DOI: 10.62520/fujece.1987849 ISSN: 2822-2881

SPI-Based Drought and Trend Analysis with Improved Seasonal Decomposition Approaches

Ramazan Acar, Mustafa Turan
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Drought is one of the most significant hydro-climatic hazards impacting water resources, agricultural production and the sustainability of ecosystems, particularly in regions with high climatic variability. In this context, reliable approaches are needed to better evaluate short- and long-term drought conditions in the Upper Euphrates Basin (UEB). In this study, the performances of the Standardized Precipitation Index (SPI), the Seasonal-Trend Decomposition (STL), and the Seasonality-Adjusted Time Series (SATS) Z-score approaches were comparatively evaluated. These indices were systematically evaluated for five meteorological stations over time scales of 1, 3, 6, and 12 months. The analysis indicated that the STL and SATS Z-score approaches provide seasonality-adjusted perspectives for identifying drought variability at shorter time scales. In contrast, due to its reliance on raw precipitation data, the Standardized Precipitation Index (SPI) was observed to capture such extreme events more sharply within similar short-term periods. A high level of concordance was observed between the proposed methods and the SPI, particularly over a 12-month timescale. The coefficient of determination (R²) values between STL Z-score and SPI were found to be between 0.91 and 0.96 for the 12-month timescale at all stations. For SATS Z-score and SPI, this value was found to be between 0.98 and 0.99. MK and Şen-ITA trend analysis methods were used to analyze the results of the three approaches for all stations. The obtained trend results generally support the consistency observed between the methods. Finally, it is suggested that using the STL, SATS Z-score, and SPI approaches together will offer a comparative perspective for drought assessment in the Upper Euphrates Basin.

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