Campbell Scientific VisualWeather Software - Manual (Page 61)

Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
where the atmospheric pressure is obtained from the following equation:
P = 101.3
293- 0.0065Z
293
5.26
⎡
⎣⎢ ⎤
⎦⎥
(18)
where, Z = Elevation in meters. In the above calculation the temperature
is assumed to be 20°C; hence,
T (Kelvin) = T (°C) + 273 =293. Note that by taking the value of
temperature to be 20 °C at all altitudes the temperature dependence of
γ
has been ignored.
At sea level Z= 0; therefore, P = 101.3 kPa. At a higher altitude the
pressure P drops below this value, since the air is less dense. As the
altitude increases, P decreases and so does
γ.
VisualWeather computes the atmospheric pressure, P, based on the user-
entered elevation.
Note that the latent heat of vaporization of water, λ is also a function of
temperature. However, its variation with temperature is small enough to
be neglected. For the FAO 56 PM equation the value of
λ is assumed to
be constant, λ = 2.45 MJ/kg.
5. Calculation of e°(T
hr
) (saturation vapor pressure at air temperature
T
hr
).
The saturation vapor pressure is related to air temperature, T
e°(T
hr
) = 0.6108 e (17.27T) / (T + 237.3) (19)
Note that the right-hand side of this equation forms a part of the
numerator in equation (3) above.
6. Calculation of e
a
(average hourly actual vapor pressure).
e
a
= e°(T
hr
)*RH
hr
/100 (20)
RH
hr
is the average hourly relative humidity (%)
Finally these individual results are combined to calculate hourly values of
ET
o
by using equation (2).
Denominator = Δ + γ (1 + 0.34 u
2
)
First term = 0.408 Δ (Rn-G) / (Δ + γ (1 + 0.34 u
2
))
Second term = γ (37/(T
hr
+ 273)) u
2
(e°(T
hr
)-e
a
)) / ( Δ + γ (1 + 0.34 u
2
))
ET
o
(mm/hour) = First term + Second term
A-7