Investigation of temperature and precipitation changes in the Iran's basins in the next 20 years based on the output of CMIP6 model

Climate change affects the hydrological cycle, available water resources and water demand by changing temperature and precipitation. In this regard, predicting temperature and precipitation changes by the models of the Sixth Climate Change Assessment Report due to more accuracy in their outputs can be helpful to plan and manage water resources in the future. In addition, identifying appropriate models among these climate models will pave the way for future research into the effects of climate change. In order to achieve these objectives, the present study was conducted.

At first Step, 138 and 113 meteorological stations were selected to study precipitation and temperature based on the appropriate location distribution in Iran, respectively. Then, monthly precipitation and temperature data of the historical period (1995-2014) were zoned in 30 basins of the country by Thiessen polygon method. In the next step, the results of simulation of precipitation and temperature of 10 CMIP6 models were extracted from website. Then, in order to equalize the time period of the base period of climate models with the observation period, the simulation results of models were used on a monthly basis.For each model, the position of network cells covering 30 basins of the country was determined and according to the percentage of the area of each cell in that basin, the average temperature and precipitation of GCM models were calculated at the level of each basin.Moreover, to increase the accuracy of the results of climatic models, the output of 10 climatic models in the historical period (2020-2039) were compared to observational data and were ranked based on KGE criteria. Then, the models were weighted based on the obtained rankings. In the next step, the forecasting results of climate models under three scenarios SSP126, SSP245 and SSP585 in the next period were obtained, after combining the output of the models based on their weight. Finally, the average changes in temperature and precipitation for the period of 2020-2039 compared to the historical period, were presented for all the basins.

The results of climate models ranking based on their ability in climate forecasting showed that the high score models are different depending on the variety climate in regions and their diversity in precipitation simulation is greater than temperature. So that, among 10 CMIP6 models, 6 high score models for simulation of precipitation and 4 high score models for temperature simulation in different parts of the country were determined. Furthermore, the results of climate forecasts indicate a change in precipitation from about +19 to -12% and an increase in temperature on average in the range of 0.1 to 0.6 degrees Celsius in different basins of the country over the next 20 years. Based on the average temperature changes of 20 next year, it can be seen that the northern regions of the country will experience more temperature increases than the southern regions of the country in the coming period. Increasing the temperature in the next period, while increasing evapotranspiration, will also increase water demand and consumption in different sectors. Therefore, it is necessary to develop policies to adapt to climate change in the agricultural, drinking and industrial sectors. Moreover, the western basins of the country will experience the greatest decrease in rainfall during this period. Due to the strategic location of some of these basins, both in terms of proximity to neighboring countries and in terms of dependence on agricultural activities to create food security and the lack of water resources, reduced rainfall can lead to several problems. Therefore, it is necessary to take measures to reduce water consumption, including changing consumption patterns and planning to compensate for economic losses and establish a balance between water supply and demand.