Estimate of river seepage by conditioning downward groundwater flow in the Toyohira River alluvial fan, Japan

River seepage serves as the main source of groundwater in alluvial fans. However, reasonable estimates are rarely obtainable due to inaccuracy of vertical flows around the losing river. In the alluvial fan of the Toyohira River, Japan, to provide a straightforward description of vertical hydraulic gradient (VHG), this study demonstrates assignment of constant head boundary conditions on the shallowest aquitard instead of no-flow conditions on the basement. Initial constant heads and layer-scale anisotropy are calibrated in terms of hydraulic heads. As a result, the simulated river seepage is in reasonable agreement with previous discharge measurements, with net water loss determined to be approximately 1 m 3 /s. Although the simulation fails for water gain in the lower section, due to a lack of sporadic intercalations of fine deposits, the conditioned VHG modelling approach is found to be effective on a regional scale, especially when losing/gaining sections have not been identified in advance. EDITOR D. Koutsoyiannis ASSOCIATE EDITOR J. Ward