Surface Runoff, Seepage, and Total Runoff from Precipitation 1990
02.13.4 Percolation from Precipitation without consideration of surface sealing
For water-economic questions and in context with the calculation of percolation the influence of sealing and the sewage system has to be considered as a crucial influence on the water supply of urbane areas. (Cf. text to maps 02.13.1-3) However, the investigation of the percolation is of particular interest for the evaluation of the capability of soils for soil protection programs or for the evaluation of interventions following the nature conservation law. On the one hand it is possible to derive the different capability of soils from this figure. On the other hand it can be appraised in the context of plannings which influence a planned future seal would have on the rate of percolation for a planned area. These statements cannot be met with the values of the map 02.13.2, because here median values for the respective reference areas from sealed and unsealed as well as from canalized and not canalized areas are portrayed.
For these reasons, the investigation and depiction of the percolation of unsealed surfaces is shown on map 02.13.4. This map portrays the percolation by precipitations on unsealed surfaces. The given values refer only to the unsealed part of the blocks and block segments. The percolation and seepage measurements from unsealed soils is also a basis for the calculation of the exchange frequency of ground water.
Details can be found in the text to map 02.13. For this investigation of percolation, the initial data was used under the assumption that all surfaces were unsealed, ie. the value was set as zero that means that surface sealing was not considered. The surface area of the streets was also not considered and taken to be zero. Thus the results apply only to unsealed soils of block areas.
The calculations were made (as with map 02.13) using the complex drain model “ABIMO” developed at the Federal Institution for Hydrography in Berlin. The procedure assesses data from the Information System City and Environment. Calculations for approximately 25,000 different areas were made with the assumption that all surfaces were unsealed. Again, street surface areas were not considered. The percentage of percolation resulting from precipitation is above all dependent on the vegetation, the soil and the depth to groundwater. A detailed description of the method is located in the text to map 02.13.
With a yearly percolation of 200-250 mm, the high anthropogenic alterated soils of the inner city and industrial areas reach the highest percolation rate in the Berlin, followed by the predominently sandy pleistocene valleys/watercourses and sand plateaus with levels of 150-200 mm. When this sandy soils are living forest areas then the average percolation sinks to 100-150 mm, as the depth of the rootsystems allows an increase in evaporation. Due to the high water holding ability of the clay soils in Teltow and Barnim, the vegetation here can increase the evaporation, leading to a percolation as low as 50-100 mm. Areas with a high ground water level are susceptible to additional water loss due to the capillary effect which pushes ground water up into a higher evaporation zone. This can give a yearly average of less than 50 mm.
If actual evaporation is higher than precipitation this leads to water depletion and the calculated percolation becomes negative. Some areas are capable of a percolation of more than 300 mm. ie. individual areas with extremely low (or no) vegetation foliage. In these cases only an extremely low amount of precipitation is due to evaporation and the predominant part is simply due to drain off.
When data for the estimation of the consequences of additional sealed surfaces in the context of planning processes are used, the following is to be noted :
The percolation rate portrayed in the map will be reduced to zero by planned seal only if the planned seal is truly unpermeable and waterproof (roofs, asphalt) and if the precipitation water drains complete into the sewage system. If partly permeable surfaces are planned or the runoff from precipitation water drains only partly into the sewage system, corresponding reductions are to be performed regarding the reduction of the percolation rate. For more exact calculations, the application of the drain model ABIMO is recommended; with the input of simulated data for planned surface structures it is possible to compare the contemporary situation with the planned situation.