Investigating Time Changes of the Reference Evapotranspiration in Tabriz

Evapotranspiration is one of the important parameters in the hydrological cycle which has a direct impact on the amount of water available for agriculture. In this study، data obtained from average mean temperature، minimum temperature، maximum temperature، minimum relative humidity، wind speed، and sunshine hours during a 57-year period (2008-1951) were used to estimate the potential evapotranspiration. Potential evapotranspiration values were calculated through introducing the geographical location، including altitude، latitude and longitude of Tabriz station and using nine reliable methods in REF-ET software. According to the obtained results، the best way to calculate this index in the study area was identified to be Blaney-Criddle method. Kendall’s graphic test was also used in order to detect time changes of this index. The results indicated that despite the decline trend of this indicator until 1390s، the potential evapotranspiration has gradually increased 4 English Abstracts over recent years. The maximum increase of evapotranspiration occurs in the middle of spring and autumn and monthly changes of the index show a significant increase for May and October. However، the potential evapotranspiration shows a non-significant trend despite its large fluctuations over summer months (July، August، and September).