Appendix B ______________________________________________________ Calculation Formulas
VAISALA______________________________________________________________________ 285
Evapotranspiration
Evapotranspiration is calculated using the Penman-Monteith equation.
The evapotranspiration calculation estimates the loss of water from a
vegetative surface trough the combined process of plant transpiration and
soil evaporation. It is calculated with the following formula:
)34.01(
)(
273
900
)(408.0
2
2
U
eeU
T
GR
Eto
dan
where
Eto = Reference crop evapotranspiration [mm/d]
R
n
= Net radiation at crop surface [MJ/m
2
/d]
G = Soil heat flux [MJ/m
2
/d]
T =
Average air temperature [C]
U
2
= Wind speed measured at 2m height [m/s]
(e
a
-e
d
) = Vapor pressure deficit [kPa]
=
Slope of vapor pressure curve [kPa(C)]
=
Psychometric constant [kPa(C)]
Slope of the Vapor Pressure Curve ()
2
)3.237(
4098
T
e
a
Net Radiation (Rn)
This is the average radiation from sensor. Net is calculated using surface
albedo.
The radiation value [W/m
2
] from measurements is converted to
[MJ/m
2
/d] by the calculation.
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