Campbell Scientific VisualWeather Software - Manual (Page 66)

Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
A-12
Stefan-Boltzmann constant SIGMA = 2.043 x 10 -10 MJ/m 2 /K 4 /hour
SIGMA*(absolute temp) 4 MJ/m 2 /hour
Long wave, low energy radiation R
nl
MJ/m 2 /hour
Net radiation (short wave-long wave radiation) R
n
MJ/m 2 /hour
Soil heat flux (if Rs>0 then 0.1*Rn, else 0.5*Rn) G MJ/m 2 /hour
Net radiation - soil heat flux (Rn-G) MJ/m 2 /hour
Denominator (Denom) in the equation for calculating ET
o
Denom = Δ + γ (1+0.34 u
2
)
First term in the equation for calculating ET
o
First term = 0.408 * Δ * (Rn-G) / Denom
Second term in the equation for calculating ET
o
Second term =( γ * (37/(T+273)) * u2* (eo-ea)) /
Denom
Hourly ET
o
value ET
o
(mm/hour) = first term + second term
REFERENCES
1. Crop Evapotranspiration, Guidelines for Computing Crop Water
Requirements, FAO Irrigation and Drainage Paper 56, Richard G. Allen,
Luis S. Pereira, Dirk Raes, and Martin Smith, Food and Agriculture
Organization of the United Nations, Rome, 1998.
2. 'Uber einige meteorologische Begriffe' , Tetens,O. , Z. GeoPhys., 6:297-
309, 1930.
3. 'On the computation of saturation vapor pressure', Murray, F.W., J. Appl.
Meteor., 6:203-204, 1967.