Vaisala AWS330, User Manual

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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 . 0 1 (
) (
273
900
) ( 408 . 0
2
2
U
e e U
T
G R
Eto
d a n
  


  



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.