streamflowassociatedwithhighflows,mainlylinkedtoeventshavingamagnitudeexceedingthe95thpercentile.

Althoughtherewasareductioninthefrequencyoftheeventsexceedingthe95thpercentileaswellasthetotalstreamﬂow oftheseevents,therewasnochangeinthefrequencyofthemostextraordinaryevents(>99.5thpercentile).Fig.4shows thetrendsintheannualfrequencyofeventsexceedingthe95th,98th,99th,99.5th,99.9th,and99.95thpercentilesatthe variousgaugestations.Forboththeheadwatersandthelowerreachesoftherivers,therewasareductioninthefrequencyof

Fig.4.Correlationbetweenthefrequencyofeventsexceedingvariouspercentiles(95th,98th,99th,99.5th,99.9th,and99.95thpercentiles)andthetime series(1950–2013).Blackbarsrepresentsigniﬁcantcorrelations.

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Fig.5.Evolutionofthenumberofﬂoweventsperyearexceedingthe95th(grayline)and99.9th(bluebars)percentilesateachofthetwogaugestations locatedintheheadwatersandinthelowerreaches.Trendlinesareincludedforeventsexceedingthe95th(blackline)and99.9th(blueline)percentiles.

dailyflowsbelowthe98thpercentile.However,thereweresomeexceptionsintheheadwaters,includingthegaugestations ofArabóandlaSeud’Urgell.Nevertheless,theanalysisofthefrequencyoftheextraordinaryfloods(i.e.thoseexceedingthe 99.5thpercentile)indicatesthatthemagnitudeofthedecreasingtrendismuchlower,althoughassessingtrendsinthe frequencyoftheseeventsisdifficultgiventheirirregularcharacterandunevensamplesize.Fig.5confirmsthesamefinding forthegaugestationslocatedintheheadwatersandthelowerreachesofthebasin(SeròsandBalaguer).Asillustrated, thereisamarkeddecreaseinthenumberofflowsexceedingthe95thpercentileinthelowerreachesoftheSegreRiver (Oliana,SeròsandBalaguer),whilethefrequencyoftheextraordinaryevents(>99.9thpercentile)showednocleartemporal pattern.Exceptionally,thereisareductioninthefrequencyofextremeandextraordinaryfloodsinPinyana,whichcanbe explainedbytheveryhighwaterregulationandthewatertransfertootherbasins.Inanycase,inthestationsoftheSegre River,itisclearlyobservedthatthemostextraordinarydailyflowsshowedalowfrequencyandwereusuallygroupedin thesameyear.Theseeventsmostlyoccurredpriortotheextensiveriverregulationofthebasinthatmostlyoccurredduring theperiod1960–1970.Nonetheless,othereventsalsooccurredinthe1980sand1990s,followingtheconstructionofthe maindamsinthebasin.

Theincreasedregulationinthebasinhasinfluencedsignificantlythefrequencyofextremeandextraordinaryfloodevents.

Fig.6showstheevolutionofthestoragecapacity,theratiobetweentheannualwaterstorageandthestoragecapacityand theannualfrequencyofeventsabovethe95thpercentileinthegaugestationslocateddownstreamthereservoirnetwork.

Thedifferentgaugestationsshowedasigniﬁcantdecreaseinthefrequencyoftheseevents,whichconcurswiththeincreased storagecapacity.Inaddition,theinterannualvariabilityoftheeventsabovethe95thpercentileisrelatedtothetemporal variabilityintheratioofreservoirstorage/capacity(Table1).Thus,inthelasttwodecades,onlythoseyearswithhigh reservoirlevelswitnessedsomeoftheseevents.Thepatternoftheevolutionofeventsabovethe99thpercentileisquite similar,albeitwithaclearreductioninthemostregulatedriversectors(Fig.7).Nevertheless,whilePinyanawasexceptionally

Table1

Pearson’srcoefficientsbetweentheannualfrequencyofdayswithastreamflowabovethe95th,99thand99.9thpercentilesandtheaverageannualvalues ofreservoirstorage,totalreservoircapacityandtheratiobetweenstorageandcapacityupstreamthegaugingstationslocatedinthelowerboundofthe basin.Statisticallysignificantcorrelations(p<0.05)aregiveninbold.

