Cascading droughts: Exploring global propagation of meteorological to hydrological droughts (1971-2001).

An understanding of the spatiotemporal behaviour of Meteorological drought (MD) and Hydrological drought (HD) is crucial for analysing how drought propagation occurs. Here, drought events were treated as three-dimensional grid structures spanning space (latitude and longitude) and time. 31 years (1971-2001) of global MD and HD events were analysed for evidence of propagation, and the most severe 20 MD events explored in detail. From the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) data archive, precipitation data was used for identifying MD events and an ensemble of simulated runoff from several global hydrological models used for detecting HD events. A technique was developed based on overlapping of the spatial and temporal coverage of MD and HD events, to establish propagation, and to calculate several propagation features. In three dimensions, the transformation from MD to HD was characterised based on delayed instigation, elongated duration, and dampened intensity of the HD event. Additionally, pooling of MD events that resulted in one or multiple branched HD events were identified. Results indicate that minor MD events with short durations and small areas generally do not exhibit propagation. The frequency of HD events with drought duration of 6-12-months is higher than that of MD events with 6-12-month duration. Out of 1740 extreme MD events identified for the 31-year period, 272 events propagated and resulted in 395 extreme HD events. Propagation features for the 20 most severe MD events show substantial variation based on geographical location highlighting the influence of regional climatic and hydrological conditions. This study advances the understanding of global drought propagation mechanisms by addressing key methodological challenges and providing a structured framework for future large-scale drought assessments.