Spatio‑temporal synergies in compound drought propagation

Drought propagation is a non‑linear, multiscale process linking meteorological, soil, and hydrological drought through temporal conditioning and spatial coherence within river basins. Understanding these interactions is essential for drought early warning and regional risk assessment, yet their quantification remains challenging, particularly in regions with strong land–atmosphere feedbacks such as Poland. In this study, we apply two complementary methodological frameworks—Causality Chain Model (CCM) and Network Correlation Analysis (NCA)—to characterize the spatio‑temporal evolution of drought over Poland using monthly SPI, SPEI and SRI datasets for 1980–2020.

The CCM identifies robust cause–effect transitions along the sequence meteorological → soil → hydrological drought, with propagation delays (DPT) ranging from 1 to 12 months, depending on regional hydroclimatic conditions and indicator aggregation scales. Metrics including DPCs (Drought Propagation Counts) and DPCs_proc reveal that while not every meteorological drought propagates further, a substantial proportion does, forming statistically significant synergistic sequences. The DIP (Drought Intensity Propagation) index indicates regions where drought intensity amplifies during propagation (DIP > 1), highlighting the role of soil moisture depletion and catchment storage deficits in reinforcing hydrological drought development across Poland.

The NCA provides a spatially explicit perspective, identifying propagation hubs, coherent clusters, and regions with strong cross‑catchment connectivity. High values of Degree Centrality and Closeness Centrality reveal locations acting as spatial initiators or transmission nodes of drought signals. The CDC (Closeness to Drought Center) metric further delineates centres of synchronized drought evolution, enabling recognition of areas with elevated susceptibility to persistent hydrological stress.

By integrating CCM and NCA, this study offers a comprehensive multiscale characterization of drought propagation over Poland, capturing both temporal causality and spatial coherence. The combined framework provides actionable indicators supporting regional drought risk assessment, hydrological regionalization, and climate adaptation planning, improving the capacity to anticipate how drought conditions evolve under future climate variability.