Çelik et al. (2025) Spatio-Temporal Analysis of Observed Drought Events in the Tigris–Euphrates Basin during the 1960–2023 Period Via SPI and SPEI Drought Indices

Identification

• Journal: Pure and Applied Geophysics
• Year: 2025
• Date: 2025-11-20
• Authors: Mehmet Ali Çelik, Adile Bilik, Murat Türkeş
• DOI: 10.1007/s00024-025-03863-3

Research Groups

• Geography Department, Faculty of Arts and Sciences, Igdir University, Igdir, Türkiye
• Center for Climate Change and Policy Studies, Boğaziçi University, Istanbul, Türkiye

Short Summary

This study analyzed spatio-temporal drought patterns in the Tigris-Euphrates Basin from 1960 to 2023 using SPI and SPEI, finding a significant increase in drought frequency and severity, especially after 1990, driven by rising temperatures and evapotranspiration.

Objective

• To analyze the spatio-temporal trends of drought events in the Tigris-Euphrates Basin over a 63-year period (1960–2023), hypothesizing an increase in drought frequency, severity, and duration since 1990 due to rising temperatures and evapotranspiration.

Study Configuration

• Spatial Scale: Tigris-Euphrates Basin (approximately 879,000 square kilometers), covering parts of Türkiye, Syria, Iraq, and Iran. Analysis conducted at specific grid points (~4 kilometers resolution) corresponding to selected districts.
• Temporal Scale: 1960–2023 (63-year period), with comparative analysis between 1960–1990 and 1991–2023.

Methodology and Data

• Models used:
  • Drought Indices: Standardized Precipitation Index (SPI) and Standardized Precipitation-Evapotranspiration Index (SPEI), calculated at 3-month (meteorological drought) and 12-month (agricultural, hydrological, ecological drought) timeframes. Potential Evapotranspiration (PET) estimated using the Thornthwaite method.
  • Statistical Methods: Mann–Kendall trend test, Least-Squares Linear Regression (LSLR), Pettitt test (for change point detection), One-way Analysis of Variance (ANOVA), K-means clustering, Rotated Principal Component Analysis (RPCA), Shapiro–Wilk test, Levene’s test, independent-samples t-test, Kruskal–Wallis test, Wilcoxon rank-sum test, Pearson’s r, Spearman’s ρ, and Kendall’s τ correlation.
  • Spatial Interpolation: Inverse Distance Weighted (IDW) interpolation.
• Data sources: TerraClimate dataset (monthly climate and water balance data, ~4 kilometers spatial resolution, 1958-present). Variables included precipitation (pr), maximum and minimum air temperatures (tmmx, tmmn), potential evapotranspiration (pet), actual evapotranspiration (aet), soil moisture (soil), and vapor pressure (vap). Data processed using Google Earth Engine (GEE).

Main Results

• Drought events in the Tigris-Euphrates Basin have significantly increased in frequency, intensity, and duration, particularly after 1990.
• SPEI consistently showed more severe drought conditions and a higher frequency of drought years compared to SPI, highlighting the critical role of increased evapotranspiration due to rising air temperatures.
• A statistically significant change point towards drier conditions was detected around 1990 (Pettitt test, p < 0.05).
• Seasonal analysis revealed spring as the most drought-prone season by both indices, with SPEI highlighting autumn as particularly severe due to evapotranspiration. Winter generally experienced the fewest drought events.
• Spatial analysis identified a distinct southward shift in drought intensity, with arid zones expanding significantly in southern districts (e.g., Şanlıurfa, Mardin, Nusaybin), while northern highland regions (e.g., Tunceli, Erzincan) remained comparatively stable.
• Statistical tests (Mann-Kendall, ANOVA, t-test) confirmed significant and accelerating drying trends, with SPEI showing a much more dramatic increase in drought severity and trend compared to SPI (e.g., SPEI Z-statistic p < 0.00001 for 1991–2023 vs. 1960–1990).
• Seasonal drought variability has changed significantly, with summer droughts becoming more stable, predictable, and persistent (decreased variance), while winter droughts show increased unpredictability in some southern locations (increased variance).

Contributions

• Provides empirical evidence at the basin scale for increasing drought frequency and severity, emphasizing that drought is driven not solely by reduced precipitation but significantly by rising temperatures and evapotranspiration.
• Offers a more continuous and detailed spatial representation of drought heterogeneity within the Tigris-Euphrates Basin than many earlier regional-scale studies.
• Highlights the added value of SPEI over SPI in semi-arid regions by capturing temperature-driven moisture deficits, which is crucial for understanding climate change impacts.
• Underscores the need for integrated drought monitoring (SPI and SPEI), enhanced transboundary water governance, and climate-responsive policies (efficient irrigation, adaptive water management) for the region.
• Shifts the focus in drought risk-reduction policy from "precipitation decline only" to a "temperature + evapotranspiration" axis.

Funding

This research received no specific grant from any funding agency.

Citation

@article{Çelik2025SpatioTemporal,
  author = {Çelik, Mehmet Ali and Bilik, Adile and Türkeş, Murat},
  title = {Spatio-Temporal Analysis of Observed Drought Events in the Tigris–Euphrates Basin during the 1960–2023 Period Via SPI and SPEI Drought Indices},
  journal = {Pure and Applied Geophysics},
  year = {2025},
  doi = {10.1007/s00024-025-03863-3},
  url = {https://doi.org/10.1007/s00024-025-03863-3}
}