The Potential Evapotranspiration Trends in Isfahan and Rasht Stations

Evapotranspiration is one of the most important components of the hydrological cycle and hydrological models for planning irrigation system water. In this study, the amount of evapotranspiration using the FAO Penman-Monteith for synoptic weather stations of Isfahan and Rasht that located in parts of central and northern Iran in the period 1970-2011. Some of the most important meteorological variables affecting it in the scales of seasonal and annual was calculated. In addition, changes were also analyzed and compared. For this purpose, non-parametric test of Mann-kendall and parametric test of regression was used. The results showed that the annual scale evapotranspiration changes in Rasht, and Isfahan stations had the significant increasing trend (5% level). At these station, the temperature increased and the minimum relative humidity decreased but of course was greater than other variables. In the seasonal scale both positive and negative trends have been observed. Isfahan Station evapotranspiration trends in other seasons except in winter is negative (5% level). Rasht Station evapotranspiration trends in other seasons except in spring was positive (5% level). Among the studied variables, seasonal and annual of number of sunshine hours was increased (level 5%). Maximum and minimum air temperature trend at both stations in the annual scale was increased (5% level). Comparing the results of the review process, potential evapotranspiration and temperature changes by both tests showed contradictory phenomena in the process of evaporation variables in Isfahan Station.