Rainfall data for urban hydrology: what do we need?

For urban hydrologists rainfall is mainly interesting if it touches the surface of the catchment and is intense enough to produce runoff. Rain data requirements for urban applications are different compared to applications in natural catchments. Especially the temporal and spatial resolution needs to be much higher. New tasks in urban drainage call for even better rain data: ideally a long data series (e.g. 20 years or more) with 1 min time resolution, 1 km(2) spatial resolution, and time synchronization errors below 1 min should be available. For some applications these requirements might be relaxed, though. It appears that only in a joint effort, hydrologists, with the support of meteorologists, might be able to supply this data. Data sources can be historical paper chart records, state-of-the-art automatic rain gauges, and radar which is attractive because it provides a high spatial resolution. As the next decades will be characterized by extremely high expenditures for sewer renovation in all industrialized countries, the rain data base to be used for proper re-dimensioning of these systems needs to be much improved.