Document Type : Original Article
1. Introduction
Drought is a climatic phenomenon with high fluctuations and widespread impacts on economic dimensions and social development. This phenomenon, unlike other natural disasters, with a gradual growth, affects the entire hydrological cycle; in such a way that its effects are evident even after the end of the drought period. Droughts are mainly classified into meteorological, agricultural, hydrological, and socio-economic subgroups based on their drivers and impacts. According to the IPCC report, drought refers to a prolonged period of reduced precipitation that can lead to water scarcity and an imbalance in the hydrological cycle. Its impacts have been intensified by global warming, such that since 1970, its severity, extent, and critical range have been increasing worldwide, having a greater economic impact than other disasters. The increasing concentration of greenhouse gases has accelerated the global warming trend. The role of solar activities, as a factor in atmospheric dynamics, is also important in climate studies. Given the importance of drought, its timely detection and accurate assessment are vital for crisis management. Therefore, the present study aimed to calculate the Standardized Precipitation-Evapotranspiration Index (SPEI) at 31 synoptic stations in Iran, identify its trends and change points, and evaluate the relationship of this index with global Atmospheric carbon dioxide concentration (GCO2) (an anthropogenic factor) and monthly sunspot frequency (MSSF) (a natural factor).
2. Materials and Methods
The required data included daily mean temperature and precipitation for these 31 stations from the Iran Meteorological Organization (IRIMO) for 1961-2023, monthly GCO2 data from NOAA, and monthly MSSF data from SIDC Belgium were collected and analyzed. The SPEI-12 index, which is based on the Standardized Precipitation Index (SPI) and also uses potential evapotranspiration (PET) (calculated by the Thornthwaite method), was computed. To examine trends and identify change points, non-parametric Mann-Kendall and Sen's slope tests were used, and for linear correlation, Pearson correlation was applied. Cross-wavelet transform (XWT), using the Morlet wavelet, were employed to reveal common power and relative phase between time series in the time-frequency domain.
3. Results and Discussion
The results of the Mann-Kendall and Sen's slope tests showed that, except for Shahrekord station (negligible upward trend), other stations experienced a significant decreasing trend in the SPEI-12 index. The most substantial decrease was observed in Yazd, Bam, Kerman, Tabriz, and Zahedan. Mann-Kendall mutation test analysis also indicated significant changes at 9 stations, including an upward shift in Shahrekord and significant downward shifts in Ramsar, Kermanshah, Shiraz, Bushehr, Bandarabbas, Birjand, Babolsar, and Khorramabad. These findings emphasize the necessity of water resource management and drought monitoring in Iran's arid and semi-arid climate. Pearson correlation between SPEI-12 and GCO2 showed an inverse correlation at most stations, especially in Bam, Yazd, Kerman, Mashhad, Khoy, Tabriz, Zahedan, and Shahrud, which is consistent with drought intensification due to global warming; however, a strong correlation between MSSF and SPEI-12 was not observed. Cross-wavelet analysis between SPEI-12 and GCO2 showed negligible signal strength, but out-of-phase cycles were observed in the 32 to 128-month periods (approximately 2.67 to 10.67 years), indicating the complexity of relationships and the delayed or non-linear effect of GCO2 on drought dynamics. In contrast, cross-wavelet analysis between SPEI-12 and MSSF revealed a stronger relationship. Out-of-phase signals in the 64 to 128-month cycles (5.33 to 10.67 years), particularly in the 1977-2010 period, were evident. These out-of-phase cycles predominated in the 128-month cycle. In-phase signals in the 32-month cycle (approximately 2.67 years) were also observed at some stations. These results are consistent with studies suggesting the role of solar activities in decadal climate changes and show that wavelet analysis can reveal hidden frequency and phase correlations.
4. Conclusion
The present study showed that drought in Iran is intensifying, and most stations (except Shahrekord) have a decreasing and significant trend in SPEI-12. This trend is more prominent in Yazd, Bam, Kerman, Tabriz, and Zahedan. The presence of downward change points also indicates sudden and long-term changes in regional drought conditions. The findings confirm the probable impact of global warming through increased GCO2 on drought intensification in Iran, which was identified by an inverse linear correlation. Although the signal strength of this relationship in cross-wavelet analysis was weak, the existence of out-of-phase cycles emphasizes the complexity of the relationship. Cross-wavelet analysis revealed a stronger relationship and distinct out-of-phase cycles between MSSF and SPEI-12. This indicates that solar activities, although they may not have a strong linear effect, influence drought dynamics at specific frequency scales. Overall, this research emphasizes the importance of comprehensive and multi-factorial approaches in drought studies and shows that in addition to climate factors related to global warming, the role of natural phenomena such as solar activities, albeit with more complex mechanisms, should also be considered. These results can be useful for long-term water resource management and adaptation to climate change in Iran.
5. Acknowledgments
The authors sincerely thank the Iran Meteorological Organization (IRIMO) for providing essential meteorological data for this research.
6. Conflict of Interest
The authors declare no conflict of interest.
