Comparative Assessment of Palmer Drought Indices for Hydrological and Agricultural Drought in the Mediterranean Ceyhan Basin, Türkiye

Hydrological drought is an increasing challenge for water security, ecosystem resilience, and water resource management in Mediterranean river basins, emphasizing the need for reliable indicators capable of capturing regional hydroclimatic variability. This study evaluates the effectiveness of the Palmer Hydrological Drought Index (PHDI) and its self-calibrating version (scPHDI) for hydrological drought assessment in the Ceyhan Basin, Türkiye. A homogenized long-term dataset of meteorological and streamflow records (1970–2024) was developed, with missing climatic data reconstructed using PRISM-based spatial interpolation, and complemented by groundwater records (2002–2015). Monthly Palmer indices were calculated using a soil water balance approach at individual stations and aggregated to the basin scale through weighted averaging. The indices were compared with SPI12 and SPEI12, which share the same meteorological forcing, and with the streamflow-based SRI12, alongside spatial and temporal analyses of drought occurrence. The results reveal a clear intensification of drought conditions after the early 2000s, with increasing frequencies of moderate and severe drought events across the basin. The self-calibrating Palmer hydrological index showed improved spatial consistency by adapting index parameters to local climatic characteristics. Agreement between scPHDI and the streamflow-based SRI12 supported the hydrological relevance of the detected drought signal, whereas agreement with SPI12 and SPEI12 was interpreted as a benchmark rather than as independent validation. The findings indicate that scPHDI is a promising indicator for retrospective hydrological drought assessment and may contribute to drought monitoring systems in Mediterranean basins, although its forecasting skill and warning lead time remain to be demonstrated.