Modeling Solar Radiation Data for Reference Evapotranspiration Estimation at a Daily Time Step for Poland

The Penman–Monteith formula (P-M) is a well-established indirect method for estimating reference evapotranspiration (ET₀). The key input for this equation is global solar radiation (H). When real data are unavailable, other weather parameters are used to estimate H. In this study, sixteen years’ worth daily registers of H, sunshine duration (S), and air temperature (t) from 10 sites across Poland were used to determine coefficients for the Angström–Prescott (A-P) and Hargreaves–Sammani (H-S) equations. The H values obtained with locally calibrated, general Polish and global A-P and H-S equations were applied to the P-M formula. The ET₀ results thus obtained were compared to those derived with the P-M method and measured solar radiation data. The method of determination of the radiation component had a significant but sometimes unexpected impact on the ET₀ values. The better predictive power of the solar radiation model usually resulted in better accuracy of the evapotranspiration estimation; however, there were exceptions to this rule.