Pinyana Oliana Balaguer Serós

Annualfrequency(events>95th)

Storage −0.24 0.19 0.03 0.00

Reservoircapacity −0.31 0.03 −0.26 −0.29

Ratiostorage/capacity 0.25 0.19 0.49 0.47

Annualfrequency(events>99th)

Storage −0.39 0.31 0.03 0.06

Reservoircapacity −0.54 0.16 −0.24 −0.19

Ratiostorage/capacity 0.46 0.31 0.45 0.39

Annualfrequency(events>99.9th)

Storage −0.45 0.17 0.09 0.12

Reservoircapacity −0.57 0.10 −0.16 −0.14

Ratiostorage/capacity −0.22 0.17 0.40 0.40

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Fig.6.Temporalevolutionofthereservoirstoragecapacity(blue),theratiobetweenannualstorageandcapacity(red)upstreamthegaugestationslocated inthelowerboundoftheriver.Blacklinerepresentstheannualfrequencyofeventsabovethe95thpercentile.

affectedbyastrongwaterregulationandtransfer,othergaugestationshadacertainfrequencyoftheseeventsduringthe lasttwodecades.Consideringthefrequencyofextraordinaryfloodevents(>99.9th),thetrendwaslessclearinthestations ofSerósandBalaguer(Fig.8),inwhichsomeextraordinaryfloodeventswereidentifiedfrom1960to1990,inspiteofthe highwaterregulationfrom1950.SincetheconstructionoftheRialbdamin2000,noeventsabovethe99.9thpercentile wereidentified.Thispatterncouldberelatedtothestrongdecreaseofrelativereservoirstoragesduringthe2000s,which coincidedwiththemostextremeclimatedroughteventsinthebasin(seebelow).Overall,Table1clearlydemonstrates howthefrequencyofeventsabovethe95thpercentileisnegativelycorrelatedwiththeevolutionofthereservoircapacity inBalaguerandSerós,althoughthiscorrelationdecreaseswiththeeventsabovethe99thand99.9thpercentiles.Onthe

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Fig.7.Temporalevolutionofthereservoirstoragecapacity(blue),theratiobetweenannualstorageandcapacity(red)upstreamthegaugestationslocated inthelowerboundoftheriver.Blacklinerepresentstheannualfrequencyofeventsabovethe99thpercentile.

contrary,thefrequencyofthethreetypesofevents(i.e.95,99and99.9thpercentiles)issigniﬁcantlycorrelatedwiththe evolutionoftheratiostorage/capacity.

4.3. Droughts

Fig.9illustratestheevolutionoftheSSI(hydrologicaldroughtindex),basedongaugestationsintheheadwatersofthe Segrebasin.Ingeneral,theseriesforthemajorityofstationsshowedseveredroughtepisodesinthe1950s,althoughthe mostextremeepisodesoccurredduringthe2000s.Acorrelationanalysis,calculatedatdifferenttimescales,betweenthe SSIandtheSPEI(climatedroughtindex)revealsthathydrologicaldroughtswerecorrelatedwithclimaticdroughtsinthe headwatersattimescalesof5–8months.TheSPEIseriesforeachsub-basincorrespondedtothedroughttimescalethat

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Fig.8.Temporalevolutionofthereservoirstoragecapacity(blue),theratiobetweenannualstorageandcapacity(red)upstreamthegaugestationslocated inthelowerboundoftheriver.Blacklinerepresentstheannualfrequencyofeventsabovethe99.9thpercentile.

showedthehighestcorrelationwiththeSSI.SimilartotheSSI,theSPEIshowedthatthemostextremedroughtepisodes occurredinthe2000s,particularlyfrom2005to2010.

Fig.10showsthesameanalysis,butforthegaugestationslocateddownstreamofthemaindams.Atthesestations,a markeddecreasewasobservedintheSSIvalues,whichwasmuchmorepronouncedthanthatobservedfortheheadwaters duringtheperiod1950–2013.Moreover,themagnitudeofthecorrelationsbetweentheSSIandtheSPEIdecreasedatvarious timescalesinthelowerreaches.ThemaximumcorrelationoccurredforlongerSPEItimescales.Nevertheless,thebehavior oftheclimatedroughtsinthelowerreacheswasquitesimilartothatobservedintheheadwaters,withthemaindrought episodesbeingrecordedinthe2000s;althoughtheyshowedlowerseverityanddurationrelativetotheSSIdroughts.

