chapter3-环境水文学

'March19,2009PrincipleofHydrology,BNU1Chapter3EvaporationandTranspirationProf.ZongxueXu March19,2009PrincipleofHydrology,BNU2ObjectiveEvaporation:ETranspiration:TPotentialevapotranspiration:ETpActualevapotranspiration:ETaEstimationofE,ETp,ETa March19,2009PrincipleofHydrology,BNU3EvaporationDefinition:Thephysicalprocessinvolvingaphasechangefromliquidtovaporbywhichwaterisreturnedtotheatmospherefromlakesandreservoirs,insomecases,fromriverchannelsinarivercatchment.(Open/freewaterevaporation)ETo March19,2009PrincipleofHydrology,BNU4PotentialEvapotranspirationDefinition:Themaximumrateofevapotranspirationfromavegetatedcatchmentunderconditionsofunlimitedmoisturesupply.ETp March19,2009PrincipleofHydrology,BNU5ActualEvapotranspirationDefinition:AlltheprocessesbywhichliquidwateratornearthelandsurfacebecomesatmosphericwatervaporETaGlobalaverage:Two-thirdsoftheprecipitationthatfallsonthecontinentsisevapotranspirated.Ofthisamount,97%isfromlandsurface,3%fromopenwatersurface. March19,2009PrincipleofHydrology,BNU6EstimationProcedureEstimatepotentialevapotranspirationbasedonmeteorologicalfactorsEstimatetheactualevapotranspirationprocessesatthecurrentstatusoftheplant-andsoil-moisture-relatedcharacteristics March19,2009PrincipleofHydrology,BNU7MeteorologicalFactorsEvaporationexample:MoisttemperateclimateOpenwater:600mmLandsurface:450mmAridclimateOpenwater:2000mmLandsurface:100mmMeteorologicalfactorsSolarradiationWindRelativehumidityTemperature March19,2009PrincipleofHydrology,BNU8WindTheboundarylayerbetweengroundsurface(water)andairmustberemovedandcontinuallyreplacedbydrierairThemovementoftheairintheboundarylayerdependsonwind March19,2009PrincipleofHydrology,BNU9RelativeHumidityReplacementoftheboundarylayerofsaturatedairbyairofequallyhighhumiditywillnotmaintaintheevaporationrateEvaporationwilloccuronlyiftheincomingairisdrierthantheairthatisdisplaced March19,2009PrincipleofHydrology,BNU10TemperatureIftheambienttemperaturesoftheairandgroundarehigh,evaporationwillproceedmorerapidlythaniftheyarelow.Thecapacityofairtoabsorbwatervaporincreasesasitstemperaturerises. March19,2009PrincipleofHydrology,BNU11FactorsaffectingevapotranspirationIncidenceofprecipitationClimaticfactors:temperature,humidity,…Type,mannerofcultivationandextentofvegetation March19,2009PrincipleofHydrology,BNU12MethodstoEstimateEvaporationWaterbudget/storageequationapproachEnergybudgetmethodEmpiricalformulaePanmethod March19,2009PrincipleofHydrology,BNU13WaterBudgetApproachP:TotalprecipitationI:SurfaceinflowU:UndergroundoutflowO:SurfaceoutflowS:Changeinstorage March19,2009PrincipleofHydrology,BNU14EnergyBudgetMethodEstimatingallsourcesandsinksofthermalenergyandleavesevaporationastheonlyunknownNeedmanydataAspecialistapproach March19,2009PrincipleofHydrology,BNU15PenmanCombinationMethodOpenwaterevaporationH:AvailableheatenergyEa:Latentheatflux(Energyforevaporation) March19,2009PrincipleofHydrology,BNU16AvailableHeatEnergyRI:IncomingradiationRO:Outgoingradiation March19,2009PrincipleofHydrology,BNU17Incoming/OutgoingRadiationed:meanvaporpressuren:brightsunshine(hr/day) March19,2009PrincipleofHydrology,BNU18LatentHeatFluxu2:meanwindspeedat2mabovethegroundsurface