Analysis of drought characteristics (severity, duration, magnitude) in Iran based on multivariate standardized drought index

Introduction
The weather has an ongoing impact on human living and working environments. Drought is a natural disaster that ranks first in frequency of occurrence, financial losses, and even human casualties among natural disasters that endanger humans and their environment. Due to its complexity and imperceptibility, this phenomenon—one of the primary and recurring features of various climates—has significantly impacted the human environment more than any other hazard. Its effects can also accumulate gradually over time and last for years afterward.

Drought cannot be avoided, but if its nature and characteristics are researched and understood, we may be able to forecast when it will occur, lessen its adverse effects through planning and preparation, and perhaps even control it.

Methodology
The political region of Iran is the subject of this study (Figure 1). Soil moisture and monthly precipitation are among the data used. The necessary data, which included 516 precipitation files and 516 soil moisture files with a spatial resolution of 0.5*0.66 in NC format, as well as drought and its characteristics calculated for all points, were acquired from the MERRA website for 43 years (1980–2022) to conduct this research.

Iran's droughts were estimated over 3, 6, 9, 12, and 24 months using the MSDI. The characteristics of droughts, such as frequency, duration, severity, and magnitude, were computed, analyzed, and presented as a map in addition to examining the actual droughts.

Results and discussion
Analyzing the drought characteristics for 1546 points across Iran revealed that the frequency and number of drought periods decrease as the time scale increases. In contrast, the values of other traits—such as continuity, intensity, and magnitude—also rise as the time scale increases. The increase in drought characteristics with increasing time scale, in terms of magnitude, duration, and severity, has been highlighted in studies by Nouri and Homai (2020) and Torabinejad et al. (2023). Geographically, the southeast and eastern parts of Iran have seen the highest frequency of droughts throughout the 43 years; in the southeastern part of the country, which is centered on the provinces of Sistan and Baluchestan, Kerman, and Hormozgan, there have been 28 to 34 periods of drought in 3 months. Conversely, the least amount of drought occurred in the northern coasts during the short-term periods of three and six months, with six to twelve periods, and in the coasts of the north and the southwestern region of the country during the long-term periods of twelve and twenty-four months, with two to six periods.

The provinces of Sistan Baluchistan, Kerman, and Hormozgan in the southeast, as well as the far east of Khorasan Razavi and South provinces in the east of the country, experienced the worst droughts in terms of severity. Severe droughts lasting 12 or 24 months are concentrated in the southwest, in Bakhtiari and Chaharmahal, Kohgiluyeh and Boyer Ahmad, and part of Khuzestan.

The findings from the computation and analysis of the duration and magnitude of droughts in Iran over the period under discussion indicated that, for two features, the country's northern regions, centered on the north coasts, mainly Mazandaran province, had more prolonged and severe droughts than other regions within the time scales of three and six months. By looking at the country's southwest from the perspective of location over periods of 12 and 24 months, the provinces of Fars, Bushehr, south of Kohgiluyeh and Boyer Ahmad, and Khuzestan province make up the central core of the most prolonged and most extensive droughts.

 Conclusions
Based on soil moisture and precipitation data, the results of a drought calculation in Iran demonstrated that droughts occur throughout the country with varying degrees of severity and weakness. Their occurrence has become a permanent feature of the country's climate, particularly in recent decades. The analysis of Iran's drought chronology revealed that, according to the MSDI index's range, most of the country's droughts are weak and moderate. It was also discovered that as time scales increase, the frequency of droughts decreases while the continuity, magnitude, and severity increase. Geographically, the country's southeast and east saw the highest frequency of droughts, while its northern coast—mainly the province of Mazandaran—and southwest—where the provinces of Chaharmahal & Bakhtiari and Kohgiluyeh & Boyer Ahmad—saw the lowest frequency. The maximum severity of drought in the country is -1.93 on the 3-month scale in the southeast and -2.2 on the 24-month scale in the southwest. Two of the study's notable findings are the continuity and high magnitude of the droughts that struck the country's north and southwest. Thus, the most extensive droughts, with values between -16 and -31 on short-term scales and between -35 and -68 on 12- and 24-month scales, have happened in the country's north and southwest, respectively. These results suggest an overall decrease in soil moisture and precipitation in these regions.