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1、Chapter3 EvaporationandTranspirationProf.ZongxueXuMarch19,20091PrincipleofHydrology,BNUMarch19,2009PrincipleofHydrology,BNU2ObjectiveEvaporation:ETranspiration:TPotentialevapotranspiration:ETpActualevapotranspiration:ETaEstimationofE,ETp,ETaMarch19,2009PrincipleofHydrolog
2、y,BNU3EvaporationDefinition:Thephysicalprocessinvolvingaphasechangefromliquidtovaporbywhichwaterisreturnedtotheatmospherefromlakesandreservoirs,insomecases,fromriverchannelsinarivercatchment.(Open/freewaterevaporation)EToMarch19,2009PrincipleofHydrology,BNU4PotentialEvapo
3、transpirationDefinition:Themaximumrateofevapotranspirationfromavegetatedcatchmentunderconditionsofunlimitedmoisturesupply.ETpMarch19,2009PrincipleofHydrology,BNU5ActualEvapotranspirationDefinition:Alltheprocessesbywhichliquidwateratornearthelandsurfacebecomesatmosphericwa
4、tervaporETaGlobalaverage:Two-thirdsoftheprecipitationthatfallsonthecontinentsisevapotranspirated.Ofthisamount,97%isfromlandsurface,3%fromopenwatersurface.March19,2009PrincipleofHydrology,BNU6EstimationProcedureEstimatepotentialevapotranspirationbasedonmeteorologicalfactor
5、sEstimatetheactualevapotranspirationprocessesatthecurrentstatusoftheplant-andsoil-moisture-relatedcharacteristicsMarch19,2009PrincipleofHydrology,BNU7MeteorologicalFactorsEvaporationexample:MoisttemperateclimateOpenwater:600mmLandsurface:450mmAridclimateOpenwater:2000mmLa
6、ndsurface:100mmMeteorologicalfactorsSolarradiationWindRelativehumidityTemperatureMarch19,2009PrincipleofHydrology,BNU8WindTheboundarylayerbetweengroundsurface(water)andairmustberemovedandcontinuallyreplacedbydrierairThemovementoftheairintheboundarylayerdependsonwindMarch1
7、9,2009PrincipleofHydrology,BNU9RelativeHumidityReplacementoftheboundarylayerofsaturatedairbyairofequallyhighhumiditywillnotmaintaintheevaporationrateEvaporationwilloccuronlyiftheincomingairisdrierthantheairthatisdisplacedMarch19,2009PrincipleofHydrology,BNU10TemperatureIf
8、theambienttemperaturesoftheairandgroundarehigh,evaporationwillproceedmorerapidlythaniftheyarelow
