Analysis of the Trend and Spatial Variation of Aridity in the Future Climate of Iran

Document Type : Full length article


1 Department of Geography, Faculty of Literature and Human Sciences, Razi University, Kermanshah, Iran

2 School of Civil Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Malaysia

3 Department of Statistics, Faculty of Science, Razi University, Kermanshah, Iran



This research aims to investigate the spatial variability and temporal trends of Iran's aridity in the future (2020–2050) based on the SSP2-4.5 and SSP5-8.5 scenarios of CMIP6 models (MRI-ESM2 and GFDL-ESM4) compared to observational data (1992–2014) using AI and IDM indices based on precipitation, evapotranspiration, and average temperature variables. The coefficient of variation and innovative trend analysis were exerted to examine the changes and trends of the average annual aridity, respectively. The results showed that during the observation period, except for the northern areas of Alborz and a part of the northwest, other areas of the country were arid and semi-arid. However, in the future, the scenarios show a decrease in humidity in the northern areas of Alborz, the northern areas of the inner plateau of Iran, and parts of the southern areas of the Zagros mountains. Aridity decreases in the northwest and parts of the central and northern regions of Zagros. The models predicted arid and semi-arid conditions in other areas, as in the past. The highest percentage of annual average spatial variation of land (71%–105%) was observed in the southeast and south coasts of the country in the period 1992–2014, and according to the models, the percentage of spatial variation of land will decrease in the future. The trend of the average annual aridity values of Iran showed that the drought has increased at significance levels of 0.05 and 0.01 in the past and will increase in the future at significance levels of 0.05 and 0.01 based on MRI-ESM2 (SSP5-8.5 scenario and IDM index). At these significant levels, aridity shows a decreasing trend in other conditions. The results of this research can be useful in planning and reducing the negative effects of climate change in Iran.
Extended Abstract
High temperatures and low precipitation characterize arid and semi-arid regions. Aridity is a permanent feature of a region's long-term hydrological and climatic conditions. Aridity is a function of precipitation, evapotranspiration, and temperature. Due to the diversity of climate, many numerical indicators have been proposed for different types of climate in different regions of the world. Indices based on the changes in precipitation and temperature, or indices calculated based on precipitation, evapotranspiration, and studying aridity or variables affecting it, are useful in environmental planning.
Iran has an area of about 1,698,195 square kilometers, between 25 and 40 degrees north latitude and 44 and 64 degrees east longitude. An arid and semi-arid climate covers about 90% of the country. The most important reason for the arid climate of Iran should be related to its geographical location because it is close to the tropical region (receives more solar radiation) and is under the influence of sub-tropical high pressure. On the other hand, the Zagros and Alborz mountain ranges prevent moisture from entering the interior, arid regions. Another factor affecting the aridity of Iran is that a large part of its territory is far from seas and oceans.
In this study, SSP2-4.5 and SSP5-8.5 scenarios data from CMIP6 models (MRI-ESM2 and GFDL-ESM4) and observed data (1992–2014) in cloud of precipitation, average, maximum, and minimum temperature, wind speed at 2-meter height, sunshine duration, and several radiation variables (from CMIP6) were prepared. The data were converted into data with a 50 x 50 km resolution in the R software with the Resample command, and calculations were made using two aridity indices, AI and IDM. Due to the lack of sunshine hours data in CMIP6 models, this variable was calculated using the input (short and long wave) and output (short and long wave) data of the models and the radiation estimation method of Li et al. After calculating the indices, zoning maps were produced in ArcGIS 10.5.
Results and Discussion
The results showed that during the observation period, except for the northern areas of Alborz and small parts of northwestern Iran, which according to AI included semi-humid to humid areas and according to IDM included Mediterranean to very humid areas, other regions of the country were placed in dry and semi-arid classes. An increase in aridity in 6.4% and 5.4% of the country's area based on the AI index and under the conditions of SSP2-4.5 and SSP5-8.5 scenarios and 4.2% and 3.4% based on the IDM index, according to the mentioned scenarios of The MRI-ESM2 model, will occur in the northern areas of Alborz (especially the southern coasts of the Caspian Sea) and the northern areas of the inner plateau of Iran in the future. According to the GFDL-ESM4 model, the increase in aridity in 4.4% and 7.1% of the country's regions based on AI and 3.3% and 6.3% of the country's regions under the SSP2-4.5 and SSP58.5 scenarios based on the IDM index and according to the mentioned scenarios, as predicted by the MRI-ESM2 model, it will happen in the Caspian coasts and the northern areas of Alborz, and in addition, in the southern areas of Zagros, in 2020–2050. 17.2% and 22.4% of the country's area under the conditions of SSP2-4.5 and SSP5-8.5 of the MRI-ESM2 model in the central Zagros areas, parts of the northeast and northwest based on AI, and also 17.2% and 20.4% The entire area of the country located in the central Zagros regions to the northwest of Iran will experience a decrease in aridity, according to the IDM index and according to the mentioned scenarios.
The highest amount of changes in the observation period and the future, based on the indicators (and scenarios of both models), related to the southeastern regions and the southern coasts of the country, especially the coasts of the Oman Sea, by 71%–105% and related to the observational data (1992–2014), which is most likely related to changes in humidity in this area following the beginning and end of monsoon activities in this area. According to the models, the number of aridity changes in the future (2020–2050) will decrease. The most decrease in aridity changes in the future is related to the coasts of the Oman Sea.
The trend of average aridity values in Iran showed that aridity increased at significance levels of 0.05 and 0.01 in the past (2014–1992), and in 2020–2050 only under SSP5-8.5 of the MRI-ESM2 model and the IDM index at significance levels of 0.05 and 0.01 will increase. In other conditions, aridity will decrease at significance levels of 0.05 and 0.01. Both models showed a decrease in humidity in the northern regions of Alborz and a decrease in aridity in the country's northwestern regions. The models had the same performance in depicting the country's future climate based on the average annual aridity for the period 2020–2050.
The results showed that during the observed period, except for the northern areas of Alborz and small parts of the northwest of Iran, other regions were classified as arid and semi-arid. However, in the future, the scenarios of the models show a decrease in humidity in the northern areas of Alborz, the northern areas of the inner plateau of Iran, and parts of the southern areas of the Zagros mountain range. Dryness decreases in the northwest and parts of the central and northern regions of Zagros. The models did not show change in other regions of Iran, and arid and semi-arid conditions will continue in these areas. The highest percentage of annual average spatial variation of aridity (71%–105%) was observed in observational data (1992–2014) in the southeast areas and south coasts of the country, and according to the models, the percentage of spatial variation of aridity will decrease in the future. The trend of average values of aridity in Iran showed that aridity increased at significance levels of 0.05 and 0.01 in the past. During 2020–2050, it will increase based on MRI-ESM2 (SSP5-8.5 scenario and the IDM index) at the significance levels of 0.05 and 0.01. In other conditions, aridity shows a decrease at these significance levels. The research results can be effective for long-term planning to reduce the negative effects of climate change in Iran, especially in its eastern and southern parts.
There is no funding support.
Authors’ Contribution
All of the authors approved the content of the manuscript and agreed on all aspects of the work.
Conflict of Interest
Authors declared no conflict of interest.
We are grateful to all the scientific consultants of this paper.


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