Runoff values determined for the by 1 × 1 km grid area show considerable regional differences. They meet expectations based on the essential factors of:
High runoff values occur at inner city areas with a high degree of sealing. A large portion of runoff water in the Berlin urban area is directly discharged into watercourses (mainly the Spree river and canals) by the storm sewer system. Groundwater recharge is thus lost. Statements about groundwater recharge in urban areas are possible only with detailed knowledge of local distribution by the rain water drainage system.
Water depletion (negative values) or minimal runoff are found where larger lakes and wetlands are connected. Runoff quantities in non-urban areas correspond to the groundwater recharge of the upper groundwater storey. The upper groundwater storeys discharge horizontally into the flat upland surface waters, and vertically into lower groundwater storeys. Runoff quantities in the glacial valley area are to be considered equal to groundwater recharge of the main groundwater aquifer.
Two examples will demonstrate how the Map of Runoff can be applied in the determination of water resources.
- To determine an average subterranean runoff:
The average subterranean runoff from the Barnim ground moraine area to the glacial valley for a closed underground balance area. (It is a watershed with areas on the flat upland area as well as in the glacial valley): The first calculation is the water supply quantity (total runoff) of the area segment on the flat upland area. It is the sum runoff of all grid areas (including areas beyond the state border, if necessary). Throughflow into watercourses leaving the upland/glacial valley area border zone is subtracted from the water stocks of the upland area. Removals from groundwater or surface waters in the upland area are subtracted from the water stocks, in as far as these removals do not remain in the upland area as sewage. The difference is the approximate average underground outflow of the upland area.
- To determine the bank-filtration portion of groundwater removals for the area just described. It is affected in the glacial valley area by a surface water:
Water stocks are calculated from the total runoff of all grid areas in the glacial valley area, plus the projected underground runoff from the flat upland area. If the available water stocks described above are subtracted from groundwater removals, the approximate quantity of water filtered through banks will be derived. Detailed investigations also require those watercourses which flow through the glacial valley area to be taken into consideration. Areas with high degrees of surface sealing cause water quantities to be diverted directly into surface waters by the storm sewer system. These quantities are subtracted from calculated water stocks.