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Fig.9.Left:TemporalevolutionoftheSSIinthegaugestationsoftheheadwaters.Central:CorrelationbetweentheSSIandthe1-to48-monthSPEItime scales.Right:EvolutionoftheSPEIattimescalewithhighercorrelation.

Aninterestingresponseofhydrologicaldroughtstoclimatedroughtswasthechangeintheresponseofhydrological droughtsatthetimescalesofclimatedroughts.Wefoundthatthemagnitudeofthechangewasgreaterinthelowerreaches thanintheheadwaters.Thisaspectwasdetectedbyapplyingmoving-windowcorrelationsbetweentheSSIandSPEIseries recordedatdifferenttimescales(Fig.11).Intheheadwaters,therewasnotrendtowardadecreaseinthemagnitudeof correlationsbetweentheSSIandtheSPEIatthe5-monthtimescale,whichcorrespondedtothemaximumcorrelation valuesinthemajorityofseries.ThiswasclearlyevidentfortheOrganyàandPontdeSuertstations,aswellasotherstations inthebasin.Incontrast,acomparisonofthecorrelationsbetweentheSSIandtheSPEIatshortandlongtimescalesin thelowerreachessuggestsatrendtowardalesserresponsetoshortSPEItimescalesandagreaterresponsetolongSPEI timescales.

Trendsofdroughtsoflongerdurationandgreatermagnitudewereassessedforboththeheadwaterandthelowerreach areas.Fig.12showstheevolutionoftheaverageannualdroughtduration(inmonths)andmagnitude(instandardized units)forthreestations(Puigcerdà,Organyà,andPontdeSuert)locatedintheheadwatersandthree(Seròs,Balaguer,and Pinyana)locatedinthelowerreaches.Forallthesestations,thecorrelationbetweentheannualaveragedroughtduration andmagnitudewaspositiveandstatisticallysigniﬁcantforbothhydrologicalandclimaticdroughts.Nevertheless,forthe headwaters,wefoundthatthetrendwasstrongerforclimatedroughtsthanforhydrologicaldroughts,whiletheopposite occurredforthelowerreaches(Table2).Thus,theevolutionoftheaverageannualdroughtdurationandmagnitudeinthe headwaterswaspositiveandstatisticallysigniﬁcantforbothhydrological(SSI)andclimate(SPEI)droughts.Nevertheless, forheadwatergaugestations,theincreaseinthemagnitudeanddurationoftheclimatedroughtepisodeswasmuchgreater thanthatobservedforhydrologicaldroughts.Incontrast,forthethreegaugestationslocateddownstreamofthemaindams

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Fig.10.Left:TemporalevolutionoftheSSIatgaugestationsinthelowerreaches.Central:CorrelationbetweentheSSIandthe1-to48-monthSPEItime scales.Right:EvolutionoftheSPEIattimescalewithhighercorrelation.

inthebasin(Seròs,Pinyana,andBalaguer),themagnitudeanddurationofhydrologicaldroughtsincreasedmorethanthat observedforclimatedroughts.

Fig.13showsthetemporalevolutionofthereservoirstoragecapacity,theratiobetweenannualstorageandcapacityand theannualSSIinthegaugestationslocatedinthelowerboundoftheriver.Themostregulatedbasins(i.e.Pinyana,Balaguer andSerós)showedapositiveandsignificantcorrelationwiththeevolutionofthereservoircapacity(Table3).Thisfinding suggeststhatdroughtseverityhasincreasedinthelowerboundofthebasin,asaconsequenceofriverregulation.Resultsalso revealthatreservoirstorageisakeydriverofstreamflowdroughtseveritydownstreamthereservoirs.Inparticular,there isahighandsignificantcorrelationbetweentheannualreservoirstoragesandtheannualSSIinthemostregulatedbasins, demonstratingasignificantinfluenceofthereservoirmanagementontheoccurrenceofhydrologicaldroughtsdownstream.

Fig.11. Evolutionof30-yearmovingcorrelationsbetweentheSSIandshort(blue)andlong(red)SPEItimescalesattwogaugestationsintheheadwaters (above)andtwostationsinthelowerreaches(below).

