Long-term flood control in central Japan: A half-century groundwater monitoring and evaluating adaptation measures for sustainable use

Groundwater depletion and quality degradation are accelerating worldwide under pressures from groundwater abstraction, land-use transitions, and climate-driven shifts in recharge. Monsoon Asia, responsible for nearly two-thirds of global freshwater withdrawals, is particularly vulnerable. At the eastern frontier of the Asian Monsoon, Japan has historically experienced groundwater crises, making it a valuable reference for anticipatory groundwater governance. This study identifies groundwater origins, quantifies recharge sources, and provides the first empirical estimate of how long-term flood-control strategies, including riverbed excavation and subsequent regulation, have influenced groundwater. Analysis combined half a century of literature with 20 years of shallow groundwater monitoring data, revealing a long-term shift in recharge pathways from river infiltration toward surface-derived inputs following riverbed excavation, with approximately 80 % of monitoring sites now exhibiting significant surface-derived recharge. A groundwater recharge project (GRP) utilizing fallow paddy fields is predicted to secure future groundwater supplies locally, and scaling seasonal GRPs nationally could potentially satisfy a fraction of Japan's freshwater demand. These findings highlight the critical role of sustained monitoring in detecting human-driven shifts in recharge dynamics and demonstrate that strategically managed agricultural land offers a flexible, transferable measure for groundwater security. Japan's experience provides actionable insight into emerging groundwater challenges across monsoon Asia and other groundwater-dependent regions, offering evidence-based guidance for integrating flood-control policies with long-term water resource management.