March19,2009PrincipleofHydrology,BNU19Penman-MonteithMethodρ:densityofairλ:latentheatofvaporizationofwaterCp:specificheatofairatconstantpressurera:aerodynamicresistancetowatervaportransportrs:bulk(canopy)surfaceresistancetowatertransport March19,2009PrincipleofHydrology,BNU20FAOPenman-MonteithMethodRn:netradiationatthecropsurfaceG:Soilheatfluxdensity March19,2009PrincipleofHydrology,BNU21LatentHeatofVaporizationTa:airtemperatureλ:MJkg-1 March19,2009PrincipleofHydrology,BNU22AtmosphericPressureZ:elevation(m)P:kPa March19,2009PrincipleofHydrology,BNU23SaturationVapourPressurees(Ta):kPa March19,2009PrincipleofHydrology,BNU24ActualVapourPressureTd:dewpointtemperatureea(Td):kPa March19,2009PrincipleofHydrology,BNU25SlopeofVapourPressureCurveΔ:kPaoC-1 March19,2009PrincipleofHydrology,BNU26PsychrometricConstantP:atmosphericpressure(kPa)λ:latentheatofvaporization(MJkg-1) March19,2009PrincipleofHydrology,BNU27NetShortwaveRadiationα:albedo/canopyreflectioncoefficientRs:Incomingsolarradiation(MJm-2day-1) March19,2009PrincipleofHydrology,BNU28NetLongwaveRadiationσ:Stefan-Boltzmannconstant(4.903×10-9MJK-4m-2day-1)Rs/Rso:relativeshortwaveradiation March19,2009PrincipleofHydrology,BNU29NetRadiationRns:IncomingnetshortwaveradiationRnl:Outgoingnetlongwaveradiation March19,2009PrincipleofHydrology,BNU30SoilHeatFluxcs:soilheatcapacity(MJm-3oC-1)Ti:airtemperatureattimeIΔz:effectivesoildepth(m) March19,2009PrincipleofHydrology,BNU31EmpiricalFormulaeObjective:freewatersurfaceevaporation March19,2009PrincipleofHydrology,BNU32SeveralExamplesu2:Windspeedataheightof2m(m/s) March19,2009PrincipleofHydrology,BNU33SeveralExamplesPenman(1948)(in./day) March19,2009PrincipleofHydrology,BNU34SeveralExamplesKuzmin(1957)(in./month) March19,2009PrincipleofHydrology,BNU35SeveralExamplesRemanenko(1961)(cm/month)hn:relativehumidity March19,2009PrincipleofHydrology,BNU36MeasurementofEvaporationbyPansEvaporationPan March19,2009PrincipleofHydrology,BNU37PanMethodCET:Pan-to-ETcoefficient(0.5-1.0) March19,2009PrincipleofHydrology,BNU38RelationshipbetweenET0andETpf:Aseasonalcorrectionfactor(wetbaresoil:f=0.9) March19,2009PrincipleofHydrology,BNU39CropEvapotranspirationkc:Cropcoefficient March19,2009PrincipleofHydrology,BNU40ActualEvapotranspirationPhysically-basedmodelPenman-MonteithmodelConceptualmodelPotentialevapotranspirationWetnessofthesoil(actualsoilmoisturestoragedividedbythemaximumstorage) March19,2009PrincipleofHydrology,BNU41EstimationofETafromETpSMT:ActualsoilmoisturestorageSMC:Soilmoisturestorageatfieldcapacity March19,2009PrincipleofHydrology,BNU42SeveralExamplesRoberts(1978)(daily)RAT=SMT/SMC March19,2009PrincipleofHydrology,BNU43SeveralExamplesBudyko&Zubenok(1961)Monthlyvalues March19,2009PrincipleofHydrology,BNU44SummariesFreewatersurfaceevaporationPotentialevapotranspirationActualevapotranspirationEstimationofETpEstimationofETafromETp March19,2009PrincipleofHydrology,BNU45Comments&Suggestions? March19,2009PrincipleofHydrology,BNU46SeeYouNextWeek!'