Dual control of bedding structure on catchment hydrology: the interplay between direct bedding-parallel recharge and indirect landslide-driven drainage in steep accretionary complexes

Empirical understanding of how geological bedding structure governs catchment hydrology is limited, particularly in high-relief, landslide-prone mountains at the meso-scale (10- 1-100 km2), a range relevant to large-scale sediment disasters. This study evaluates the dual control of bedding structure-direct deep recharge and indirect shallow drainage-through multi-period snapshot observations of discharge and electrical conductivity under rainless conditions in a Japanese accretionary complex. Results suggest that the effect of the direct control of bedding structure on baseflow spatial variation is limited at this study site, characterized by geotechnical conditions where bedding dip generally exceeds topographic gradients. Under these conditions, the results are consistent with a mechanism where bedding-parallel flow serves as a dominant, spatially uniform process leading to deep infiltration regardless of the slope types (anti-dip or dip slope). Conversely, the influence of indirect control associated with landslide deformation appears to predominate in dip-slope-dominated basins. The higher temporal variability of hydrological indicators in these basins suggests a dynamic drainage mechanism -potentially a physical causal chain facilitated by landslide-driven fracture networks-that manifests under wetter conditions. Integrating these direct and indirect controls, we propose the 'structural ground system' as a novel framework that combines structural geology and hydrological variability. This approach offers a new perspective for interpreting the spatially anisotropic hydrological processes that characterize sedimentary mountain ranges, providing an interpretative framework potentially applicable to other humid, tectonically active, high-relief regions under similar geometric conditions.