Evapotranspiration (ET) is the largest water consumer in arid and semi- arid regions. Therefore, acquiring the knowledge of the factors and processes leading to ET is of utmost importance in decision – making water resources management. In this study, the long time trend in mean monthly temperature, rainfall and ET was investigated to this end, all synoptic stations were categorized according to the De Martonne climate classification method. As we were mostly concerned with the semi- arid region of the Land of Iran, only 14 stations with at least 30 years of data were selected for this purpose. The least squares error, the Man – Kendall, and the Vald-Volfoyts methods were used in analyzing the data. The FAO-Penman-Monteith equation was used in the reference crop ET estimation. It was observed that 24% of the stations showed positive and 14% showed negative trends, and the remaining stations did not follow any trend. As for the rainfall, no meaningful trend was detected in this parameter. Of the applied statistical methods, the least squares and error the Mann-Kendall methods showed closer similarities than the other procedures. In conclusion, it is certain that some regions shall experience higher temperatures, thus ET, in the coming years.
)
