In 1948 Penman introduced the combination method to calculate the potential evapotranspiration (ETP) as a single explicit expression, containing two unknowns: the evaporation rate and the surface temperature. The Penman equation was based on assuming that the air at the surface is saturated and that the ratio of the humidity gradient to the temperature gradient equals the slope of the saturation humidity over the temperature relation at the ambient temperature, divided by the psychrometric constant. However, by using a recursive combination method (RCM) with commercially available software (Mathcad v. 12) it is possible to calculate ETP iteratively without the assumption introduced by Penman. The difference between the two methods on clear, hot and windy days can be as much as 20%, with the RCM calculating ETP values that are larger. We also compare the above two methods with the ASCE standardized reference evapotranspiration (ETR) equation.
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