地下水影响的土壤和地表水分蒸发

*ClimateandBio-AtmosphericSciencesGroup,SchoolofNaturalResourceSciences,UniversityofNebraska-Lincoln,237L.W.ChaseHall,Lincoln,NE68583-0728,USAReceived11September2003;revised6April2004;accepted16April2004AbstractSoilhydrologicalprocessesplayanimportantroleinland-atmospheresystem.Inmostclimatemodels,theseprocessesaredescribedbysoilmoisturevariationsinthefirst2mofsoilresultingfromprecipitation,evaporation,andtranspiration.Groundwatereffectsonsoilmoisturevariationsandsurfaceevaporationareeitherneglectedornotexplicitlytreated.Althoughgroundwatermayhaveasmalleffectonsoilmoistureinareaswithadeepgroundwatertable,groundwatercanactasasoilwatersourceandhavesubstantialeffectsinareaswherethewatertableisnearorwithinamodel’ssoilcolumn.Howgroundwateraffectssoilmoisture,itsverticaldistribution,aswellasthesurfacewaterfluxaretheissuesaddressedinthisstudy.Asoilhydrologicalmodelwasdevelopedtoincludegroundwatereffectsbyallowingwaterexchangebetweentheunsaturatedzoneandgroundwater.Themodelusesaverticallyvaryingsaturationhydraulicconductivity,andisevaluatedusingobservationsatonestationintheNebraskaSandHills.Modelresultsshowitsabilitytodescribetherolesofgroundwaterinmaintainingtheobservedsoilmoisture,especiallyindeeplayers.Inaddition,comparisonsshowthatthesoilmoisturecontentinthefirstmeterofthesoilcolumnfromthemodelwithgroundwateris21%greaterthanthatfromamodelwithoutgroundwater.Highsoilmoisturecontentintherootzoneresultsinincreasedevapotranspiration(ET).TheaverageETinthreeperiodsfrom1998to2000is7–21%higherwhengroundwaterisconsideredinthemodel.Becauseofthegroundwatereffects,spatialvariationsinthegroundwatertablecancreateanadditionalspatialvariabilityofsoilmoistureandsurfacewaterflux.Thisadditionalvariabilitycouldbeimportantindevelopmentofstormsinregionswhosedomainhasalargeportionwithhighgroundwatertable.q2004ElsevierB.V.Allrightsreserved.Keywords:Groundwater;Soilmoisture;Surfaceevaporation;Model;NebraskaSandHills1.IntroductionTemporalandspatialvariationsofsoilmoisturearereceivingincreasedattentioninclimatestudiesbecausesoilmoistureisanessentialelementinprocessesthatdrivelandsurfacewaterandenergyfluxes,whichaffectecosystemdynamicsandbiogeo-chemicalcyclesintheland-atmospheresystem.Soilmoistureintheunsaturatedzonechangesasaresultofprecipitationrechargeandwaterexchangewithboththeatmosphereandgroundwater.Moststudiesofwaterexchangebetweentheunsaturatedzoneandtheatmospherehavefocusedonunderstandingsoilmoisturevariationsandtheireffectsonatmos-phericboundarylayerprocessesaffectingweather0022-1694/$-seefrontmatterq2004ElsevierB.V.Allrightsreserved.doi:10.1016/j.jhydrol.2004.04.019JournalofHydrology297(2004)285–300:LaboratoryofWaterResourcesDevelopment,HehaiUniversity,Nanjing,China.*Correspondingauthor.Tel.:þ1-402-472-6642;fax:þ1-402-472-6614.E-mailaddress:[e-mail](Q.Hu).(e.g.Manabe,1969;Ookouchietal.,1984;Dickinson,1984;PielkeandAvissar,1990;Pielkeetal.,1991).Inthosestudies,groundwatereffectsonsoilmoisturevariationhavebeenneglected,however.Inthemeantime,severallandsurfaceandsoilhydrologicalmodels(LSM)havebeendevelopedandusedinmesoscaleatmosphericmodelsandgeneralcirculationmodels(GCM).SomeofthoseLSMhavesophisticatedverticalexchangeprocessesofmoistureaswellastemperaturebetweenthesoil,thebiosphere,andtheatmosphere(Dickinson,1984;Dickinsonetal.,1986;Sellersetal.,1986;PanandMahrt,1987;Xueetal.,1991;ChenandDudhia,2001),whileothersusesimplifiedrepresentationsofverticalexchangephy-sicsthatincorporatetheeffectsofspatialheterogen-eityintopography,soil,andvegetationonsoilmoisturevariationandrelatedhydrologicalprocesses(e.g.EntekhabiandEagleson,1989;FamigliettiandWood,1990,1994;Woodetal.,1992;Schaakeetal.,1996;Stieglitzetal.,1997;Kosteretal.,2000).InmostoftheLSM,thesoilcolumnisdividedintonumberoflayerstodescribe,invariousdegreesandindifferentways,verticalsoilmoistureexchangeprocesses,suchasinfiltrationandpercolation,surfaceandundergroundrunoff,andeffectoftherootdensityprofileonsoilmoisture(Acs,1994).Theselayerscomprisearootzone,usually1mthick,andadeepsoilzonebeneathit.Thethicknessofthelayerbeneaththerootzonevaries;itis1mintheOSUmodel(PanandMahrt,1987)andtheLSMusedinthePennState-NCARMM5(ChenandDudhia,2001).Itvariesbetween1and2mintheSSiBmodel(Xueetal.,1991),andisfixedat10mintheBATSscheme(Dickinsonetal.,1986).ManyoftheseLSM,particularlytheonesbasedontheTOPMODEL(BevenandKirkby,1979),arefocusedontheeffectsofspatialheterogeneityofsoilmoistureonthesurfacehydrologyandtheatmosphere(e.g.EntekhabiandEagleson,1989;FamigliettiandWood,1994;Stieglitzetal.,1997;Kosteretal.,2000).Thesemodelsemployverticallayersthatextendtothedepthofthegroundwatertableanddo,infact,accountfortheimpactofdistributionofthegroundwatertableonspatialheterogeneityofsoilmoistureintheupperlayers.However,fewofthesemodelsexplicitlyaccountfortheeffectsof