Global assessment of karst drought propagation from multisource Earth Observation: Epikarst-mediated rapid shallow coupling and delayed deep responses

The pronounced heterogeneity of karst hydrogeological structures complicates drought propagation. However, as traditional ground-based observations are often limited by complex subterranean environments, the regulatory function of the epikarst—a critical interface linking surface and subsurface systems—remains poorly understood at the global scale. To address this gap by leveraging multisource Earth Observation datasets, we constructed the standardized shallow groundwater drought index (SGDI) to characterize epikarst droughts and evaluate the full cascading drought propagation chain. Results revealed distinct vertical heterogeneity in propagation times. Meteorological-to-soil propagation took 6.5–12.7 months, reflecting root-zone buffering capacity. By contrast, rapid soil-to-epikarst propagation (1.2–6.0 months) underscored strong hydraulic coupling within the shallow system, whereas epikarst-to-deep groundwater responses were considerably delayed (10.6–16.6 months). Drought propagation exhibited distinct progressive probability patterns. Specifically, under severe soil droughts, the probability of inducing secondary epikarst droughts was relatively high (0.98–0.77), whereas the probability for other pathways was lower. Trigger thresholds decreased (became more negative) with increasing drought severity, showing notable spatial heterogeneity. Furthermore, soil-to-epikarst propagation exhibited higher thresholds for moderate and severe droughts, whereas deep groundwater thresholds were generally comparable. Conversely, the spatial extent of identifiable thresholds in epikarst-to-deep groundwater propagation diminished with increasing severity, confirming deep groundwater’s buffered and attenuated response. These findings demonstrate the value of integrating EO data to constrain heterogeneous subterranean processes. The proposed SGDI serves as an effective geoinformation proxy for large-scale epikarst drought monitoring, providing a spatial basis for differentiated water management in karst regions.