2 CLIMATEANDSTREAMFLOWVARIABILITY INTHETANARIVERBASIN 2.1 Introduction Changesinclimatewillultimatelyaffectriverrunoff.Sincethelasticeageclimatein NorthernFennoscandiahasfluctuatedconsiderably.Paleoecologicalevidencesuggests thatduringthemidHolocene(ca.8000-5000BP)summerswerewarmeranddrierthan today(Seppä&Hammarlund,2000).Thereappearstobeageneralcoolingtrend throughouttherestoftheHolocene,uptothe20thcentury(Eronen&Zetterberg,1996; Seppä&Birks,2002).DirectmeasurementsonclimateanddischargeinNorthern Fennoscandiaare,however,onlyavailableforthelastcentury.Leeetal.(2000)analysed dataontemperatureandprecipitationfornorthernLapland,andconcludedthata significantwarmingoccurredinthefirsthalfofthe20thcentury,whileannual precipitationhasincreasedsince1880.Fewstudieshaveanalysedlong-termtrendsin riverdischargefornorthernLapland.Hyvärinen&Leppäjärvi(1989)analysed observationsondischargeinFinland,butcouldnotfindacleartrendforriversin northernLapland.Roaldetal.(1997)reportedaslight,thoughnotsignificant,decreasing trendforriversinnorthernFinland,SwedenandnortheastNorwayovertheperiod1930- 1980.Inthischapter,dataseriesoftemperature,precipitationandriverdischargeinthe TanaBasinareanalysed,inordertoidentifythevariationsinclimateandhydrologythat occurredoverthepastcentury.Thisanalysisisprecededbyageneraldescriptionofthe studyarea. 2.2 Studyarea 2.2.1 Location TheTanaRiverBasinislocatedinthenorthernmostpartofFennoscandia(i.e. ScandinaviaandFinland,seefigure2.1),between68º28'and70º28'Nand23º30'and 28º30'E.Thecatchmentareaisapproximately16000km2,ofwhich32%belongsto Finlandandtheremaining68%toNorway(Mansikkaniemi,1970).TheTanaRiver(in FinnishTenojoki)flowsfromtheconfluenceoftheKarasjåkkaandInarijokiriversinto theTanafjord,andisoneofthelargestriversinScandinaviadrainingintotheBarents sea.Itdrainsanextensiveuplandarea,includingalargepartoftheFinnmarksviddain Norway,andbelongstothesub-arcticzoneofFennoscandia. 2.2.2 Geologyandtopography TheTanaBasinisunderlainbyPrecambrianbedrock,consistingmainlyofgranites, granulites,andvarioustypesofgneissesandschists.Eocambriansedimentaryrockscan befoundinthenorthernmostpart,consistingofsiltstones,shalesandsandstones 31 (Mansikkaniemi,1970).Exceptforafewplacesthebedrockiscoveredwithglacialtills, mainlydepositedduringthelastglaciation(Weichselian),althoughinsomeareas depositsofpre-Weichselianagepredominate(Olsenetal.,1996).Sincethelast glaciationextensivepeat-bogshavebeenformed.PermafrostinNorthernFennoscandia isdiscontinuous,mainlyoccurringinpalsamires,andinbedrockonfellsummitswhere itmayreachadepthofseveraltensofmeters(King&Seppälä,1988;Seppälä,1997, 1998;Kukkonen&Šafanda,2001). Thetopographyoftheuplandareaisgenerallysmoothwithgentlyslopingfells(treeless areas)andveryflatsummits(Seppälä&Rastas,1980).Thishighlandiscutthroughby severalsteep-sidedfaultvalleys,suchasthevalleysoftheUtsjokiandTana,whichare tectonicinorigin(Mansikkaniemi,1970).Althoughelevationsaregenerallynothigher than500m,someNorwegianmountainpeaksinthenorthernpartreachtojustmorethan 1000mabovesealevel.AdigitalelevationmodeloftheTanaBasin,derivedfromthe USGeologicalSurvey(USGS)GTOPO30globaldataset(USGS,2001),andafrequency distributionoftheelevationclasses,areshowninfigure2.2. Figure2.1Location,topographyanddrainagenetworkoftheTanaRiverBasin 