Long-term estimation of changes in station climate parameters under the CanESM2 model (Case study: Boroujerd synoptic station)

General atmospheric circulation models show an increase in greenhouse gas concentrations. These models predict global climate change by simulating the earth's climate. General atmospheric circulation models are not accurate enough for hydrological and water resources studies due to the large temporal and spatial scale of the simulated climate variables compared to the case model. Therefore, they must be scaled. There are different methods for exponential downscale of large-scale variables, and general atmospheric circulation models. In this research, the SDSM model is used to downscaling climatic data.

Boroujerd Synoptic Station with an altitude of 1629 meters above sea level is located at latitude 33◦ 55′ East and longitude 48◦ 45′ North. Boroujerd city is located at the foot of the highest wall of the Zagros at an altitude of 1550 to 1571 meters above sea level and the highest point is in the Garin mountain range with an altitude of 3623 meters in the west and its lowest area is in Silakhor plain with an altitude of 1500 meters. In this study, the CanESM2 climate model under three scenarios RCP26, RCP45, and RCP85 in three time periods 2040-2021, 2060-2041, and 2080-2061; and SDSM model version 4.2 were used for downscale (micro-scale) exponential climatic data. For calibration and validation of the SDSM microscale model, R2 and RMSE calibration indices were used. In this study, 30% of the data were used for validation and 70% of the data were used for calibration.

In the SDSM model, the maximum and minimum temperature values are better predicted than the precipitation values, and the simulated data are closer to the observational values. In all scenarios and periods, the precipitation trend is decreasing. The largest decrease in precipitation is related to January in the period 2021-2040 and the RCP8.5scenario, with a decrease of 69.22 percent. The temperature in all scenarios and periods had an increasing trend compared to the base period. The highest increase in the minimum temperature data is related to the RCP4.5 scenario in October for the period 2061-2080 and it was equal to 4.90 ° C, respectively, and in the maximum temperature data related to the RCP4.5 scenario in October for the period, 2061-2080 was predicted to be equal to 7.02 ° C. Calibration of SDSM model for Boroujerd station for each of the minimum and maximum temperature and precipitation showed the mean values of coefficient of determination 0.99, 0.98 and 0.67, respectively.

The highest decrease in rainfall is related to January in the period 2040-2021 and the RCP8.5 scenario, with a decrease of 69.22%. Also, the SDSM exponential downscale (microscale) model for the minimum and maximum temperature parameters predicted an upward trend. Calibration of SDSM model for Boroujerd synoptic station shows the efficiency of SDSM model in microcompalation of parameters. As a result, we will face a decrease in hydrological reserves in future periods.