Measurements for Water: Evaporation

Measurements for

water

A.M.J. Coenders

Waterbalans: Evaporation

Challenge the future

DelftUniversity ofTechnology

Measuring evaporation

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Evaporation is …

E = total evaporation

Eo = open water evaporation

Et = transpiration

Es = soil evaporation

Ei = interception evaporation

o t s iE E E E E= + + +

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Potential vs Actual

• Potential:

• No water stress

• Atmospheric demand for moisture

• Actual:

• Reality

• Net result of atmospheric demand for moisture from a surface and

the ability of the surface to supply moisture

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Evaporation measurements

Measurement Eo Et Es Ei

Potential Class A pan X

Penman (-Monteith) X

Actual Lysimeter X

Sapflow X

Bowen ratio X

Eddy correlation X

Scintillometer X

Energy balance X

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Class A-pan

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Penman (-Monteith)

∆ (Rn – G) + ρair cp vpd / raλE = _________________________

∆ + γ (1 + rs / ra)

= 0 for Penman (no vegetation)

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Radiation Rn = (Rs,in - Rs,out) - (“Rl,in” – Rl,out)

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Radiation – Short wave radiation

The absorber (1) absorbs radiation (2). The radiation is converted into heat. The heat flux to the heat sink (4) is measured by the thermopile (3).

Hukseflux LP02 (Pyranometer)

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Radiation – Long wave radiation

Spectral Range 4.5 to 42 µm (50% points)

Sensitivity (nominal) 5 to 7 µV/W /m2

Response time (63 %) < 6 s

Response Time (99 %) <18 s

Temperature Dependence of Sensitivity (–20 to +50 ºC) -10 to +40 ºC ±5 %

Field of View 150°

Irradiance -250 to +250 W /m2 (net irradiance)

Non stability change / year < 1%

Temperature sensor Thermistor (YSI 44031)

Kipp & Zonen CGR3 Pyrgeometer

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Radiation – Net radiation

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Radiation – Remote sensing

Meteosat incoming radiation

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Soil heat flux dTG K

dz=

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( ) ( ) ( )wawsaa tttete −−= γ

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t+

t =tes 237

3.17exp61.0)(

VPD= Vapor Pressure Deficit (es-ea)

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Vapor Pressure Deficit (es-ea)

This image cannot currently be displayed.

t+

t =tes 237

3.17exp61.0)(

This image cannot currently be displayed.

)(

)(

te

te = h

s

a

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Aerodynamic resistance

( )2

245 1

0.54 0.5 86400ar u= ∗

+

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Summary Penman

• Net radiation

• Ground heat flux (G=0 on daily time scale)

• Temperature

• Humidity

• Wind

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Lysimeters

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Energy Balance

DTS Flux tower

Rn = ρλE + H + G

e.g. Bowen ratio

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Sonic anemometer

Eddy covariance

Vertical wind *moisture=>

vertical moisture flux

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Scintillometer :

Transmitter

ReceiverTurbulent Atmosphere

measures H in Rn = ρλE + H + G

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Scintillometer