Effectsofmicrotopographyonsurfacesubsurfaceexchang

Effectsofmicro-topographyonsurface–subsurfaceexchangeandrunoffgenerationinavirtualriparianwetland—AmodelingstudyS.Freia,G.Lischeidb,J.H.Fleckensteina,c,⁎aDepartmentofHydrology,UniversityofBayreuth,GermanybLeibnitzCentreforAgriculturalLandscapeResearch,(ZALF),GermanycHelmholtzCenterforEnvironmentalResearch(UFZ),GermanyabstractarticleinfoArticlehistory:Received4December2009Receivedinrevisedform20July2010Accepted20July2010Availableonline27July2010Keywords:Micro-topographyIntegratedsurface–subsurfacesimulationThresholdprocessSpillandfillTransmissivityfeedbackRunoffgenerationinwetlandsInhumiduplandcatchmentswetlandsareoftenaprominentfeatureinthevicinityofstreamsandhavepotentialimplicationsforrunoffgenerationandnutrientexport.Wetlandsurfacesareoftencharacterizedbydistinctmicro-topography(hollowsandhummocks).Theeffectsofsuchmicro-topographyonsurface–subsurfaceexchangeandrunoffgenerationfora10by20msyntheticsectionofariparianwetlandwereinvestigatedinavirtualmodelingexperiment.Areferencemodelwithaplanarsurfacewasrunforcomparison.Thegeostatisticallysimulatedstructureofthemicro-topographyreplicatesthetopographyofapeat-formingriparianwetlandinasmallmountainouscatchmentinSouth-EastGermany(Lehstenbach).Flowwasmodeledwiththefully-integratedsurface–subsurfacecodeHydroGeoSphere.Simulationresultsshowedthatthespecificstructureofthewetlandsurfaceresultedindistinctshiftsbetweensurfaceandsubsurfaceflowdominance.Surfacedepressionsfilledandstartedtodrainviaconnectedchannelnetworksinathresholdcontrolledprocess,whengroundwaterlevelsintersectedthelandsurface.Thesenetworksexpandedandshrunkinaspillandfillmechanismwhentheshallowwatertablefluctuatedaroundthemeansurfaceelevationundervariablerainfallinputs.Themicro-topographyefficientlybufferedrainfallinputsandproducedahydrographthatwascharacterizedbysubsurfaceflowduringmostoftheyearandonlytemporarilyshiftedtosurfaceflowdominance(N80%oftotaldischarge)duringintenserainstorms.Incontrastthehydrographintheplanarreferencemodelwasmuch“flashier”andmorecontrolledbysurfacerunoff.Anon-linear,hystereticrelationshipbetweengroundwaterlevelanddischargeobservedatthestudysitewasreproducedwiththemicro-topographymodel.Hysteresiswasalsoobservedintherelationshipbetweensurfacewaterstorageanddischarge,butoverarelativelynarrowrangeofsurfacewaterstoragevalues.Thereforeitwasconcludedthatsurfacewaterstoragewasabetterpredictorfortheoccurrenceofsurfacerunoffthangroundwaterlevels.©2010ElsevierLtd.Allrightsreserved.1.IntroductionRiparianzonescontaindynamicinterfacesbetweenground-andsurfacewaterflowpaths[10,27].Itisimportanttounderstandthemechanismsthatgovernhydrologicflowpathsandstreamflowgenerationinriparianzonesbecausenutrienttransformationsandexportareintegrallyrelatedtothehydrologicdynamics[8,18,34,53].However,thesedynamicscanbequitecomplex[27,53]andaregenerallypoorlyunderstood[28,49].Inhumidtemperateclimatesriparianzonesareoftenoccupiedbywetlands[27,34,36].Rapidsurfaceandshallowsubsurfaceflowstypicallydominaterunoffgenerationinriparianwetlandsduringrainstorms[8,34].Gibsonetal.[17]showedthatrunoffdynamicshighlydependonsurfacestorageandinteractionsbetweensurfacewaterandshallowgroundwater.KværnerandKløve[27]identifieddistinctlydifferentrunoffgenerationprocesseswithshiftsbetweensubsurfaceandsurfaceflowdominanceforlowandhighflowevents.Non-linearrelationshipsbetweenriparianwatertabledepthandstreamflowhaveoftenbeenobserved[5,15,38,44].Forcatchmentsdominatedbymatrixflow,theserelationshipshavebeenattributedtothetransmissivityfeedbackmechanism[3,4,44].Streamfloworigi-natingfrommatrixflowincreasesexponentiallywhenthewatertablerisesintosoillayerswithprogressivelyincreasinglateralhydraulicconductivity[3,44].Insystemswhereshiftsbetweenmatrixflowandsurfaceflowdominanceoccur,additionaldynamicsandnon-linearitieshavebeenobserved(e.g.[27]).Peat-formingwetlandsareoftencharacterizedbyahummockytopographywithsequencesofhighpoints(hummocks)anddepres-sions(hollows)atthesub-meterscale,whichwillaffectrunoffgenerationduringtransitionsbetweensurfaceandsubsurfaceflowAdvancesinWaterResources33(2010)1388–1401⁎Correspondingauthor.E-mailaddress:jan.fl[e-mail](J.H.Fleckenstein).0309-1708/$–seefrontmatter©2010ElsevierLtd.Allrightsreserved.doi:10.1016/j.advwatres.2010.07.006ContentslistsavailableatScienceDirectAdvancesinWaterResourcesjournalhomepage:filtrationandrunoffgenerationprocesseswerefirstinvestigatedbyDunneetal.[9].Theyshowedthathillsloperunoffwascontrolledbyanintricateinterplaybetweenrainfallintensity,surfaceflowdepth,vegetationcoverandthespecificmicro-topographyoftheslope.Micro-topographycanattenuateanddelaysurfaceflows[27],becausesurfacedepressionsfirstneedtobefilleduntilaspecificsurfacewaterstoragethresholdisexceededandsurfaceflowtowardsthestreamchannelcanbeinitiated[2,12].Tromp-vanMeerveldandMcDonnell[51]and[52]termedsimilarthresholddynamicsinthegenerationofsubsurfacestormflowsonbedrocksurfaceswithmicro-topographythe“fillandspillmechanism”.QuandDuffy[40]reporteddistinctdoublepeaksinhydrographsf