Evaluation of Effective Meteorological Variables on Reference Evapotranspiration Trend in Lake Urmia Basin

Reference Evapotranspiration (ETo) has an important role in irrigation management  and scheduling. Evaluating the sensitivity of ET o to different meteorological variables and contribution of each variable in ET o trend is essential for better management of water crisis in Lake Urmia basin. In this study, ETo in 13 synoptic stations in and around Lake Urmia basin for the period 1990-2018 was estimated by the FAO Penman-Monteith method. Trend analysis showed that the average annual ETo of stations has significant increasing trend of 8.0 mm year-2. In addition, the trend analysis of the average annual meteorological variables of the stations reveals significant increasing trends in minimum temperature (Tmin) (0.07 ℃ year-1), maximum temperature (Tmax) (0.09 ℃ year-1), and wind speed (u2) (0.02 m s-1 year-1). Sensitivity analysis of ETo to meteorological variables indicate that on an annual temporal scale, the maximum sensitivity of ETo is to relative humidity (RH) and the minimum sensitivity of ETo is to Tmin. Also, during this period, the sensitivity of ETo to Tmax, u2, and RH increased and the sensitivity of ETo to solar radiation (Rs) decreased. The contribution analysis of meteorological variables to ETo variations indicate that in most stations (with arid and semi-arid climate), u2 has the largest contribution in ETo variations. However, in Sardasht station with a very humid climate, the contribution of other factors in the ETo trend (decrease in RH and increase in Tmin and Tmax) is greater than the effect of decrease in u2. The results of this study highlights the need to pay attention to the different response of ETo to meteorological variables changes in different climates in planning water resources systems and studies that are dependent on climate change scenarios.