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'Chapter3EvaporationandTranspirationProf.ZongxueXuMarch19,20091PrincipleofHydrology,BNU
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!'
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