Evaporation• What is evaporation?• How is evaporation measured?• How is evaporation estimated?
• Reading: Applied Hydrology Sections 4.1 and 4.2 on Infiltration and soil water movement
EvaporationEvaporation – process by which liquid water becomes water vapor– Transpiration – process by which liquid water
passes from liquid to vapor through plant metabolism
– Evapotranspiration – evaporation through plants and trees, and directly from the soil and land surface
– Potential Evaporation – evaporation from an open water surface or from a well-watered grass surface
Factors Influencing Evaporation• Energy supply for
vaporization (latent heat)– Solar radiation
• Transport of vapor away from evaporative surface– Wind velocity over surface– Specific humidity gradient
above surface• Vegetated surfaces
– Supply of moisture to the surface
– Evapotranspiration (ET)• Potential Evapotranspiration
(PET) – moisture supply is not limited
nR
E
Net radiation
Evaporation
Air Flowu
Evapotranspiration (ET)
Over land surfaces, we cannot distinguish between water vapor that evaporated from the soil and water vapor that was transpired through plants
Evaporation from an Open Water Surface
• Simplest form of evapotranspiration– This is the amount of water lost from lakes and reservoirs– Often estimating by measuring the loss from a National
Weather Service Class A pan
• This is referred to as Potential Evapotranspiration (ETp) because it is the maximum potential rate of ET under the given meteorological conditions
Lysimeters
Measurement of evapotranspiration
Flux Towers (Marcy Litvak)
Flux tower instruments
IRGA
3-D Sonic anemometer
Net radiometerPyrronometer
Quantumsensor
Flux tower instruments
Air Temperature at 1m and 10m
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5
10
15
20
25
30
35
40
t_hmp_10m
Freeman Ranc Flux Tower (Marcy Litvak)
Vapor Pressure and Saturated Vapor Pressure (kPa)
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1
2
3
4
5
6
e_Avge_sat_Avg
Relative Humidity at 1m and 10m
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0.2
0.4
0.6
0.8
1
rh_hmp_10m
Average = 0.61
Average = 0.71
Wind Speed (m/s)
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5
10
15
20
25
Net Radiation (W/m2)
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-100
0
100
200
300
400
500
600
700
800
ET -Eddy covariance method• Measurement of vertical
transfer of water vapor driven by convective motion
• Directly measure flux by sensing properties of eddies as they pass through a measurement level on an instantaneous basis
• Statistical tool
Basic Theory
Mean
Fluctuation
Instantaneous signal
InstantaneousPerturbation from
The mean
All atmospheric entities show short-period fluctuations about their long term mean value
Time averaged property
Turbulent mixing
Propterties carried by eddies:Mass, density ρVertical velocity w
Volumetric content qv
Temperature T
Sensible Heat Flux (W/m2)
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-200
-100
0
100
200
300
400
500
Latent Heat Flux (W/m2)
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-200
-100
0
100
200
300
400
500
Evaporation (mm/day)
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-4
0
4
8
12
16
20
Average = 3.15 mm/day
Energy Balance Method
Can directly measure these variables
How do you partition H and E??
Energy Balance Method
28.4W 𝑚2
× 𝐽 /𝑠𝑊 × 1𝑔
2450 𝐽 ×3600 𝑠1h𝑟 × 24 h𝑟
1𝑑𝑎𝑦 ×𝑚3
1000𝑘𝑔 ×1𝑘𝑔1000𝑔 ×
1000𝑚𝑚1𝑚 =1𝑚𝑚
𝑑𝑎𝑦
𝜌𝑤𝐸𝑇=
E28.4=
128.4 (𝑅𝑛−𝐺−𝐻−𝑊 )
The maximum radiative evaporation rate Er =
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