Three bioretention cells in Norway were monitored for 23 to 36 months to evaluate the hydrological performance of bioretention cells operated in regions with cold climates and to test if cell size equations can be used to predict hydrological performance. Values of saturated hydraulic conductivity (Ksat) were determined for separate events by analyzing the observed infiltration rates and via infiltration tests. The two cells with the highest Ksat values (15.9 and 45.0 cm/h) performed excellently during the study period infiltrating nearly all of the incoming runoff. In contrast, the cell with low Ksat value (1.3 cm/h) infiltrated barely half of the incoming runoff. The latter cell had a clear seasonal variation in hydrological performance relating to changes in the Ksat values over the year. The size equation that gave the best predictions of the observed hydrological performance accounts for both surface storage and infiltration. By using this equation to evaluate various bioretention cell designs, it was found that the most effective way to increase the hydrologic performance is to have a Ksat value above 10 cm/h.

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