Inundation analysis using coupling storage function model with a distributed hydrological model in Kushiro marsh, Japan

: In Kushiro River basin, inundation is likely to occur due to heavy rain, since the river-bed slope is very gentle in the downstream portion, which has natural levees. Conse‐ quently, in past floods, river discharge was observed to increase slowly and a delay of a few days in peak river dis‐ charge was observed compared to the peak precipitation. Therefore, we applied a distributed hydrological model, Geophysical fluid CIRCulation model (GeoCIRC), to reproduce such flood discharge in Kushiro River. GeoCIRC is based on object-oriented programming and various hydrological processes, such as infiltration flow, under‐ ground water flow, surface flow, and river flow can be implemented easily. We proposed a model that can incorpo‐ rate the effect of return flow using storage function by introducing new parameters, such as storage time and time lag. This was done to consider not only the flood inunda‐ tion in Kushiro River but also the return flow from flood inundation to the river flow. As a result, we obtained high Nash-Sutcliffe coefficients for river discharge for two large flood events.