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Berlin Environmental Atlas

02.13 Surface Runoff, Percolation, Total Runoff and Evaporation from Precipitation (Edition 2013)

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Surface Runoff

After the average total runoff has been calculated as a difference between precipitation and real evaporation, surface runoff is determined in a second work step. Surface runoff corresponds to the total runoff on roof areas which drains into the wastewater/sewage system. Areas not connected to the sewage system thus produce no surface runoff. Non-built-up impervious areas infiltrate a part of their drainage into the sub-surface, depending on the type of surface (surface-coverage types). This Infiltration factor is dependent on the width, age and type of the seams. The non-percolating runoff is passed to the wastewater system as surface runoff - depending on the degree of connection to the system - or, if the system does not receive it, percolates into the soil at the edge of the impervious areas. Those portions of the precipitation onto roof areas not connected to the wastewater system also percolate into the soil (cf. Tab. 1). The difference between total runoff and surface runoff thus corresponds to percolation as a basic quantum for new groundwater formation.

For the application of the method for urban areas, the parameters n and the infiltration factors had to be determined for the various impervious paving materials. Both lysimetre tests were evaluated with different impervious-paving materials and calculations for wetting loss (cf. Wessolek/Facklam 1997). The quanta selected for the stated parameters are listed in Tab. 2. The change of these parameters due to compression and silting of the joints associated with the ageing process has been taken into account. However, due to still insufficient scientific bases, this information still involves certain uncertainties. Moreover, a different grouping of surface coverage types into surface coverage classes would be desirable from a hydrological point of view.

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Tab. 2: Effectivity Parameter n und Infiltration Factor Fi for Various Surface Imperviousness Classes

Excel
[Table is also available as Excel-File (MS-Excel is required).]

In order to provide an impression as to how the various area uses, imperviousness parameters and conditions of the wastewater/sewage system would affect the water balance, the ABIMO model was used for approx. 35 model sections with different input quantities; the results are shown in Table 3. The relationship between surface runoff, imperviousness and evaporation is decisively dependent of the extent of impervious coverage and the passage of rainwater to the wastewater system.

Tab. 3 : Relationship Between Surface Runoff, Percolation and Evaporation for Areas of Various Types, Imperviousness etc. (cf. Köppel/Deiwick 2004)


Excel
[Table is also available as Excel-File (MS-Excel is required).]

A new version of ABIMO programme was used for the present calculation. This version differs from the old one primarily in its improved parameter control in the assignment of values for the degree of connection of roof surfaces with the wastewater system.

As a result of these calculations, updated long-term average values for total runoff, evaporation, surface runoff and percolation are available for each of the 25,000 separate sections. These values have been shown classified in mm/year in these maps; the totals in cu. m./year have also been calculated and averaged. It must be taken into account that the values shown are average values covering the sections represented as uniform areas; in fact, however, they have non-homogeneous structures. The runoffs of impervious and pervious areas have been standardized to average values per block. In addition, the runoff of roadways has been attributed to the adjacent blocks. The maps do not show, for instance, how great the percolation capacity of a square meter of pervious soil is. For this purpose, another full-coverage and block-referenced calculation has therefore been carried out with changed marginal parameters, i.e., assuming completely pervious conditions. The results of this calculation are shown in Map 02.13.4.

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