The reed-bed filter, widely used to treat wastewater, is adapted here to treat stormwater. Modelling such systems poses problems linked to the unsteady-state behaviour, itself a consequence of the flow variability during storm events. Under those conditions, hydraulic head variations play an important role in the infiltration processes. The goal of this study is thus to develop simplified models which can be used for the sizing and design configuration of constructed reed-bed wetlands for CSO treatment. The paper proposes five models based on Darcy and Richards approaches, but also accounting for evapotranspiration and capillarity. Those models have been validated against data for a field biofiltration systems located at Monash University Car park. Two models based on the concept of barrier filters and simplified Richards equations seem to perform well with good experimental agreement obtained. These conceptual models were then applied to catchments in Lyon and Nantes (France), in order to test whether such simplified models can be feasibly applied to optimise the key design and sizing parameters for constructed reed-bed wetlands. Real inflow time series data collected on these sites were used to develop and test the models. Models have shown to be sensitive to drainage layer and diverted weir level.