Aim of the study:
In this study, a novel approach was undertaken, employing the concept of SSF isochrones in tandem with the time-area histogram (TAH) method as a fresh model for SSF estimation within catchments.

Material and methods:
Leveraging the subsurface travel time equation of hillslopes and GIS software, the spatial delineation of isochrones within the soil was accomplished. Furthermore, a methodology was introduced to compute the TAH of SSF. Subsequently, the subsurface hydrograph of the catchment was derived, by convoluting the TAH of SSF with the infiltration hyetograph. To validate the findings, the subsurface hydrographs calculated using the geomorphologic instantaneous unit hydrograph (GIUH) model were juxtaposed with those from two catchments in northern Taiwan – Heng-Chi and San-Hsia.

Results and conclusions:
The mean error in comparison with the SSF time-area model stood at 10.5%, affirming the robustness of the proposed approach.