Evaluation of temporal spatial changes of reference evapotranspiration under the influence of climate change in Gorganroud watershed in northern Iran

Reference evapotranspiration (ET0), as one of the main components of the hydrological cycle, plays an important role in water resources management and agricultural planning. This study was conducted with the aim of predicting the temporal and spatial changes of ET0 in the Gorganroud watershed in northern Iran. The minimum and maximum temperatures were predicted using the output of five CMIP6 climate models under two climate scenarios of SSP2-4.5 and SSP5- 8.5 for the historical base (1985-2014), near future (2025- 2054) and far future (2071-2100) periods. The bias correction of the simulation data was performed using the linear scaling method. To reduce the uncertainty of climate models, a multi-model ensemble based on the application of Bayesian Model Averaging (BMA) was created and the reference evapotranspiration was calculated using the Hargreaves-Samani method. The results showed that under the SSP2-4.5 scenario, the minimum and maximum temperatures will increase by 1.65 and 1.8 degrees C, respectively, whereas under the SSP5-8.5 scenario, the minimum and maximum temperatures will increase by 2.5 and 2.7 degrees C, respectively. Similarly, the projections show that the reference evapotranspiration will increase on seasonal and annual scales in the future climate compared to the base period. The largest increase in ET0 is estimated to be 12.4% under the SSP5-8.5 scenario in the period 2071-2100 compared to the base period. The largest increase in evapotranspiration is in summer with values of 5.8-8% and 7.8-13.3% for the SSP2-4.5 and SSP5-8.5 scenarios, respectively. Analysis of the zonation of changes in evapotranspiration showed that most of the changes occur in the eastern regions and at Gharehbil and Cheshmehkhan stations. Our results indicate that future climate change will cause a significant increase in ET0 at high altitudes.