32 2.2.3 Climate TheclimateoftheTanabasinisinfluencedbytheproximityoftheArcticOceanandthe AtlanticGulfstream,andthereforerelativelymildandmaritime,consideringitslatitude. SeveralmeteorologicalstationsarelocatedinandaroundtheTanabasin,mostofwhich haveobservationrecordssincethe1950sor1960s.Thesestationsarelistedintables2.1 and2.2(forlocationsseefigure2.3).Seasonaldifferencesintemperature,precipitation andsnowdepthareillustratedinfigure2.4forKarasjok,Norway.TheclimateofKevo, Finland,hasbeendescribedinmoredetailbySeppälä(1976).IntheKöppenclimate classificationsystemthisstationbelongstothesubpolar(Dfc)climates,withshort,cool summers,severewinters,andnoclearseasonalityinprecipitation.Polartundra(ET) climatesmaybefoundathigheraltitudesintheregion(Seppälä,1976). Table2.1MeanannualtemperaturesformeteorologicalstationsinandaroundtheTanabasin Elevation Meanannual Maximumannual Minimumannual temperature temperature temperature m ºC period ºC period ºC Rustefjelbma 9 -0.46 1957-1998 1.62 1990 -2.28 1971 Kevo(Finland) 107 -1.85 1962-1998 0.65 1974 -3.81 1966 Karasjok 129 -2.15 1957-1998 0.36 1974 -4.43 1966 Cuovddatmohkki 286 -2.46 1966-1998 -0.38 1974 -4.33 1985 1957-1970, Kautokeino 307 -2.54 -0.52 1959 -4.83 1966 1996-1998 KautokeinoII 330 -1.79 1970-1996 -0.25 1989 -4.25 1985 Sihcajavri 382 -2.88 1957-1998 -0.76 1974 -5.10 1985 Annualprecipitationisgenerallylowforallweatherstations,rangingfromabout 340-360mminKautokeino(southwestoftheTanaBasin)to460mminRustefjelbmain thenortheast.Therearesignificantdifferencesbetweenindividualyears,butingeneral theannualprecipitationamountisnotlowerthan200mm/year,orhigherthan700 mm/year.Precipitationisusuallyhighestinthesummermonths.However,ithaslong beenrecognisedthatgaugerecordsintheArcticmayseriouslyunderestimateactual precipitationamounts,especiallyinwinter.Duetowind-inducedundercatch,wettingand evaporationeffects,measurementofsnowusingprecipitationgaugeshasbeenshownto havesystematiclossesofupto100%,dependingongaugetypeandtheobservationsite (Goodisonetal.,1998;Yangetal.,2001). 33 Table2.2MeanannualprecipitationformeteorologicalstationsinandaroundtheTanabasin. Elevation Meanannual Maximumannual Minimumannual precipitation precipitation precipitation m mm period mm year mm year Rustefjelbma 9 459 1957-1998 600 1992 327 1980 Polmak 21 397 1968-1980 569 1975 291 1969 Sirbma 51 384 1968-1998 609 1992 210 1980 SkoganvarreII 74 424 1957-1997 586 1989 289 1969 Kevo(Finland) 107 407 1962-1998 582 1964 263 1980 Port 115 380 1981-1998 487 1982 289 1994 Karasjok 129 367 1957-1998 513 1964 252 1969 Valjok 132 444 1957-1998 681 1964 283 1969 Iskorasjohka 153 404 1974-1998 543 1982 308 1996 Jergol 230 362 1981-1998 484 1982 270 1994 1966-1980, Cuovddatmohkki 286 371 494 1992 273 1986 1982-1998 KautokeinoII 307 361 1970-1996 497 1992 202 1980 Kautokeino 330 342 1957-1970 460 1961 226 1968 Mollesjohka 382 360 1974-1998 506 1992 248 1980 Sihcajavri 382 377 1957-1998 601 1957 199 1976 Jotkajavre 389 458 1957-1998 619 1959 320 1969 Meanannualtemperaturesrangefrom-2.88ºCforSihcajavritoslightlybelow0ºCin Rustefjelbma,whichislocatedclosetotheTanafjord.Temperaturesareusuallybelow0 ºCfromthemiddleofOctoberuntiltheendofMay.Likeinmostsub-arctic environments,snowmeltintheTanabasinisusuallyaveryrapidprocess.AtKevo, Finland,thedepletionofthesnowcovermayamounttomorethan20cmofsnowdepth perday.Almost50%ofthesnowpackmaydisappearinonly10days,andin30dayson average85%ofthesnowpackismeltedaway.Mostmeteorologicalstations,usually locatedintherivervalleys,becomefreeofsnowinMayoroccasionallyinearlyJune, butthiscanbeseveralweekslaterinthesurroundinguplands. 