Spatiotemporal Patterns and Scale-dependent Correlations of Drought Indices in Southeastern Iran with Emphasis on Recent Temperature and Precipitation Conditions

Document Type : Research Paper

Authors

• Haniye Mohammadi ¹
• Javad Bazrafshan ¹
• Arezoo Nazi Ghameshlou ¹
• Shahin Rafiee ²

¹ Department of Irrigation and Reclamation Engineering, College of Agricultural and Natural Resources, University of Tehran, Karaj, Iran.

² Department of Biosystems Mechanical Engineering, Faculty of Agriculture and Natural Resources, University of Tehran, Karaj, Iran.

Abstract

Keywords

• Mann–Kendall test
• Meteorological drought
• SPEI
• SPI
• Sistan and Baluchestan

Main Subjects

• ...

Introduction

Drought is one of the most destructive natural hazards, causing far‑reaching environmental, economic, and social consequences. It disrupts regional water balances through reduced precipitation, increased temperatures, and intensified evapotranspiration, leading to secondary crises such as food insecurity, groundwater depletion, and ecosystem degradation. In Iran, Sistan and Baluchestan province — characterised by an arid to hyper‑arid climate, extreme precipitation variability, and a local economy heavily dependent on water‑sensitive sectors (agriculture, livestock, and wetlands) — exhibits high vulnerability to drought. Previous studies in this region have predominantly relied on univariate indices such as the Standardised Precipitation Index (SPI) and have generally covered statistical periods ending around 2018 or 2020. Consequently, the influence of the unprecedented warming observed during the last decade (2014–2024) on drought intensification remains underexplored. Moreover, a comparative assessment of SPI and the Standardised Precipitation Evapotranspiration Index (SPEI) across multiple time scales (1, 3, 6, and 12 months) — and under the specific thermodynamic conditions of southeastern Iran — is still lacking. Against this background, the present study has four main objectives: (1) to characterise meteorological drought in Sistan and Baluchestan over the period 1990–2024 using SPI and SPEI; (2) to detect temporal trends and abrupt change points in drought severity; (3) to compare the behaviour of the two indices across different accumulation scales; and (4) to evaluate the degree of convergence/divergence between SPI and SPEI as a function of climate type (coastal vs. interior arid).

Method

This applied research follows a quantitative‑analytical design. The study area is Sistan and Baluchestan province (≈180,726 km²), located in southeastern Iran, bordering Afghanistan and Pakistan. Daily meteorological data — precipitation, minimum and maximum temperature, relative humidity, wind speed, and sunshine hours — were obtained from six synoptic stations (Zabol, Zahedan, Iranshahr, Khash, Saravan, and Chabahar) for the period 1990–2024 from the Iran Meteorological Organization. Missing values were reconstructed using linear interpolation and correlation‑based infilling. Homogeneity of the time series was verified using the SNHT (Standard Normal Homogeneity Test) at α = 0.05. The FAO Penman‑Monteith method, which accounts for radiation, humidity, and wind, was used to estimate reference evapotranspiration (ET₀) for SPEI calculation. SPI and SPEI were computed at four-time scales (1, 3, 6, and 12 months) following the widely accepted gamma and log‑logistic distribution fits, respectively.

To ensure robust trend analysis, the pre‑whitening procedure was applied to remove the effect of serial correlation. The non‑parametric Mann–Kendall test was then used to detect monotonic trends, and Sen’s slope estimator quantified the magnitude of change. The Pettitt test (α = 0.05 and 0.01) identified potential abrupt change points in mean annual temperature and total precipitation. Pearson’s correlation coefficient was calculated between SPI and SPEI at each station and time scale to examine behavioural convergence.

Results and Discussion

Mean annual temperature exhibited a significant increasing trend (p < 0.01) at all six stations. The Pettitt test detected a coherent breakpoint around 1998–2001 at Zabol, Zahedan, Khash, Saravan, and Iranshahr, while Chabahar (coastal) showed a later shift in 2007. In contrast, annual precipitation displayed no significant structural trend; only Zabol had a detectable breakpoint (1998). The lack of a precipitation trend coexists with a marked rise in potential evapotranspiration (PET). The highest PET values were recorded at Zabol (summer mean 8.65 mm day⁻¹, with daily extremes exceeding 21 mm day⁻¹), followed by the interior arid stations (Iranshahr, Zahedan, Khash, Saravan ≈5.7–6.7 mm day⁻¹). Chabahar showed the lowest PET and the smallest seasonal range due to maritime moderation.

