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Measurements for
water
A.M.J. Coenders
Waterbalans: Evaporation
Challenge the future
DelftUniversity ofTechnology
Measuring evaporation
2
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
This image cannot currently be displayed.
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