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Fig.12.(A)Evolutionoftheannualdroughtduration(blue:hydrologicaldroughts;red:climatedroughts)atthreegaugestationsintheheadwatersand threeinthelowerreaches.(B)Evolutionoftheannualmagnitude(blue:hydrologicaldroughts;red:climatedroughts)atthreegaugestationsinthe headwatersandthreeinthelowerreaches.

Table2

Magnitudeoftrendsindroughtdurationandmagnitudeintheheadwa- ters(blue)andlowerreaches(orange).Bold:signiﬁcanttrends(p<0.05).

Table3

Pearson’srcoefﬁcientsbetweentheannualSSIandtheaverageannualvaluesofreservoirstorage,totalreservoircapacityandtheratiobetweenstorage andcapacityupstreamthegaugestationslocatedinthelowerboundofthebasin.Statisticallysigniﬁcantcorrelations(p<0.05)aregiveninbold.

Pinyana Oliana Balaguer Serós

Storage −0.13 0.18 −0.30 −0.22

Reservoircapacity −0.31 −0.02 −0.61 −0.55

Ratiostorage/capacity 0.28 0.18 0.49 0.56

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Fig.13.Temporalevolutionofthereservoirstoragecapacity(blue),theratiobetweenannualstorageandcapacity(red)upstreamthegaugestations locatedinthelowerboundoftheriver.BlacklinerepresentstheannualSSI.

5. Discussionandconclusions

WeanalyzedtheevolutionofclimateandhydrologicalextremeeventsintheSegrebasin(northeasternSpain),where streamﬂowshavebeenhighlyregulatedbyadensenetworkofreservoirsconstructedduringthesecondhalfofthe20th century.Between1950and2013,therewasageneralreductionintheoccurrenceofextremeprecipitationeventsinthis basin,whichthisstudydeﬁnedasthoseexceedingthe95thpercentileinprecipitationseries.Thispatternisconsistent withpreviousanalysesundertakenatthenational(Rodrigo,2010;Gallegoetal.,2011)andregional(López-Morenoetal., 2010;TurcoandLlasat,2011)scales.Thus,thepercentageofannualprecipitationexplainedbyeventsexceedingthe95th percentilegenerallydecreased,butwithfewexceptionswheretherewasadecreaseinthefrequencyofsuchevents.The

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trendfortheextraordinaryeventsperyear(i.e.thoseexceedingthe99.9thpercentile)wasnotsoclear.Theanalysisofthe trendfortheseeventswasdifﬁcultbecausetheyoccurveryirregularly.RecentstudiesinSpainfocusingontheseeventshave providednoevidenceforageneralizedtrend(e.g.Aceroetal.,2011;Begueríaetal.,2011).Nevertheless,theheadwaters oftheSegrebasinshowedanincreaseinthefrequencyoftheextraordinaryprecipitationevents,whiletherehasbeenno signiﬁcantchangeintherestofthebasin.

Theevolution ofthestreamﬂowatthegaugestationsinthebasinalsoindicatedageneralreductioninhighﬂows.

Therehasbeenageneralreductionintheannualstreamflowsincethe1950s,thoughbeingmorepronouncedinthelower areasofthebasinthanintheheadwaters(Vicente-Serranoetal.,2016).Nevertheless,thepatternofstreamflowdecrease differedbetweentheheadwatersandthelowerpartofthebasin.Intheheadwaters,therewasageneralreductioninthe streamflowvolumeassociatedwitheventsexceedingthe95thpercentile.Incontrast,therewerenomajorchangesinthe volumeassociatedwithlowflows.Followingtheconstructionofadensenetworkofreservoirsinthelowerpartsofthe basin,therewasanincreaseinthestreamflowvolumeassociatedwiththelowflowcategories,combinedwithamarked decreaseinthehighflowcategories.ThispatternwasmainlyrecordedattheSeròs,Balaguer,andPinyanagaugestations, whicharelocatedinthelowerpartoftheSegrebasin.

Thisstudyalsonotedageneraldecreaseinthevolumeassociatedwithhighﬂowsduringrecentdecades,particularly inthenorthern(Renardetal.,2008;Giuntolietal.,2013)andsouthern(López-Morenoetal.,2006)slopesofthePyrenees.

