Understanding surface and subsurface flow dynamics in a coastal subsided region

Coastal regions are dynamic environments where surface and subsurface hydrological processes interact in complex ways, significantly affecting water resources and ecosystem sustainability. In regions affected by land subsidence, the surface water/groundwater interaction is further complicated by possible changes in groundwater flow paths, surface water dynamics, and so on. These changes may exacerbate flood risks and saline water inundations. In addition, human interventions, such as engineered infrastructure, is likely to intensify these challenges. Despite the progress in understanding these processes, developing models to capture the coupled dynamics of surface and subsurface flows still needs further investigations for quantitative discussion. For this purpose, we use numerical code HydroGeoSphere to develop coupled dynamics of surface and subsurface flow model in the Kujukuri Plain, the eastern coastal region of Japan. Groundwater levels and river levels show a similar trend during non-irrigation periods, primarily influenced by rainfall patterns. This similarity is likely to indicate a hydrological connection between the two systems, highlighting the dominant role of precipitation in these periods. River levels are additionally affected by tidal fluctuations, which are not observed in measured groundwater levels. This can be understood by the amplitude decay of high frequency signal in subsurface environment. The simulation results show a reasonably good agreement with observed river levels and groundwater levels. The sensitive parameters identified in the analysis are hydraulic conductivity, surface roughness coefficient, and recharge which mainly affect model results. These findings highlight the need for careful calibration of these parameters to ensure model reliability for the model results to be applied to effective water resource management strategies.