A multi-indicator, data-driven framework for spatiotemporal analysis of compound droughts in the Southern Caspian Basin.

A temporal-lag-informed, multi-indicator framework was developed to assess the integrated drought dynamics in the Southern Caspian Basin (SCB) from 1985 to 2024. To construct a consistent Composite Drought Index (CDI), the framework integrates meteorological (SPEI, derived from gridded precipitation and temperature data), hydrological (SRI, based on runoff variability), and vegetation (NDVI, derived from satellite observations) indicators using principal component analysis (PCA) applied to objectively weighted standardized anomalies. This study represents the first basin-scale application of a lag-sensitive, multi-domain PCA-based framework in the SCB, thereby enabling a systematic assessment of drought propagation and delayed interactions across climatic, hydrological, and ecological systems, supported by the correlation analysis (r2 > 0.8). The results emphasize a notable temporal variability within the different regions, where Atrak and Qarasu-Gorgan sub-basins had the most severe and prolonged drought conditions. The spatial variability of the ecosystem sensitivity and temporally delayed vegetation responses is captured by lagged NDVI signals, which contribute 2.5-26% to the variance of the first principal component. The CDI dataset from 1985 to 2024 quantifies drought-related lag effects, identifies dominant propagation pathways, and highlights persistent sub-basin-scale disparities. Such knowledge provides a robust foundation for regional drought assessment, supports adaptive and basin-specific water management strategies, and strengthens the development of early-warning systems under heterogeneous hydroclimatic conditions.