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

8/5/04 8/6/04 8/7/04 8/8/04 8/9/04 8/10/04 8/11/04 8/12/04 8/13/04 8/14/04 8/15/040

5

10

15

20

25

30

35

40

t_hmp_10m

Freeman Ranc Flux Tower (Marcy Litvak)

Vapor Pressure and Saturated Vapor Pressure (kPa)

8/5/04 8/6/04 8/7/04 8/8/04 8/9/04 8/10/04 8/11/04 8/12/04 8/13/04 8/14/04 8/15/040

1

2

3

4

5

6

e_Avge_sat_Avg

Relative Humidity at 1m and 10m

8/5/04 8/6/04 8/7/04 8/8/04 8/9/04 8/10/04 8/11/04 8/12/04 8/13/04 8/14/04 8/15/040

0.2

0.4

0.6

0.8

1

rh_hmp_10m

Average = 0.61

Average = 0.71

Wind Speed (m/s)

8/5/04 8/6/04 8/7/04 8/8/04 8/9/04 8/10/04 8/11/04 8/12/04 8/13/04 8/14/04 8/15/040

5

10

15

20

25

Net Radiation (W/m2)

8/5/04 8/6/04 8/7/04 8/8/04 8/9/04 8/10/04 8/11/04 8/12/04 8/13/04 8/14/04 8/15/04

-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)

8/5/04 8/6/04 8/7/04 8/8/04 8/9/04 8/10/04 8/11/04 8/12/04 8/13/04 8/14/04 8/15/04

-200

-100

0

100

200

300

400

500

Latent Heat Flux (W/m2)

8/5/04 8/6/04 8/7/04 8/8/04 8/9/04 8/10/04 8/11/04 8/12/04 8/13/04 8/14/04 8/15/04

-200

-100

0

100

200

300

400

500

Evaporation (mm/day)

8/5/04 8/6/04 8/7/04 8/8/04 8/9/04 8/10/04 8/11/04 8/12/04 8/13/04 8/14/04 8/15/04

-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 =