Bivariate Analysis of the Impact of Climate Change on Drought with SPEI Index and Coppola Functions (Case Study: Dugonbadan)

Droughts affected by climate change are among the most devastating weather events, causing significant damage both in natural resources and in human life. In this study, in order to investigate the effect of climate change on drought, both precipitation and temperature future period (2030-2050) were calculated with CanESM2 model under RCP8.5 scenario in the Douganbodan station in Gachsaran city. Then, drought was evaluated with Standardized Precipitation-Evapotranspiration Index (SPEI) during the base (1985-2005) and future periods in different time series. Finally, severity and duration variables of drought were analyzed for the calculation of joint probability and return period with the multivariate copula function. The results show that monthly temperature under RCP8.5 scenario has increased by 2.2°C and precipitation has been reduced by 5.5%. Drought evaluation results show that the number of dry and wet periods is reduced by increasing the length of the scale, but the longest period increases. Also, the number of drought periods relative to the base period have increased under the RCP8.5 scenario. The Frank function was selected as the best fitted function with the mean values of the three fitting indicators, RMSE (0.38), NSE (0.9) and AIC (-101.9). The results of analysis of joint return period of a drought event with a severity of 10 and duration of 10 months for the base period and RCP8.5, respectively, were 5 and 4 years, that indicate a decrease in the return period of severe drought and their increase in future under RCP8.5 scenario relative to the base period.