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 straightforward description of vertical hydraulic gradient, this study demonstrates to assign constant head boundary conditions on the shallowest aquitard instead of no flow conditions on the basement. Initial constant heads and layer-scale anisotropy

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... re 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 water gain in the lower section, due to a lack of sporadic intercalations of fine deposits, the conditioned vertical hydraulic gradient modeling approach is found to be effective on a regional scale, especially when losing/gaining sections have not been identified in advance.