Cities are becoming increasingly vulnerable to flooding because of rapid urbanization, installation of complex infrastructure, and changes in the precipitation patterns caused by anthropogenic climate change. While there are many previous works that have dealt with the effects of urbanization, the number of quantitative assessment studies related to the impacts of climate change on urban drainage remains, however, rather limited. This is partly due to the particular difficulties of dealing with this type of climate impact assessment for urban catchments. In particular, downscaling of outputs from global circulation models or regional climate models to urban catchment scales are needed because these models do not contain an adequate description of the rainfall governing processes at relevant high temporal and spatial resolutions. More specifically, for urban catchments these resolutions could be as small as a few kilometres spatially and as small as a few minutes temporally. Consequently, the expected results from such impact studies can be highly uncertain and dependent on the feasibility and reliability of the downscaling process. This problem becomes more challenging when dealing with the extreme runoff events since the properties of such extreme events do not necessarily reflect those of the average precipitation.