Chapter3-环境水文学.ppt

1、March 19, 2009,Principle of Hydrology, BNU,1,Chapter 3Evaporation and Transpiration,Prof. Zongxue Xu,March 19, 2009,Principle of Hydrology, BNU,2,Objective,Evaporation: E Transpiration: T Potential evapotranspiration: ETp Actual evapotranspiration: ETa Estimation of E, ETp, ETa,March 19, 2009,Princi

2、ple of Hydrology, BNU,3,Evaporation,Definition: 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) ETo,March 19, 2009,Princip

3、le of Hydrology, BNU,4,Potential Evapotranspiration,Definition: The maximum rate of evapotranspiration from a vegetated catchment under conditions of unlimited moisture supply. ETp,March 19, 2009,Principle of Hydrology, BNU,5,Actual Evapotranspiration,Definition: All the processes by which liquid wa

4、ter at or near the land surface becomes atmospheric water vapor ETa Global 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, 2009,Principle of Hydrology, BNU,6,Estimation Procedu

5、re,Estimate potential evapotranspiration based on meteorological factors Estimate the actual evapotranspiration processes at the current status of the plant- and soil-moisture-related characteristics,March 19, 2009,Principle of Hydrology, BNU,7,Meteorological Factors,Evaporation example: Moist tempe

6、rate climate Open water: 600 mm Land surface: 450 mm Arid climate Open water: 2000 mm Land surface: 100 mm Meteorological factors Solar radiation Wind Relative humidity Temperature,March 19, 2009,Principle of Hydrology, BNU,8,Wind,The boundary layer between ground surface (water) and air must be rem

7、oved and continually replaced by drier air The movement of the air in the boundary layer depends on wind,March 19, 2009,Principle of Hydrology, BNU,9,Relative Humidity,Replacement of the boundary layer of saturated air by air of equally high humidity will not maintain the evaporation rate Evaporatio

8、n will occur only if the incoming air is drier than the air that is displaced,March 19, 2009,Principle of Hydrology, BNU,10,Temperature,If 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 in

9、creases as its temperature rises.,March 19, 2009,Principle of Hydrology, BNU,11,Factors affecting evapotranspiration,Incidence of precipitation Climatic factors: temperature, humidity, Type, manner of cultivation and extent of vegetation,March 19, 2009,Principle of Hydrology, BNU,12,Methods to Estim

10、ate Evaporation,Water budget / storage equation approach Energy budget method Empirical formulae Pan method,March 19, 2009,Principle of Hydrology, BNU,13,Water Budget Approach,P: Total precipitation I: Surface inflow U: Underground outflow O: Surface outflow S: Change in storage,March 19, 2009,Princ

11、iple of Hydrology, BNU,14,Energy Budget Method,Estimating all sources and sinks of thermal energy and leaves evaporation as the only unknown Need many data A specialist approach,March 19, 2009,Principle of Hydrology, BNU,15,Penman Combination Method,Open water evaporation H: Available heat energy Ea

12、: Latent heat flux (Energy for evaporation),March 19, 2009,Principle of Hydrology, BNU,16,Available Heat Energy,RI: Incoming radiation RO: Outgoing radiation,March 19, 2009,Principle of Hydrology, BNU,17,Incoming/Outgoing Radiation,ed: mean vapor pressure n: bright sunshine (hr/day),March 19, 2009,P

13、rinciple of Hydrology, BNU,18,Latent Heat Flux,u2: mean wind speed at 2 m above the ground surface,March 19, 2009,Principle of Hydrology, BNU,19,Penman-Monteith Method,: density of air : latent heat of vaporization of water Cp: specific heat of air at constant pressure ra: aerodynamic resistance to

14、water vapor transport rs: bulk (canopy) surface resistance to water transport,March 19, 2009,Principle of Hydrology, BNU,20,FAO Penman-Monteith Method,Rn: net radiation at the crop surface G: Soil heat flux density,March 19, 2009,Principle of Hydrology, BNU,21,Latent Heat of Vaporization,Ta: air tem

15、perature : MJkg-1,March 19, 2009,Principle of Hydrology, BNU,22,Atmospheric Pressure,Z: elevation (m) P: kPa,March 19, 2009,Principle of Hydrology, BNU,23,Saturation Vapour Pressure,es(Ta): kPa,March 19, 2009,Principle of Hydrology, BNU,24,Actual Vapour Pressure,Td: dew point temperature ea(Td): kPa

16、,March 19, 2009,Principle of Hydrology, BNU,25,Slope of Vapour Pressure Curve,: kPaoC-1,March 19, 2009,Principle of Hydrology, BNU,26,Psychrometric Constant,P: atmospheric pressure (kPa) : latent heat of vaporization (MJkg-1),March 19, 2009,Principle of Hydrology, BNU,27,Net Shortwave Radiation,: al

17、bedo/ canopy reflection coefficient Rs: Incoming solar radiation (MJm-2day-1),March 19, 2009,Principle of Hydrology, BNU,28,Net Longwave Radiation,: Stefan-Boltzmann constant (4.90310-9 MJK-4m-2day-1) Rs/Rso: relative shortwave radiation,March 19, 2009,Principle of Hydrology, BNU,29,Net Radiation,Rn

18、s: Incoming net shortwave radiation Rnl: Outgoing net longwave radiation,March 19, 2009,Principle of Hydrology, BNU,30,Soil Heat Flux,cs: soil heat capacity (MJm-3oC-1) Ti: air temperature at time I z: effective soil depth (m),March 19, 2009,Principle of Hydrology, BNU,31,Empirical Formulae,Objectiv

19、e: free water surface evaporation,March 19, 2009,Principle of Hydrology, BNU,32,Several Examples,u2: Wind speed at a height of 2 m (m/s),March 19, 2009,Principle of Hydrology, BNU,33,Several Examples,Penman (1948) (in./day),March 19, 2009,Principle of Hydrology, BNU,34,Several Examples,Kuzmin (1957)

20、 (in./month),March 19, 2009,Principle of Hydrology, BNU,35,Several Examples,Remanenko (1961) (cm/month) hn: relative humidity,March 19, 2009,Principle of Hydrology, BNU,36,Measurement of Evaporation by Pans,Evaporation Pan,March 19, 2009,Principle of Hydrology, BNU,37,Pan Method,CET: Pan-to-ET coeff

21、icient (0.5-1.0),March 19, 2009,Principle of Hydrology, BNU,38,Relationship between ET0 and ETp,f: A seasonal correction factor (wet bare soil: f=0.9),March 19, 2009,Principle of Hydrology, BNU,39,Crop Evapotranspiration,kc: Crop coefficient,March 19, 2009,Principle of Hydrology, BNU,40,Actual Evapotranspiration,Physically-based model Penman-Monteith model Conceptual model Potential evapotranspiration Wetness of the soil (actual soil moisture