Coupling MODFLOW and distributed hydrologic model BTOP in the Fujikawa River basin

Distributed hydrologic models (DHMs) are widely utilized for simulating hydrological processes on a river basin scale while having greatly simplified groundwater flow dynamics. This study is the first attempt to investigate a coupling of MODFLOW and BTOP models and we used an existing BTOP model in the Fujikawa River basin, Japan. The BTOP model, which is based on a topographic index approach of TOPMODEL with river flow routing, has been applied globally with coarse grids of about 20-km as well as at much smaller scales at several river basins. In our first investigation we attempted sequential linkage and full coupling between these two models. For the sequential linkage, the BTOP model is simulated on a 30-arcsecond (about 1-km) grid in the Fujikawa basin and provides recharge to the MODFLOW model for the groundwater simulations and baseflow contribution to river discharge at each grid. Having iterative coupling between these two models would allow us to further investigate surface-groundwater interactions, groundwater abstraction as well as movement of environmental isotopes such as tritium in various river basins in the future studies.