2.2.4 Vegetation InNorthernFennoscandiaextensiveforestsofmountainbirch(Betulapubescensssp. tortuosa)extentbeyondthearcticconiferoustreeline.MostoftheTanabasintherefore 34 (a) (b) Figure2.2(a)DigitalElevationModel(DEM)oftheTanaRiverBasin;(b)histogramandcumulative distributionofelevationclasses.DatadistributedbytheEROSDataCenterDistributedActiveArchive Center(LPDAAC),locatedattheUSGeologicalSurvey'sEROSDataCenterinSiouxFalls,South Dakota,USA.Seetheappendixforacolourversion 35 Figure2.3MeteorologicalanddischargestationsintheTanaBasin belongstothesub-arcticdeciduousbirchzone(Hustich,1961).Someisolatedpine (Pinussylvestris)forestscanbefoundintheInarijoki,KarasjåkkaandUtsjokivalleys, whichcouldbesaidtobelongtotheborealconiferousregion.NeartheArcticOceanthe upperlimitofbirchoccurrenceisonly20-30mabovesealevel,butintheKarasjåkka valleythetreelineisfoundatanaltitudeofnearly400m(Mansikkaniemi,1970). Tundraheathsdominatethelandscapeabovethebirchforests;typicalplantspecieshere areBetulanana,EmpetrumhermaphroditumandVacciniummyrtillus.Thehighestfells andmountaintopsarebarren.Miresandpeatlandsintheareaarecharacterisedby willows(Salix),Sphagnum,anddifferentgrassesandsedges.Seppälä&Rastas(1980) distinguishedaseparatevegetationclassforbirchforestsdamagedbybutterflylarvaeof thegeometridOporiniaautumnata.RegularoutbreaksofOpiriniaareseenasoneofthe additionalfactorsthatdeterminethelimitsofbirchoccurrences.Humaninfluenceis believedtobeinsignificant(Mansikkaniemi,1970).Somemeadowsandfieldscanbe foundinthemainvalleys,whileintherestoftheareareindeerherdingisthemainland use. AgeneralisedvegetationmapoftheTanabasinisshowninfigure2.5.Thismapwas madebasedonaLandsatTMsatelliteimageofJuly18th,1987.Thisimagewas classifiedusingfielddescriptions,collectedduringseveralfieldtripsin1999and2000. TheclassesresemblethoseofSeppälä&Rastas(1980).Amixedpine/birchclasscould 36 howevernotbediscerned,asinmid-summerthespectralresponseofthefreshbirch leavesispredominatingthelowerreflectivityofthepinetrees.Spectralclassesthatcould beseparatedincludealpineheaths,blockfields,sandandgravel,areascoveredwith reindeerlichens(whichhaveaverycharacteristic,brightreflectance),andmeadowsin therivervalleys.Theclassificationprocedurewascomplicatedbyseveralfactors,suchas shadowedsteepvalleyslopes,thesimilarspectralresponseatpixelscaleofpineforests andmires,andthehighdegreeofmixingofvegetationspeciesinthearea,inparticularof birchandpinetrees.Nevertheless,itisbelievedthatthismaprepresentsthegeneral distributionofvegetationtypesintheTanaRiverBasincorrectly. 2.2.5 Riverdischarge MeanannualrunoffoftheTanaRiver,asmeasuredatPolmak,Norway,is166m3/s.The interannualvariabilityindischargeishigh,rangingfrom103m3/s(in1941)to269m3/s (1932).ThestreamflowregimeoftheTanathroughouttheyear(figure2.6)istypicalfor