The map of Total Runoff (Map 02.13.3) shows that the total runoff for the highly sealed inner-city areas (Urban-Rail Ring Line) is in the range of 350-450 mm/yr.; the values are even higher in the very dense center-city area and in some industrial areas. Here, only about 150 mm/yr. (Map 02.13.6) evaporate, referenced to the precipitation measurements (at 1 m height), which are about 10-15% less than the ground-level precipitation. The less densely built-up areas in the outskirts of the city show runoffs of 250-350 mm/yr. Compared with the runoffs of the unsealed areas on the outskirts. or in the areas surrounding Berlin, where the values are approximately 150 mm/yr., Berlin can be considered an island of greatly increased runoff. The reduction of the evaporation by sealing and lack of vegetation – as can be seen in the map Evaporation (Map 02.13.6) – leads to runoff double or triple the natural run-off.
Groundwater net consumption occurs only in few areas, caused by low precipitations with simultaneously low depths to water table, which produces negative runoff formation values, since here, the vegetation, which withdraws groundwater, can evaporate more water than can subsequently be supplied by precipitation.
The map of Surface Runoff (02.13.1) shows that in the inner-city areas connected to the sewage system, an average of about 250 mm/yr. is fed to the sewage system. Peaks values are more than 350 mm/yr. In outlying areas connected to the sewage system, the values are around 100 mm/yr.
The Seepage map (02.13.2) shows a picture that is at first surprising. According to this, the inner city shows precipitation seepage of about 120 mm/yr. – roughly as much as the woodlands. The non-densely built-up residential areas on the outskirts show considerably higher seepage capacities of 200 mm/yr.; the values for the areas with low degrees of connection to the sewage system in fact climb to 300 mm/yr. In the residential areas with no connection to the sewage system, all the runoff seeps away, averaging about 300-350 mm/yr. and reaching maximum values of over 400 mm/yr.
In conclusion, the following can be stated:
- The effect of the reduced permeability of the soil caused by the high degree of sealing in the inner city is to a large extent counteracted by the effect of reduced evaporation, so that the inner-city seepage capacities are higher than initially assumed, and almost reach “natural” levels.
- The extent of sealing is only secondarily important for seepage capacity; of primary importance is the actual degree of connection to the sewage system. The type of sealing, i.e. the differing seepage capacity of the various surface-cover types, also plays an important role.
- The reduction of evaporation due to sealing in low-density areas with simultaneously low degrees of connection to the sewage system causes the seepage capacities in these areas to be the highest, and approximately double those of “natural” seepage.
In the glacial spillway area, the seepage water can percolate directly and completely to the groundwater table, due to the permeable sands which overlay the groundwater. Here, the calculated seepage is equal to new groundwater formation. On the ground-moraine highlands of Barnim and Teltow, however, loamy and thus poorly water-permeable layers overlay the mostly confined groundwater. Here, the deeply cut flows are largely fed by confined groundwater or from sandy and thus permeable layers in the ground moraine. Only that part of the seepage water (calculated seepage) not passed on by the tributaries can be considered a true recharging of the main aquifer underneath the ground moraine. These water quantities reach the glacial spillway area as sub-surface runoff. The break-down is respectively dependent on the concrete hydrogeological conditions. A comparison of the runoffs measured and calculated shows that e.g. in the catchment area of the Neuenhagen Mill Stream, some 35% the calculated seepage percolates sub-surface to the glacial spillway area, while the Tegel Creek passes on at the surface virtually all of the runoff it receives from the seepage of its catchment area. On the basis of the seepage water rates determined during the ‘90s with the ABIMO model, a map of new groundwater formation has also been developed (Environmental Atlas Map 02.17).
The evaporation from bodies of waters surfaces, which are not shown on the map, is approx. 160 mm/yr. more than the precipitation which falls on them, so that Berlin’s bodies of water lose a total of approx. 9 million cu.m. of water per year due to evaporation.
For some very highly sealed areas, no information was available as to whether the rainwater from them is passed on via the sewage system. For this reason, runoff for these areas has been certified in the maps as seepage. However, the degree of sealing and the amount of runoff makes it seem improbable in some cases that the water actually seeps away. As a result, it is likely that the share of surface runoff tends to be underestimated, and that that of seepage to be overestimated.
With the aid of the area sizes of the reference surfaces, the runoff volumes could also be calculated and then totaled (cf. Tab. 4).