Generalising relationships between runoff-rainfall coefficients and impervious areas: an integration of data from case studies in Israel with data sets from Australia and the USA

This study explores the generalised relationships between the proportion of impervious areas and storm runoff coefficient (RR) that characterises the current global trends of expansion and densification of urban zones. For this purpose there is an integrated database representing about 800 urban storm events in diverse locations around the world: from Israel and from detailed rainfall-runoff measurements in Australia and USA. Special attention is given here to the experience gathered in two case studies in Israel with information of surface imperviousness derived from remote sensing data, including air photographs and satellite images. Despite differences in measurement systems and methodologies between sites, there was evidential support for a new generalised three-phase model where in up to 20% of impervious area there are no expected major runoff events, a major enhancement in runoff-rainfall response to impervious area increase between 20% and 40%, and 1:1 relationship between 40% and 100% impervious area. This model may be implemented on a wide regional scale, based on remote sensing derived impervious areas for mapping urban areas with high flooding risk. Urban planning may incorporate this model for determining expected storm runoff levels and incorporating this information to derive appropriate hydrological solutions.