AsimilarpatternhasalsobeenobservedinotherbasinsacrosstheIberianPeninsula(e.g.Silvaetal.,2012;Morán-Tejeda etal.,2012;Medieroetal.,2014).ThispatterncanbeexplainedbythegeneralreductioninprecipitationintheSpanish Pyrenees(López-Morenoetal.,2010),andparticularlyintheSegrebasin(Vicente-Serranoetal.,2016).However,thegeneral increaseinvegetationactivityandcoverassociatedwiththeabandonmentofagriculturalpracticesinthesemountainareas duringthesecondhalfofthe20thcenturymayalsohavebeenimportant(Lasantaetal.,2005;García-RuizandLana-Renault, 2011).Medieroetal.(2014)relatedchangesinclimateprocessestothegeneralreductioninhighflowsinseveralbasins ofSpainnotaffectedbydamming.TheysuggestedthattrendsinfloodscouldalsoberelatedtotheevolutionoftheAED, whichhasincreasedmarkedlyinSpaininthelastfivedecades(Vicente-Serranoetal.,2014).Inhumidareas,includingthe Pyreneanheadwaters,anincreaseinAEDwouldcontributetogreatertranspirationinareashavingdensevegetationcover, whichwouldcontributetodepletionofthesoilwatercontent.Thus,studiesinexperimentalbasinsinthePyreneeshave shownthatthegenerationoffloodsishighlyrelatedtothesoilmoistureconditionsinthebasins(Lana-Renaultetal.,2007;

Serrano-Muelaetal.,2015),althoughadirectconnectionbetweenparticularﬂoodanomaliesandairtemperatureconditions isdifﬁculttoestablish.

Themarkeddifferencesbetweentheheadwatersandthelowerreachesofthebasinwithrespecttothetrendsinthe contributionoflowandhighflowstotheannualstreamflowvolumearelikelytobeassociatedwiththeintenseregulation ofwatertomeetthewatersupplydemandsforirrigatedareasandlivestockfarms,whicharethemaineconomicactivities inthebasin.Severalstudieshaveindicatedthattherehasbeenareductioninthemagnitudeofhighflowsassociatedwith thepresenceofdams(Benke,1990;Ligonetal.,1995;ThomsandSheldon,2000;Songetal.,2015;Baietal.,2015),but alsoanincreaseinthecontributionoflowflows(Nislowetal.,2002;CowellandStoudt,2002).Forexample,Magilliganand Nislow(2005)analyzedtheimpactofdamsin21riverbasinsintheUSA,concludingthat,forlowflows,the1-to90-day minimumflowsincreasedsignificantlyfollowingimpoundment.

IntheSegrebasin,thedamshaveclearlymoderatedthefloodsthatoccurinthebasin.Thisisevidentfromtheanalysis ofthefrequencyofstreamflowsexceedingcertainthresholdseachyear.Nevertheless,whilethispatternisevidentfor thresholdscorrespondingtothe95thto99.5thpercentiles,therehasbeennoreductioninthefrequencyofextraordinary floods.Thispatternisparticularlyevidentintheheadwaters,whereextraordinaryprecipitationeventsarerandominspace andtime,mainlycontributingtofloodgeneration(García-Ruizetal.,2000).Thus,therehasbeenageneralincreaseinthe frequencyoftheextraordinaryprecipitationevents(>99.9thpercentile)intheheadwatersofthebasin,whichcouldaffect thefrequencyoftheextraordinaryfloodsoccurringinthisarea.Anexplanationoftheabsenceofatrendinextremefloodsis anavenueforfutureresearch,particularlywiththeobservedincreaseinthefrequencyofthemostextremerainfallevents.

However,revegetationofpartsofthebasin,asreportedinPiquéetal.(2016),couldaffectwaterinterceptionandsoil moistureviaevapotranspiration,andthusinﬂuencetherelationshipbetweenextremeprecipitationeventsandﬂoods.The resultsofstudiesinexperimentalbasinsacrossthewesternPyreneesskillfullyvalidatedthishypothesis(Serrano-Muela etal.,2015).

Interestingly,r,theobservedpatternin theheadwatersoftheSegrebasin wasalsorecordedin thelowerreaches.

