Campbell Scientific VisualWeather Software - Manual (Page 65)

Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
Latent heat of vaporization of water λ=2.45 MJ/kg
Atmospheric pressure P kPa
Psychrometric constant γ kPa/ o C
Saturation vapor pressure at T eo(T) kPa
Actual vapor pressure ea kPa
Vapor pressure deficit of the air at T eo(T)-ea kPa
Solar constant GSc= 0.082 MJ/m 2 /min
Constant used in seasonal correction b radians
sin (b) none
cos(b) none
sin(2b) none
Seasonal correction Sc hour
Inverse relative sun-earth distance dr meters
Solar declination δ radians
Solar time angle at the beginning of period ω
1
radians
sin( ω1)
Solar time angle at the ending of period ω
2
radians
sin( ω2)
Solar time angle at midpoint of period ω radians
Length of the step (applies to hourly step only) t
1
= 1 hour
Extraterrestrial radiation Ra MJ/m 2 /hour
Clear sky radiation R
SO
MJ/m 2 /hour
Ratio of radiation Rs/R
SO
no units
Ratio of radiation with total cloud cover Rs/R
SO
= 0.3 no units
Ratio of radiation at nighttime in humid, subhumid climates Rs/R
SO
= 0.4 to 0.6 no units
Ratio of radiation at nighttime in arid, semi-arid climates Rs/R
SO
= 0.7 to 0.8 no units
Constant used in calculating Ra a
s
= 0.75
Constant used in calculating Ra, b
s
*Z = 2 x 10 -5 *Z
(Z = elevation in meters)
Short wave, high energy radiation R
ns
MJ/m 2 /hour
A-11