By reference to the types, courses, and slopes of sewage drains, it was possible mark off catchment areas of combined sewage drains and of separate rainwater and sewage drains, respectively. It is not possible to exactly match individual roof and courtyard areas to the catchment areas of rainwater sewers by reference to maps of the sewerage network. Entire units and subunits as represented on the map were always allocated in their entirety to one of the categories or one of the first receiving water bodies, even if individual properties or parts of a unit are drained differently from others under specific circumstances. It is merely stated whether, given the close spatial proximity of a property to a rainwater sewer, it would be possible to provide a connection to this system. There are – at least in many places – considerable differences regarding the extent to which properties are actually connected, and whether rainwater from impervious surfaces is actually discharged into rainwater sewers. The text accompanying Map 02.13.1 Surface Runoff from Precipitation documents the corresponding data, based on information from the BWB.
To some extent, data on the type of sewers cannot be derived from the maps and documents available. This is especially true of large-scale industrial and commercial areas, where maps of the sewerage networks of the BWB do not provide any information as to the the type of disposal of rainwater. Some of these surfaces are drained directly into water bodies via private drains.
Parts of the city with green and open spaces which may be considered unconnected to sewers ere not included in this analysis. They were not entered onto the map, and are therefore not included in any catchment areas.
In areas with separate sewerage, rainwater is drained into water bodies. For every unit or subunit with rainwater drains, the water body into which the rainwater drain discharges was defined. This is the so-called first receiving water body. For easier reference and for the purposes of this map, these first receiving water bodies were combined into water body sections or groups. So what is shown are the catchment areas of the respective water bodies.
Areas in which rainwater is fed into decentralized percolation facilites (such as basin or trenching systems etc.) and which only have a spillway into the sewerage system or into water bodies used in case of heavy rainfall were still assigned to these water bodies (such as Karow North).