Surface Runoff, Percolation, Total Runoff and Evaporation from Precipitation 2001
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).
The calculations show that about 60% of the precipitation evaporates, and thus about 200 million cu.m./year are available as total runoff. Three quarters of this seep away to the sub-surface, a quarter is passed through the sewage system. Although the mixed-sewage system covers only about one quarter of the total area served by sewage systems, it accounts for more than a third of total surface runoff. If annual seepage quantity of approx. 150 million cu.m., which as stated above, is not entirely fed into the groundwater, is juxtaposed to the drinking water consumption of approx. 250 million cu.m. per year, it is obvious that there is a considerable deficit. This deficit is compensated for by surface influxes (the Spree, and Havel rivers) as well as sub-surface influx of groundwater from the surrounding countryside. The surface water is used as shore filtrate at near-shore groundwater withdrawal facilities, and for groundwater charging at the waterworks facilities.
The Information System registers for every area served by the separate-sewage system an indication as to which tributary or which body of water or body-of-water segment that area is drained to (cf. Map 02.09.2.) As a result, it is possible to generate balance sheets stating the amount of rainwater which each body of water has to receive, on average. About 180 bodies of water or body-of-water segments are involved. Table 5 shows the amounts of introduction into the Berlin bodies of water, summarized by segments. Except for those quantities which, at heavy rainfall, flow through the emergency outlets of the pumping stations and through the rain overflows of the sewage network, and then also flow directly into the bodies of water, the surface runoffs in the area of the mixed system are passed to the sewage treatment plants, of the channel net, from where they are fed into the bodies of waters after an appropriate sewage-treatment process, together with the treated sewage.
The ABIMO model or program is an instrument with which simulations can also be carried out under modified circumstances. In particular, this could include an assessment of the likely changes in the water balance to be expected due to urban development projects, or of de-sealing measures, including the disconnection of areas from the sewage system, to permit seepage of precipitation there. With appropriately differentiated data, the program can also be applied on a small scale for projects at the single-lot level. Changes of the model parameters by incorporation of current findings are possible at any time.
The map Seepage without Consideration for Sealing (02.13.4) shows conditions which are in some cases greatly different from those shown in the Seepage map for which sealing was taken into account.
With 200-250 mm of annual seepage, the greatly anthropogenically transformed, yet unsealed, surfaces of the inner city and the industrial areas achieve the highest seepage capacities in the municipal area, followed by the predominantly sandy areas of the glacial spillway and the sandy parts of the highlands, with approx. 150-200 mm. If the sandy soils are woodlands, the average annual seepage drops to 100-150 mm, since the trees, due to their root depth, allow considerably more water to evaporate. Due to the higher retention capacity of the loamy soils of the highland areas of Teltow and Barnim, considerably more water can also be evaporated by the vegetation there, so that only approx. 50-100 mm seep away. In areas with surface-near groundwater, increased evaporation is caused by the capillary rise of groundwater into the evaporation-influenced soil zone, so that only an annual average of less than 50 mm seeps into the soil. If real evaporation is higher than precipitation, net water consumption occurs, i.e. the calculated values are negative.
Certain areas have seepage capacities of more than 300 mm; these are areas with low vegetation covering, or without vegetation. Therefore only small portions of the precipitation are able to evaporate there; the greater share seeps away.
If the data of the Map 02.13.4 are used to estimate the results of additional sealing in the context of Planning Procedures, the following should be considered:
The seepage capacity indicated in the map are only reduced to zero by sealing if the planned sealing is actually completely impervious to water (roof areas, asphalt) and the precipitation water of these areas is passed entirely to the sewage system. If partially water-permeable sealing is planned, or if the precipitation water runoff is to be only partially passed into the sewage system, corresponding modification must be incorporated into the calculations with regard to the reduction of sealing performance. For more exact calculations, application of the ABIMO runoff model is recommended, in which simulated data for planned surface structures can be entered as input data, so that the actual state and the plan can be compared.