Potential Evapotranspiration (following Maidment D. R. (editor), 1993, Handbook of Hydrology, Chapter 4 on Evaporation by W J Shuttleworth.)
A = Available energy = (1-a)Tf So - f e’ s T4
f = Tf/a Cloudiness factor
D = gradient of saturated vapor pressure - temperature curve at air temperature
g = psychometric constant at air temperature and pressure
e’ = net emmissivity based upon dew point
Temperature and dew point lapsed from measurements at sea level