Rain-Runoff Parameters for Six Small Gauged Urban Catchments

The interpretation of rain-runoff measurements from 6 small (less than 17 ha) urban catchments is described. The recording period covers mostly 1979-1983. Relations between rain- and runoff depths were developed using the traditional linear regression model as well as a new continuous model. Both models compute runoff from impervious surfaces in the same way. Calculation of runoff from semipervious surfaces accounts for infiltration through cracks, percolation from a sublayer and evaporation during dry weather. These phenomena are related to water content of the sublayer. The 10 parameters in the continuous model are calibrated and show values in good agreement with data from the literature. The continuous model fits the measured runoff depths somewhat better for the largest runoff events. For more frequent events, however, the two models are equally good. Rain intensity – duration – frequency curves have been computed. Variations of up to 20 %, for return periods 1/5 and 1/2 yr., are seen for catchments with distances approximately 5 km from each other. Peak flow statistics are compared with rain intensity. The peak runoff coefficient (cf. rational method) is mostly constant or slightly increasing with the return period.