Regardlessoftheextensiveregulationofthebasinsincethe1950s,extraordinaryfloodswererecordedinthe1980sand 1990s,asaconsequenceofthereconstructionofthemainreservoirsofthebasin. Thisexceptionisrepresentedinthe Pinyanastation,whichwasaffectedbyahighlevelofwaterregulationandawatertransferupstream.Basedonatem- poralperspectiveofseveralcenturies,Barrera-EscodaandLlasat(2015)suggestedthatfloodsintheSegrebasindidnot decreaseduringthesecondhalfofthe20thcentury.OurresultssuggestthatthereservoirsoftheSegrebasinhavehada markedinfluenceontheregulationofordinaryandextraordinaryfloods,butthecapacityofthereservoirnetworktoreduce extraordinaryfloodsmaydependonalargenumberoffactors,includingreservoircapacityandstorageandthedamopera- tionrules.UsingvariouscasestudiesbasedonsmallreservoirsinEurope,Salazaretal.(2012)analyzedtheeffectivenessof floodmanagementmeasures,demonstratingthatthesereservoirsareeffectiveinreducingthedownstreammagnitudeof smallandmediumevents,buthavealmostnoeffectonthelargestfloods.Similarresultshavebeenfoundinotherstudies (e.g.Smithetal.,2010).IntheSegrebasin,itislikelythatthereservoirswouldreducethemagnitudeofextremefloods,

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astheirstoragecapacityisveryhigh.Nevertheless,otherphysiographicandclimaticfactorscouldalsoplayimportant roles.

ThebasinsofnortheasternSpainareaffectedbytorrentialrainfallinresponse tovariousatmospheric mechanisms (LlasatandPuigcerver,1997).Extremeprecipitationeventscanoccurfollowingperiodsofextensiveprecipitation,gen- eratinglargevolumesofsurfacerunoff,particularlywhensoilissaturated(Lana-Renaultetal.,2007;García-Ruizetal., 2005,2008).Numerousstudieshaveshownthatannualand/orseasonalclimateconditionscanmarkedlyaffectthecapacity ofreservoirstomanagethelargestfloods.Morán-Tejedaetal.(2012)investigatedthis issueintheDourobasin (cen- tralSpain),concluding thatformostreservoirs,thelevelof alterationtoflowswashighlycorrelated withtheannual inflowvolume.A representativeexampleis thereservoirs, whichwerenot regulatedexcessively, duringyears ofrel- atively highlevelsof water input.Furthermore, López-Moreno et al. (2002)indicated that theinfluence of theYesa reservoir ondownstream floodsinAragonbasin (thewesternPyrenees)dependslargelyonwaterstoragelevel.Thus, theynotedthatwhen thedamwasatgreaterthan 90%capacity,there wasalmostnofloodcontrol,and evenhigher peakflowscouldoccurdownstreambecauseofthesuddenreleasesofthewaternecessaryfordamsafety.Theseresults highlightthedifficultiesinfloodmanagementinhighlycomplexMediterraneanbasins,includingtheSegrebasin,where climatevariability overlapsat various temporal scales(from daily toannual) in determiningthe occurrence offlood events.

ThisstudyindicatesthatreservoirconstructionandwaterusesintheSegrebasinhaveimpactedthefrequency,duration, andseverityofhydrologicaldroughtsdownstreamofthemaindams.Therehasbeenanincreaseinhydrologicaldroughts associatedwiththeobservedevolutionofclimatedroughts,whichisconsistentwiththegeneralpatternfoundfortheIberian Peninsula(Vicente-Serranoetal.,2014).Thisstudysuggestsahighlevelofagreementbetweenthetemporalevolutionof climateandhydrologicaldroughtindicesintheSegrebasin.Thus,bothrecordsindicatedthatthemostseveredrought eventsoccurredinthe2000s,inaccordancewithobservationsinotherbasinsofnortheasternSpain(López-Bustinsetal., 2013).Nevertheless,theresponseofhydrologicaldroughtstoclimatedroughtsdifferedmarkedlybetweentheheadwaters andthelowerareasofthebasin.Intheheadwaters,hydrologicaldroughtsmostlyrespondtoshorttimescalesofclimate droughts,whichisacharacteristicofareaswithlimitedcapacitytowaterstorageaswellasarapidstreamﬂowresponse toprecipitationvariability(López-Morenoetal.,2013;Barkeretal.,2015).Incontrast,inthelowerareasofthebasin, theresponseofhydrologicaldroughtsoccurredatlongerclimatedroughttimescales,becauseofthehighwaterstorage capacityupstream.ThispatternisalsoacharacteristicofotherregionsoftheIberianPeninsula,wheredamminghasclearly alteredthetimescalesofresponseofhydrologicaldroughtstoclimatevariability(VicenteSerranoandLópez-Moreno,2005;

