Hotspots of drought–flood abrupt transitions from hydroclimatic volatility to hydrologic extremes under land cover change

Drought–flood abrupt transitions are a manifestation of hydroclimatic volatility and can lead to hydrologic extremes that are influenced by the amplification of wet-dry anomalies through land-surface dynamics and watershed storage. In addition to meteorological forcing, land cover changes can modify infiltration–runoff partitioning, evapotranspiration feedback, and soil-moisture persistence, thereby reshaping where transition hotspots emerge and how abruptly they evolve. This study quantifies drought–flood transition regimes in the semi-arid Ebro basin and evaluates how land cover changes modulate the propagation of hydroclimatic variability into hydrologic regime transitions. 

The Precipitation–Evaporation Anomaly Index (PEAI) is computed from HYPER-P 1km precipitation and GLEAM evapotranspiration, providing a high-resolution description of hydroclimatic deficit–surplus anomalies over 2016–2022. PEAI is integrated with surface soil-moisture anomaly dynamics derived from Sentinel-1 radar to compute high-frequency ΔSM anomalies. These ΔSM anomaly sequences provide the basis for deriving soil-moisture memory (SMM) at fine spatial and temporal scales, with persistence, decay, and instability quantified. Drought–flood transitions are delineated by persistent PEAI anomaly reversals and retained only when accompanied by a coherent ΔSM response, after which events are mapped to delineate hotspots and summarized using metrics of frequency, duration, and abruptness. Land cover change is derived from the Copernicus datasets, which provide annual land-cover maps from 2016 to 2022, and is used to stratify SMM properties and transition metrics to compare transition behavior under similar PEAI variability across areas with different land-cover patterns. 

Results show pronounced dry–wet alternations and spatially heterogeneous soil-moisture memory, with short persistence and elevated instability in recurrent transition zones. These SMM signatures sharpen the delineation of hydroclimatic volatility hotspots and improve the spatial identification of rapid drought–flood abrupt transition events. Transition frequency and abruptness are not explained by PEAI intensity alone; instead, they vary systematically with land-cover patterns, revealing distinct transition regimes across cropland-dominated areas, natural vegetation, and expanding built-up surfaces. Overall, integrating PEAI derived from HYPER-P precipitation and GLEAM evapotranspiration with radar-based ΔSM and SMM provides a physically consistent, high-resolution framework to explain how hydroclimatic volatility propagates into hydrologic extremes through drought–flood abrupt transitions shaped by land cover change in a semi-arid Mediterranean basin.