Campbell Scientific VisualWeather Software - Manual (Page 60)

Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
Lz = Longitude of the center of the station's time zone (west of the
Greenwich line as 0
o ) around the earth in clockwise direction (degrees
west of Greenwich)
Lm = Longitude of the measurement site (west of the Greenwich line as
0
o )
S
c
= seasonal correction for the solar time (hour) =
0.1645 sin (2b) -0.1255 cos(b) -0.025 sin(b) (14)
b = 2 π (J-81)/364 where J is day of the year between
1 (January 1) and 365 or 366 (December 31) (15)
3. Calculation of G (soil heat flux).
The equation to calculate soil heat flux changes depending upon the ET
O
method used, and whether it is night or day. The following table describes the
equation to use for each method and daytime/nighttime.
Nighttime:
Standardized Short Grass 0.1Rn
Standardized Tall Grass 0.4Rn
FAO56 0.1Rn
Daytime:
Standardized Short Grass 0.5Rn
Standardized Tall Grass 0.5Rn
FAO56 0.2Rn
4. Calculation of γ (psychrometric constant).
γ = c
P
P/ ελ (16)
where, c
P
= specific heat capacity at constant pressure = 1.013 x 10 -3
MJ/kg/°C)
ε = ratio of molecular weight of water to molecular weight of dry
air = 0.622
λ = latent heat of vaporization of water = 2.45 MJ/kg
P = atmospheric pressure (kPa)
Substituting values of constants, γ becomes
γ = 0.000665 P (17)
A-6