Assessment of Potential Reference Crop Evapotranspiration Trend in Ghezel Ozan River Basin under Climate Change Conditions

Ghezel Ozan River Basin is one of the important basin in Iran, which supply people grains requirements. The amount of potential reference crop evapotranspiration (ET0) was evaluated with RCP4.5 (low emission) and RCP8.5 (high emission) scenarios on the horizons 2030, 2050, and 2070. The output of four GCM models in CMIP5 and the LARS-WG6 statistical downscaling were used. In this study, the daily historical records of six synoptic stations (namely Zanjan, Mianeh, Khalkhal, Zarrineh, Qorveh, and Bijar) from 1989-2016 were used. Differences of mean ET0 time series in the base and future time periods were tested using the t-test method in three-time scales (i.e. monthly, seasonal, and annual scales) at 5% significance level. Trends of ET0 in the proposed three-time scales were analyzed in the base and 2021-2080 periods with both RCP scenarios using the Mann-Kendall (MK) method at 5% significance level. The effect of significant autocorrelation coefficients was eliminated in MK method. The slope of trend lines was estimated by Sen’s estimator. Results showed in the whole basin, based on the RCP4.5 scenario in the horizons of 2030, 2050, and 2070, the amount of ET0 will be increased by 1.8%, 3.7%, and 5.7%, respectively. These records were about 1.7, 5.4, and 9.1 percent using the RCP8.5 scenario, respectively. The most increase in ET0 was observed for July. The annual ET0 values would be increased in the future in all stations. The mean differences of ET0 in June, July, August, summer, and annual time series with respect to the base time period were significant for all the stations and for all the future periods (under two RCP scenarios). In the future period, according to the both scenarios at all stations, the annual ET0 trend was upward.