The effect of different meteorological parameters on the temporal variations of reference evapotranspiration

Temporal changes of meteorological variables can affect reference evapotranspiration (ET0). The goal of the present research is to analyze the changes of ET0 and identify the impact of effective meteorological parameters to the changes of ET0. For this purpose, daily meteorological data recorded in 30 synoptic stations of Iran during 1960–2014 were used. The annual and seasonal values of ET0 were calculated by the recorded data. To calculate ET0, FAO56 Penman–Monteith method (standard method) was used. The annual and seasonal trends of ET0 and its eight effective parameters were analyzed. Then the contributions of effective parameters changes on ET0 were determined. To analyze ET0 trend at annual and seasonal scales, two common methods, Spearman’s Rho and Mann–Kendall tests, were used. The R2 = 0.99 showed that the results of the mentioned methods were similar and on the basis of T-statistic <0.057, their difference was not significant (95% confidence level). Therefore, only one method’s results (Spearman’s Rho) were reported. On the basis of Spearman’s Rho results, the annual and seasonal values of ET0 had negative trend in most of arid and semi-arid stations while the trend of this parameter was positive in humid and very humid stations. At annual and seasonal scales, decreasing in wind speed (W), temperature (T), sunshine hours (n), minimum temperature (TN), dew point temperature (TD), maximum temperature (TX), saturation vapor pressure deficit (SVPD) and solar radiation (RS) was observed in 58, 54, 39, 43, 56, 65, 65 and 37% studied stations, respectively. In many scales, the results showed that TX and W were the most effective meteorological variables on ET0 changes and then SVPD was located in second step in arid and semi-arid stations. In humid and very humid stations, W was the first effective parameter at all scales, except autumn.