Spatial clusters and trends of change in drought co-occurrence regions in Iran

ABSTRACT
Precipitation anomalies, especially drought and its changes are among the most important topics in climatology. The current study was carried out to regionalize drought events and evaluate the changes in their extent in annual, seasonal and monthly time scales. For this purpose, EAR5 monthly rainfall data and the RAI drought index were used. Hierarchical cluster analysis based on Ward's correlation-integration distance Mann-Kendall's test and the slope of the regression line were used to extract spatial clusters and their changes. The number of spatial clusters (areas) of co-occurrence of drought in the rainy season and months is wider and more homogeneous than in the less rainy months such as the summer season. Certain geographical-climatic areas such as Southeast, Center, Northwest and West-Southwest are seen most of the time. The annual drought changes in none of the areas show a significant trend, which could be due to intra-seasonal and monthly contrasts in drought changes. So in winter and spring (the main rainfall period of the country), an increasing trend was observed, and on the other hand, in autumn, a decreasing trend of drought was observed. On a monthly scale, the most obvious increase trend has occurred in the two main months of rainfall in the country, namely January and March, and a decrease in November. The results of the research can indicate the temporal shift of rainfall or the change in the country's rainfall regime
Extended abstract
Introduction
Drought is a global challenge with profound economic, social and environmental impacts. The amalgamation of climate change and socio-economic dynamics has exacerbated the frequency and severity of drought occurrences. Delving into the nexus of these factors and scrutinizing the temporal variability of drought as a fundamental priority has fostering advancements in drought management, forecasting and the quest for efficacious solutions.
Iran is a country that constantly confronting the challenges of drought. Numerous research underscores the variegated precipitation oscillations and drought intensities pervading different regions of Iran.
In addition, investigations into the environmental and agriculture repercussions of drought have been conducted, with scholars endeavoring to forecasting future drought trends through forecasting and simulation models. The introduction of drought indicators, modification, and development of methods for drought assessment are of great importance for the management of this phenomenon. Resource optimization and crafting adaptive strategies tailored to the drought patterns across disparate regions can hold promise in surmounting this challenge. Moreover, an incisive assessment of drought risks is imperative for the codification of preemptive policies and the efficacious management of this phenomenon.
However, further research is needed on the temporal and spatial variability of drought. While extant research, has gravitated towards regionalization and impact of drought risks the spatial stability across divergent timeframes remains inadequately explored. Studying spatial clustering elucidating homogeneous drought patterns can significantly improve drought forecasting, warning and management processes. This challenge propelling researchers towards novel inquiries and innovative solutions in the realm of drought dynamics.
 
Methodology
The scope of research is Iran's expansive terrain, from the Zagros and Alborz mountain ranges to the vast internal plains and coastal margins of the Persian Gulf, the Oman Sea in the south and the Caspian Sea in the north. The extensive environmental diversity, resulting from the country's complex topography, engender a significant difference in the spatial and temporal patterns of precipitation among different regions. Different precipitation patterns and spatial changes along with temporal changes lead to phenomena such as drought, heavy rains and floods in Iran
To study the spatial and temporal patterns of precipitation in Iran, ERA5 data have been used as a new and powerful data source. In pursuit of comprehending Iran's spatial and temporal patterns of precipitation, ERA5 data have been used as a new and powerful data source. These data are from the ECMWF database and have high spatial accuracy and different time intervals. Prior research has shown that these databases considered as a robust and dependable source for precipitation-related studies in Iran. By utilizing this database, this investigation explore drought patterns, identifying co-occurring drought clusters analysis using drought indices. Furthermore, cluster analysis is used to examine spatial clusters of drought at annual, seasonal, and monthly scales, and to assess changes in drought extent within each cluster.
 
Results and Discussion
Annual drought clusters in Iran, delineating seven different spatial clusters that are associated with different geographical and climatic features, including topography, geographical latitude, and precipitation system trajectories. These clusters include different regions from the Caspian Sea's southern coast to the central plains and deserts, the northern Zagros and foothills, the southwest from the Persian Gulf coasts to the foothills of the central Zagros, and the southeast and east. These clusters indicate the simultaneous occurrence of annual droughts, with notable regional differences, especially along the Caspian Sea, where variations between the western and northwestern regions, as well as the western and southwestern regions, are evident.
Analyses proffers discernible disparities in the magnitude of drought changes within these annual clusters. While the central region and the southern coasts of the Caspian Sea show a decreasing trend, the central plains and deserts show increasing trends underscoring the disparate climatic and geographical influences. Furthermore, seasonal cluster analysis indicates that different seasonal drought regions are observed in Iran during different seasons of the year. For example, in summer, despite reduced precipitation in the northern and southeastern regions, the number of drought clusters increases, possibly due to occasional and scattered convective rainfall. In contrast, in fall, spring, and even summer, the seasonal drought regions expand along the geographical length, with significant differences in the northwest, which is divided into two regions in fall and expands widely in winter, covering the northern part of Zagros and the western part of the Caspian Sea and Zagros in spring.
In the analysis of seasonal drought changes in Iran, an overall increase in winter drought and a decrease in fall drought are observed. This trend is associated with significant changes in different regions of Iran. Southern Iran, southwestern Iran, and the northern half of the central plateau are experiencing a decrease in fall drought. Conversely, an increase in winter drought is observed in Kerman-east and northeastern regions of Iran. In spring, changes in seasonal drought are observed, but these changes are not significant and have occurred in two regions: the southwest Zagros in southern and central Iran. Analyses propose that these changes may be due to changes in atmospheric circulation and increased atmospheric moisture during this season. Spatial maps illustrating the monthly occurrence of drought clusters show significant differences across various geographical regions throughout the year. This diversity escalates from a minimum of six regions during consecutive cold months to nine regions during two consecutive warm months. Geographical features, such as the Zagros and Alborz mountain ranges, are implicated in shaping monthly drought clusters, particularly during months such as October, December, April, and May.
While analyzing the changes in extent of drought across Iran's different regions, a noticeable pattern observed throughout the year. In January, drought extent notably increases in the northeast and east of Iran and along the Caspian Sea coasts. While the increase in drought extent is not significant in the central plains and the southern coasts up to the eastern border. In November, this pattern reverses, with a more pronounced reduction in extent of drought observed in the central plains and southern coasts extending to the eastern border. In March and April, an increase in drought extent evident in the central, eastern, northeastern, and southwestern regions, while a significant decrease is observed in southeastern Iran. The observed pattern indicates that monthly changes in drought in Iran are related to increased precipitation variability and changes in precipitation patterns throughout the year.
 
Conclusion
This study pursued two primary objectives. Firstly, it employed spatial clustering at monthly, seasonal, and annual scales to delineate regions experiencing concurrent drought conditions. The findings revealed a spatial and temporal alignment of drought patterns in Iran with its geographical and climatic attributes The emergence of spatial clusters with different drought behaviors at varying times suggests that the influence of changes in atmospheric circulation and precipitation systems outweighs geographical constancy. Notably, while discrepancies in the patterns of monthly and seasonal drought variations do not significantly reflect the annual changes, this feature becomes a distinctive characteristic of the drought variation pattern in Iran. Such differences may be due to changes in the atmospheric circulation pattern and the activity of precipitation systems. Ultimately, the transition in precipitation regimes, characterized by a decrease in the average number of rainy days and an increase in precipitation intensity, underscores to future climate challenges in Iran. These challenges could profoundly impact the region's water resources and economy.
 
Funding
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.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.