The surface runoff prediction model specified for the Hortonian overland flow at the fine-tephra mantled slope is proposed in this study. Based on the numerical solution of the Richard's equation, this model proposes the new surface boundary condition that can describe the temporal change in an infiltration capacity. The calculation to obtain the infiltration excess and the runoff hydrograph, which is routined by the kinematic-wave method, is verified by the actual hydrographs obtained by the hydraulic monitoring at the newly tephra-mantled slope in the Miyakejima volcano, which erupted in July 2000 and deposited thick fine-grained tephra on its slope. All the parameters of the unsaturated soil properties of the deposited volcanic ash are obtained by the laboratory tests of the undisturbed soil sample of the deposited volcanic ash. The verification shows that the result of the calculation by the proposed prediction model shows good accordance with the actual hydrographs though the proposed method has no controlling factor to adjust to the actual hydrograph.
