Impact of small-scale rainfall uncertainty on urban discharge forecasts

We used a multifractal characterization of two heavy rainfall events in the London area to quantify the uncertainty associated with the rainfall variability at scales smaller than the usual C-band radar resolution (1 km(2) x 5 min) and how it transfers to sewer discharge forecasts. The radar data are downscaled to a higher resolution with the help of a multifractal cascade whose exponent values correspond to the estimates obtained from the radar data. A hundred downscaled realizations are thus obtained and input into a semi-distributed urban hydrological model. Both probability distributions of the extremes are shown to follow a power-law, which corresponds to a rather high dispersion of the results, and therefore to a large uncertainty. We also discuss the relationship between the respective exponents. In conclusion, we emphasize the corresponding gain obtained by higher resolution radar data.