One topic within the field of deterministic flood modelling that appears to have been relatively neglected is the level of model sophistication that is needed to satisfy the requirements of different model applications. This study considers two isolated event models, both of which can be operated in lumped and semi-distributed format. The results of modelling 174 rainfall-runoff events from 16 medium sized catchments in the USA and South Africa are compared for these models. The lumped models perform as well as the semi-distributed when the rainfall input is relatively spatially uniform. The semi-distributed models show demonstrable improvements for storms with spatially variable rainfall if the rainfall input is adequately defined by the available data. The simpler model appears to perform as well as the more complex one. However, this is achieved at the expense of consistency of parameter values for a single catchment.