Simplified Reference Evapotranspiration Formula Using an Empirical Impact Factor for Penman’s Aerodynamic Term: Fid-Bau Portal

Simplified Reference Evapotranspiration Formula Using an Empirical Impact Factor for Penman’s Aerodynamic Term

Valiantzas, John D.

Recently, a simple algebraic formula, equivalent in accuracy to the Penman equation, was derived for computing evapotranspiration from readily available measured data. The derivation of the formula was based on simplifications made to the standardized form of the Penman equation. In this paper, a weighted coefficient is introduced that accounts for different impacts of the aerodynamic component at two different ranges of relative humidity. The new formula for estimating reference evapotranspiration ( ${E T}_{0}$ ) is obtained by calibration using meteorological data from the CLIMWAT global database. The performance of the new derived formula was tested under various climatic conditions using daily data for 12 weather stations obtained from the CIMIS database. For many places where reliable wind speed data are not available, another expression was also suggested. An alternative method not requiring wind data was also tested: the reduced-set food and agriculture organization Penman-Monteith method, according to the standardized procedure for estimating missing wind data. Comparison of the two methods not requiring wind data indicated that the proposed formula is a better option than the reduced-set Penman-Monteith method to estimate ${E T}_{0}$ .