Projection of future frequency of severe and extreme droughts over Iran country

Abstract

Drought is one of the most destructive environmental hazards posing negative economic and social consequences. The country of Iran, which is located in the dry and semi-arid belt, is involved in much damage caused by drought every year, which makes it necessary to investigate. In this study, an attempt was made to investigate the frequency (number of occurrences) of severe and extreme droughts in the future. We considered of monthly averaged precipitation of 10 climate models of the fifth report of the Intergovernmental Panel on Climate Change (IPCC) for the basic period (1976–2005) and future periods (2020–2049, 2070–2099) under two scenarios (RCP4.5, 8.5). Using a new method, the difference between the average monthly precipitation of the models in the base period with the observed data and the inverse of the difference of each model was divided by the sum of the inverse of all models in each month (WP). Next, the average monthly precipitation of each model in the future period and the corresponding scenario was divided into the base period of that model (PCF), and at the end, for each month, the amount of WP was multiplied by the PCF of each model and their sum was obtained (\(\Delta P\)). The value \(\Delta P\), which is a 12-month time series, is introduced to the lars-wg model as a scenario file, and this model builds precipitation data based on this file. In the following, using the 12-month SPI index, according to the SPI index classification (values between − 1.5 and − 2 as severe drought and greater than − 2 as extreme drought), the total number of events in which the 12-month SPI (during the examined period in each station) being placed in the severe and extreme category was calculated. The estimation of error indices, especially R_Sqr (0.95), on average shows the accuracy of the combined weighted method and the Lars-Wg model in simulating precipitation. Also, the result presented in box plots shows an increase in the frequency of severe and extreme droughts in most of the country’s stations. Except group 3 (Southwestern and Western regions of the country), where the frequency of severe drought has decreased, in other groups, especially group 4 (60% on average), there is an obvious increase. The frequency of extreme drought in areas with good rainfall in the western and northern half of the country (especially groups 3 and 4) has declined (86% on average), while extreme events has decreased in group 1 with low rainfall. Considering that these areas are the main agricultural poles in the country, increasing the frequency of extreme drought can create harmful economic, social, and environmental consequences.