Chapter3-环境水文学.ppt
March 19,2009Principle of Hydrology,BNU1Chapter 3Chapter 3Evaporation and Evaporation and TranspirationTranspirationProf.Zongxue XuMarch 19,2009Principle of Hydrology,BNU2ObjectiveEvaporation:ETranspiration:TPotential evapotranspiration:ETpActual evapotranspiration:ETaEstimation of E,ETp,ETaMarch 19,2009Principle of Hydrology,BNU3EvaporationDefinition:The physical process involving a phase change from liquid to vapor by which water is returned to the atmosphere from lakes and reservoirs,in some cases,from river channels in a river catchment.(Open/free water evaporation)EToMarch 19,2009Principle of Hydrology,BNU4Potential EvapotranspirationDefinition:The maximum rate of evapotranspiration from a vegetated catchment under conditions of unlimited moisture supply.ETpMarch 19,2009Principle of Hydrology,BNU5Actual EvapotranspirationDefinition:All the processes by which liquid water at or near the land surface becomes atmospheric water vaporETaGlobal average:Two-thirds of the precipitation that falls on the continents is evapotranspirated.Of this amount,97%is from land surface,3%from open water surface.March 19,2009Principle of Hydrology,BNU6Estimation Procedure1.Estimate potential evapotranspiration based on meteorological factors2.Estimate the actual evapotranspiration processes at the current status of the plant-and soil-moisture-related characteristicsMarch 19,2009Principle of Hydrology,BNU7Meteorological FactorsEvaporation example:Moist temperate climateOpen water:600 mmLand surface:450 mmArid climateOpen water:2000 mmLand surface:100 mmMeteorological factorsSolar radiationWindRelative humidityTemperature March 19,2009Principle of Hydrology,BNU8WindThe boundary layer between ground surface(water)and air must be removed and continually replaced by drier airThe movement of the air in the boundary layer depends on windMarch 19,2009Principle of Hydrology,BNU9Relative HumidityReplacement of the boundary layer of saturated air by air of equally high humidity will not maintain the evaporation rateEvaporation will occur only if the incoming air is drier than the air that is displacedMarch 19,2009Principle of Hydrology,BNU10TemperatureIf the ambient temperatures of the air and ground are high,evaporation will proceed more rapidly than if they are low.The capacity of air to absorb water vapor increases as its temperature rises.March 19,2009Principle of Hydrology,BNU11Factors affecting evapotranspirationIncidence of precipitationClimatic factors:temperature,humidity,Type,manner of cultivation and extent of vegetationMarch 19,2009Principle of Hydrology,BNU12Methods to Estimate EvaporationWater budget/storage equation approachEnergy budget methodEmpirical formulaePan methodMarch 19,2009Principle of Hydrology,BNU13Water Budget ApproachP:Total precipitationI:Surface inflowU:Underground outflowO:Surface outflowS:Change in storageMarch 19,2009Principle of Hydrology,BNU14Energy Budget MethodEstimating all sources and sinks of thermal energy and leaves evaporation as the only unknownNeed many dataA specialist approachMarch 19,2009Principle of Hydrology,BNU15Penman Combination MethodOpen water evaporationH:Available heat energyEa:Latent heat flux(Energy for evaporation)March 19,2009Principle of Hydrology,BNU16Available Heat EnergyRI:Incoming radiationRO:Outgoing radiationMarch 19,2009Principle of Hydrology,BNU17Incoming/Outgoing Radiation ed:mean vapor pressure n:bright sunshine(hr/day)March 19,2009Principle of Hydrology,BNU18Latent Heat Flux u2:mean wind speed at 2 m above the ground surfaceMarch 19,2009Principle of Hydrology,BNU19Penman-Monteith Method:density of air:latent heat of vaporization of waterCp:specific heat of air at constant pressurera:aerodynamic resistance to water vapor transportrs:bulk(canopy)surface resistance to water transport March 19,2009Principle of Hydrology,BNU20FAO Penman-Monteith MethodRn:net radiation at the crop surfaceG:Soil heat flux densityMarch 19,2009Principle of Hydrology,BNU21Latent Heat of VaporizationTa:air temperature:MJkg-1March 19,2009Principle of Hydrology,BNU22Atmospheric PressureZ:elevation(m)P:kPaMarch 19,2009Principle of Hydrology,BNU23Saturation Vapour Pressurees(Ta):kPaMarch 19,2009Principle of Hydrology,BNU24Actual Vapour PressureTd:dew point temperatureea(Td):kPaMarch 19,2009Principle of Hydrology,BNU25Slope of Vapour Pressure Curve:kPaoC-1March 19,2009Principle of Hydrology,BNU26Psychrometric ConstantP:atmospheric pressure(kPa):latent heat of vaporization(MJkg-1)March 19,2009Principle of Hydrology,BNU27Net Shortwave Radiation:albedo/canopy reflection coefficientRs:Incoming solar radiation(MJm-2day-1)March 19,2009Principle of Hydrology,BNU28Net Longwave Radiation:Stefan-Boltzmann constant(4.90310-9 MJK-4m-2day-1)Rs/Rso:relative shortwave radiationMarch 19,2009Principle of Hydrology,BNU29Net RadiationRns:Incoming net shortwave radiationRnl:Outgoing net longwave radiationMarch 19,2009Principle of Hydrology,BNU30Soil Heat Fluxcs:soil heat capacity(MJm-3oC-1)Ti:air temperature at time Iz:effective soil depth(m)March 19,2009Principle of Hydrology,BNU31Empirical FormulaeObjective:free water surface evaporationMarch 19,2009Principle of Hydrology,BNU32Several Examplesu2:Wind speed at a height of 2 m(m/s)March 19,2009Principle of Hydrology,BNU33Several ExamplesPenman(1948)(in./day)March 19,2009Principle of Hydrology,BNU34Several ExamplesKuzmin(1957)(in./month)March 19,2009Principle of Hydrology,BNU35Several ExamplesRemanenko(1961)(cm/month)hn:relative humidity March 19,2009Principle of Hydrology,BNU36Measurement of Evaporation by PansEvaporation PanMarch 19,2009Principle of Hydrology,BNU37Pan MethodCET:Pan-to-ET coefficient(0.5-1.0)March 19,2009Principle of Hydrology,BNU38Relationship between ET0 and ETpf:A seasonal correction factor(wet bare soil:f=0.9)March 19,2009Principle of Hydrology,BNU39Crop Evapotranspirationkc:Crop coefficientMarch 19,2009Principle of Hydrology,BNU40Actual EvapotranspirationPhysically-based modelPenman-Monteith modelConceptual modelPotential evapotranspirationWetness of the soil(actual soil moisture storage divided by the maximum storage)March 19,2009Principle of Hydrology,BNU41Estimation of ETa from ETpSMT:Actual soil moisture storageSMC:Soil moisture storage at field capacityMarch 19,2009Principle of Hydrology,BNU42Several ExamplesRoberts(1978)(daily)RAT=SMT/SMCMarch 19,2009Principle of Hydrology,BNU43Several ExamplesBudyko&Zubenok(1961)Monthly valuesMarch 19,2009Principle of Hydrology,BNU44SummariesFree water surface evaporationPotential evapotranspirationActual evapotranspirationEstimation of ETpEstimation of ETa from ETp March 19,2009Principle of Hydrology,BNU45March 19,2009Principle of Hydrology,BNU46
