This paper aims at quantifying the uncertainty on urban runoff associated with the unmeasured small scale rainfall variability, i.e. at a resolution finer than 1. km. ×. 1. km. ×. 5. min which is usually available with C-band radar networks. A case study is done on the 900. ha urban catchment of Cranbrook (London). A frontal and a convective rainfall event are analysed. An ensemble prediction approach is implemented, that is to say an ensemble of realistic downscaled rainfall fields is generated with the help of universal multifractals, and the corresponding ensemble of hydrographs is simulated. It appears that the uncertainty on the simulated peak flow is significant, reaching for some conduits 25% and 40% respectively for the frontal and the convective events. The flow corresponding the 90% quantile, the one simulated with radar distributed rainfall, and the spatial resolution are power law related. © 2012 Elsevier B.V.