Lorenzo-Lacruzetal.,2010;López-Morenoetal.,2013).Thus,theincreasedstreamflowregulationintheSegrebasinhas markedlyalteredthetimescalesofresponseofhydrologicaldroughtstoclimatedroughts.Whiletherehasbeensignificant changeinthecorrelationbetweenhydrologicalandclimatedroughtsatvarioustimescalesintheheadwaters,therehas beenaclearincreaseintheinfluenceoflongclimatedroughttimescalesinthelowerareasofthebasin,andaccordinglya reductionintheinfluenceofshortclimatedroughttimescales;thisisconsistentwiththeincreasedstoragecapacityinthe basin.

Numerousstudieshadstressedthepotentialofwaterregulationtoreducetheseverityofdroughts,basedonwater storagecapacity(YehandBecker,1982;McMahonetal.,2006).ThisapproachisapplicableintheSegrebasin,wherealarge reservoirnetworkguaranteesthewatersupplyforlargeirrigatedareasinthelowerpartsofthebasin.Nevertheless,infew instances,theSegrebasinnetworkfailedtoadequatelymeetwaterdemandsforirrigation.Arepresentativeexampleisthe mostextremedroughteventsoccurredduringthe2000s.

Nevertheless,fromahydrologicalperspective,thewaterregulationsystemdoesnotappeartobesoefficientformanag- inghydrologicaldroughts.IntheheadwatersoftheSegrebasin,theincreaseinthedurationandmagnitudeofhydrological droughtswasless,comparedtoclimatedroughts.Thiswasprobablybecauseofthecapacityofthemountainheadwaters tosplitlongdurationclimatedroughtsinresponsetointenseshortdurationprecipitationevents.Incontrast,inthelower areasofthebasin,theoppositepatternwasobserved,withamarkedincreaseinthedurationandmagnitudeofhydrological droughtsrelativetothatofclimatedroughts.IntheSegrebasin,therehasbeenalargedecreaseinstreamflow,asacon- sequenceoftherecentdecreaseinprecipitationandincreaseinAED.However,thedecreaseinstreamflowismuchmore pronouncedinthelowerareasofthebasin,duetothehighandincreasingdemandsfordifferentdomesticandagricultural uses(Vicente-Serranoetal.,2016).

Thehighdemandforwaterforagriculture,urban,andtouristusesintheIberianPeninsulahavemadetheaccentu- ation ofhydrologicaldroughts downstream largereservoir systemsa commonmanagementpractice.Thisfeaturehas alreadyobservedintheDourobasin (Morán-Tejedaetal.,2012)and theheadwaters(Lorenzo-Lacruzetal.,2010)and lowerreaches(López-Morenoetal.,2009)oftheTagusbasin.Theobjectiveofreservoirmanagementinanybasinispri- marilytosupplywaterforvarioususes,besidesreleasingwatertorivers.Becauseoftheneedtomeetthedemandsof waterusers,thebaseflowinmostregulatedriversismuchlowerinmagnitudethanthatinriverswithnaturalstream- flow(Ibà ñezetal.,1996;Batallaetal.,2004).Thisfeaturelargelyexplainstheobservedincreaseinhydrologicaldroughts downstreamofmajordams,whichinaccordanceincreaseswaterregulationcapacitytosupplyirrigatedlands,urbanareas andlivestockfarms.Thisisexacerbatedduringextremeclimatedroughtperiods,suchasthoseaffectedtheSegrebasinin 2007–2009,whenthesupplyofavailablewaterresourcestothevariouswateruserswasprioroverstreamflows.However, thestreamflowreductioncannotaffectaminimumenvironmentalflow,establishedbythecurrentenvironmentallawsin Spain.