Abstract
The Penman-Monteith (PM) method is the most recommended method for estimating reference evapotranspiration (ETo). The PM equation requires several parameters to be available, either measured or computed. Some of these parameters are conventionally calculated by some slightly sophisticated formulas, especially for handy calculations. This paper aimed to derive some simpler statistical equivalents to these formulas. Simplifications were performed to the formulas of the saturation vapor pressure e (o)[T], slope of vapor pressure (a dagger), atmospheric pressure (P), the psychrometric constant (gamma), wind speed correction, the long-wave radiation, R (nl); the sunset hour angle, omega (s); and the extraterrestrial radiation, R (a). For the first five parameters, the parameter-independent factor was analyzed for its extremes, then fitted by interpolation to a simpler equivalent formula. The last three parameters were fitted to simpler form through data from the FAO-CLIMWAT database. Each of the simplified formulas was compared to the conventional one; some correlation indices were applied to validate the new formulas. The ETo was calculated for all stations in the CLIMWAT database by both simplified and conventional formulas. All the correlation results were excellent, with a minimum correlation coefficient of 0.9966. The simplified formulas were proven to be equivalent in performance, with almost no loss in accuracy but simpler in form and faster in execution in the online database applications.