Investigation drought conditions and soil moisture of Southeastern Anatolia Project (GAP) region (Türkiye)

This study investigates drought and soil moisture dynamics between 1963 and 2022 in seven provinces of Southeastern Anatolia (Adıyaman, Diyarbakır, Gaziantep, Kilis, Mardin, Siirt, Şanlıurfa), using four drought indexes (SPI, SPEI, CZI, RDI) at 1-, 3-, and 12-month time scales; allows for a comprehensive assessment, which is not commonly done before in studied provinces. ERA5-Land soil moisture data (1950-2022) were also analyzed to provide additional insight. The results reveal Diyarbakır and Şanlıurfa as the most drought-prone provinces, exhibiting the lowest median index values and highest frequency of extreme droughts. SPEI-12 (THW) median values ranged from -0.0714 (Adıyaman) to 0.188 (Şanlıurfa), suggesting a near-neutral long-term balance with strong seasonal variability. Soil moisture levels declined sharply in summer months, reaching as low as 2-3 mm, particularly after 2009, indicating increased evapotranspiration stress. RDI-12 showed strong correlations (0.87-0.97) with other indexes, proving its reliability in capturing long-term drought patterns. Spatial analysis showed that while mild drought frequency reached up to 39% in the southwest, severe and extreme droughts were more frequent (up to 8%) in the east. Conversely, wet events were more dominant in eastern areas such as Siirt, with frequencies exceeding 53%. These findings emphasize the necessity of multi-index drought monitoring and the importance of soil moisture data in supporting water management, agricultural planning, and climate adaptation in semi-arid regions.