Comparative analysis of reference evapotranspiration estimation between various methods and the FAO56 Penman - Monteith procedure

This study aimed to assess the performance of nine empirical methods [FAO-24 Penman (1977), Makkink (1957), Turc (1961), Penman (1963), Priestley-Taylor (1972), Linacre (1977), Kimberly Penman (1982), Hargreaves-Samani (1985), Turc (1961), Penman (1963), Priestley-Taylor (1972), Linacre (1977), Kimberly Penman (1982), Hargreaves-Samani (1985), Copais (2006)], for the daily reference evapotranspiration (ETo) estimation in comparison with the Penman Monteith method standardized by the Food and Agriculture Organization (FAO56 – PM). The analysis, used data of two meteorological stations at Krania and Kozane, located at Western Macedonia, Greece. Daily values of ETo were calculated using meteorological data for a time-period of 34 and 48 years of the two stations respectively. Since none of the solar radiation variables was measured on the stations, the net radiation variable (Rnet) was derived empirically following the procedure outlined in the FAO-56 paper (Allen et al., 1998). Such values were compared using linear regression and statistical indices of quantitative approaches to model performance evaluation. All the statistical indices used were calculated on a daily basis. However, the root mean square error (RMSE) was additionally calculated on a monthly basis in order to evaluate the seasonality differences of the methods to be compared. In regard to the regression equations, the Priestley-Taylor method had the best correlation to the FAO56-PM method at Krania station, while at Kozane station the Turc method gave the best predicted values. By comparing the monthly accumulated values of ETo it may be concluded that not only on a daily but on a monthly basis as well, all of the methods compared perform good during the winter season (October-February) with smaller deviations in absolute values of ETo and lower RMSE, but show poor performance during the summer season (March-September) with the opposite characteristics