Evapotranspiration – Vaisala AWS330 User Manual
Page 287
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
+
+
Δ
−
+
+
−
Δ
=
γ
γ
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]
Δ
= Slope of vapor pressure curve [kPa(
°
C)]
γ
= Psychometric constant [kPa(
°
C)]
Slope of the Vapor Pressure Curve (
Δ
)
2
)
3
.
237
(
4098
+
=
Δ
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.