Abstract Hydrological process connectivity (HPC) is a key control on rainfall‐runoff transformation in headwater catchments, as it governs when, where, and how subsurface flow pathways are dynamically activated through interactions with the catchment's hydrological structural connectivity (HSC). However, cross‐catchment comparisons and mechanistic understanding of HPC remain limited by the lack of a unified, process‐based metric. Here, we propose a framework that quantifies HPC by integrating temporal similarity in soil moisture dynamics between sites with their spatial distance. This framework introduces two indices: (a) the spatiotemporal fusion similarity index (SFSI), which quantifies the likelihood of hydrological linkage between monitoring sites, and (b) the hydrological connectivity index (HCI), which aggregates SFSI to quantify the magnitude of HPC across scales ranging from flow pathways to hillslopes and the entire catchment. Application to the well‐instrumened Shale Hills catchment demonstrated the framework's robustness. Across three rainfall events with contrasting runoff coefficients, both SFSI and HCI increased systematically with runoff efficiency ( p < 0.01). Long‐term analysis (2011–2014) revaled pronounced spatial heterogeneity in HPC with higher connectivity on the sunny hillslope (HCI = 0.720) than on the shady hillslope (HCI = 0.686; p < 0.001), and identified swales as persistent hubs of subsurface connectivity. HSC explained approximately 40% of the spatial variability in HPC, with a stronger association on the sunny hillslope (41.1%) than on the shady hillslope (34.5%). Overall, this study provides a transferrable, process‐based framework for quantifying subsurface hydrological connectivity, advancing mechanistic understanding of runoff generation in headwater catchments and supporting improved hydrological modeling.
Zhang et al. (Wed,) studied this question.