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FUNDAMENTALS OF AGROMETEOROLOGY
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- University Of The Freestate (UFS)
give basic knowledge of agrometeorology studies
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Appendix to Theme 9: Additional equations needed for FAOPenman-Monteith ETo calculations
900
0.408( Rn G ) u 2 (e s e a )
ETo T 273
(1 0.34u 2 )
where: ETo is the reference evapotranspiration [mm d-1]
Rn is the net radiation at the crop surface [MJ m -2 d-1]
G is the soil heat flux density [MJ m-2 d-1]
T is the average air temperature at 2 m height [ºC]
u2 is the wind speed at 2 m height [m s-1]
es is the saturation vapour pressure [kPa]
ea is the actual vapour pressure [kPa]
es – ea is the saturation vapour pressure deficit [kPa]
is the slope of the vapour pressure curve [kPa ºC-1]
is the psychrometric constant [kPa ºC-1]
RH max RH min
eº (Tmin ) eº (Tmax )
100 100 eº (Tmax ) eº (Tmin ) ea
ea es RH 100
2 2 es
Rn Rns Rnl where: Rn is net radiation [MJ m-2 d-1]
Rns is net incoming shortwave radiation [MJ m-2 d-1]
Rnl is net outgoing long-wave radiation [MJ m-2 d-1]
Rns (1 ) Rs is the albedo, which is 0.23 for the hypothetical grass
reference crop [dimensionless]
Rs is solar or shortwave radiation [MJ m-2 d-1]
n
Rs a s bs Ra Ra is extraterrestrial radiation [MJ m-2 d-1]
N
as + bs is the fraction of Ra reaching the earth on clear days
Where no calibrated values are available, the values a s = 0.25 and bs = 0.50 are recommended.
Rso (0.75 2 10 5 z ) Ra Rso is clear-sky solar radiation [MJ m-2 d-1]
z is altitude [m]
T Tmin
Rnl max R
0.34 0.14 ea 1.35 s 0.35
2 Rso
G 0 for day and 10-day periods
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