The SPI‑12 series revealed that long‑term drought episodes dominate the study period; pluvial events, although occasionally intense, were short‑lived and insufficient to offset the cumulative moisture deficit. SPEI‑12 indicated consistently more severe and prolonged droughts than SPI, especially after 2000, highlighting the exacerbating role of increased evaporative demand. The Mann–Kendall trend analysis on the 12‑month scale showed a significant decreasing trend for both SPI and SPEI only at Zabol (Z = –2.165 for SPI, and –2.149 for SPEI; p < 0.05). At Zahedan, Khash, Saravan, and Iranshahr, SPEI exhibited negative Sen’s slopes (e.g., –0.0041 year⁻¹ at Zahedan) that were not statistically significant, but the contrast with SPI (which had near‑zero slopes) suggests a detectable thermal‑evaporative signal. No significant trends were found at short scales (1‑ and 3‑month) after pre‑whitening. Interestingly, at Chabahar, a significant positive trend was observed for SPI‑6 (Z = 2.168, p < 0.05), indicating a wetting tendency in mid‑scale precipitation, but this was cancelled in SPEI‑6 (non‑significant negative slope), implying that increased temperature and PET have offset the rainfall gain.

Pearson correlation between SPI and SPEI increased systematically from short to long accumulation scales: from ≈0.21–0.54 at 1–3 months to ≈0.66–0.73 at 12 months. The highest 12‑month correlation was found at Khash (0.732) and Zahedan (0.729), and the lowest at Chabahar and Zabol at short scales. This scale‑dependent convergence confirms that at longer time scales, precipitation becomes the dominant driver of both indices, whereas at short scales, surface energy fluxes control SPEI more strongly.

Conclusions

Drought in Sistan and Baluchestan province results from the interaction of three key processes: (i) structural temperature increase and thermal regime shift (with a breakpoint in the late 1990s), (ii) sustained growth of potential evapotranspiration (especially in Zabol and interior areas), and (iii) reduced precipitation efficiency in restoring long-term moisture. These factors have led to an energy-amplified drought. SPEI provides a more realistic picture of drought severity because it accounts for temperature. Significant decreasing trends were observed only at Zabol (at long scales), indicating a structural aridity effect in the northern part of the province. At other stations, although trends are non-significant, the difference between SPI and SPEI reveals the decisive role of temperature. The increasing correlation between the two indices at long scales indicates that precipitation dominates over long periods, but divergence at short scales suggests that surface energy controls drought in its early stages. Overall, water resource management in this province should focus on reducing water demand, curbing evapotranspiration, and enhancing resilience to warming, because precipitation alone cannot compensate for the moisture deficit. It is recommended to establish an early warning and monitoring system based on SPEI (prioritizing Zabol station), revise cropping patterns, and adopt modern irrigation technologies.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Authors’ contributions

1. Mohammadi: Student, Data curation, Software, Formal analysis, Investigation (including statistical analysis, interpretation of data and results), Writing – original draft, Writing-Reviewing and Editing.
2. Bazrafshan: Supervision, Conceptualization, Methodology, Validation, Writing-Reviewing and Editing, Project administration, Finalization of the manuscript.
3. Nazi Ghameshlou: Supervision, Conceptualization, Methodology, Validation, Writing-Reviewing and Editing, Project administration, Finalization of the manuscript.
4. Rafiee: Thesis Advisor, Participation in research design, Supervision of research, Study and revision of the manuscript.

All authors have read and agreed to the published version of the manuscript. All authors contributed equally to the conceptualization of the article and the writing of the original and subsequent drafts.

Declaration of Generative AI and AI-assisted technologies in the writing process

The authors did not use any artificial intelligence tools in preparing this manuscript.

Data Availability Statement

Data available on request from the authors.

Acknowledgements

This research was supported by the University of Tehran. The authors express their special thanks to the Vice Chancellor for Research Affairs of the University of Tehran. They are also sincerely grateful to the Iran Meteorological Organization for providing the meteorological data required for this study.

Ethical considerations

The authors avoided data fabrication, falsification, plagiarism, and misconduct.

Conflict of interest

All authors declare that they have no conflict of interest.