Campbell Scientific VisualWeather Software User 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