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

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