Importance of bedrock hydrology for the rainfall–runoff process in a forested mountain catchment

Physically based, distributed hydrology–vegetation models are promising tools to understand the rainfall–runoff process, a complex system involving numerous interactions between hydrological components such as soil, bedrock, topography, vegetation, and atmosphere. Although the influence of bedrock groundwater dynamics on the water yield from headwater catchments has been demonstrated, its mechanisms remain poorly described in physically realistic runoff generation models. Here, we developed a distributed hydrological model aggregating soil and bedrock blocks with explicit water balance calculations for both the soil and bedrock modules, based on the Buckingham–Darcy law. Thus, our mechanistic model allows for direct evaluation of the importance of bedrock groundwater flow for catchment runoff generation. We validated the model against comprehensive observations of runoff, bedrock groundwater table, soil water retention, and hydraulic conductivity acquired in a forested mountain catchment of the Kanto region, Japan, in 2010–2017. Because of its ability to explicitly calculate spatiotemporal variations in the bedrock groundwater table depth, the model successfully reproduced the catchment baseflow. Refining the analysis, we demonstrated that accounting for bedrock hydrology was necessary to correctly evaluate soil water dynamics. From our model simulation, we inferred that bedrock groundwater, recharged by rainwater infiltration, dominated catchment runoff generation during low-flow periods. Insights from our catchment hydrology study should be extremely useful to optimize forest watershed management strategies toward water resource sustainability.