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Berlin Environmental Atlas

02.13 Surface Runoff, Seepage, Total Runoff and Evaporation from Precipitation (Edition 2005)

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Methodology

Almost ten years ago, a model for calculating the most important quanta of the water balance was developed, programmed and used in cooperation with the Berlin office of the Federal Hydrology Agency. The approximately 25 required basic data or input parameters could be provided by the City and Environment Information System (ISU) for each of the approx. 25,000 single areas. This model has now been used unchanged, albeit with updated data (cf. Data Base).

The runoff model ABIMO developed by Glugla has been created on the basis of models developed as early as the '70s for the calculation of groundwater supply, and been expanded to include modules which take into account the special situation of urban areas. This expansion was supported by an expert report by the Institute for Ecology (soil science) of the Berlin University of Technology, and a master's thesis at the Department of Geography at the Free University of Berlin. The arithmetic implementation carried out by an external software company in addition adapted it to the specific data situation of Berlin.

The calculation method first of all ascertains the actual evaporation, in order to calculate total runoff (precipitation minus evaporation). In the second work stage, the surface runoff is determined as a share of total runoff. The difference between total runoff and surface runoff then constitutes the seepage. Fig.2. conveys an impression of the complexity of this procedure.

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Fig. 2: Flow Chart of the ABIMO Model (from Bach 1997, modified)

Total Runoff

Total runoff is calculated from the difference between long-term annual mean precipitation values and real evaporation. Real evaporation as it is actually encountered, as a mean, at sites and in areas, is calculated from the most important quanta precipitation and potential evaporation, and the mean storage qualities of the evaporating areas. Given sufficient moisture input into the evaporation area, the real evaporation value will approach that of the potential evaporation. The real evaporation is additionally modified by the storage qualities of the evaporation area. A higher storage effect (e.g. greater binding capacity of the soil and greater perracination depth) causes higher evaporation. The connection shown between the mean value of real evaporation over several years on the one hand and precipitation, and potential evaporation and evaporation effectivity of the site on the other fulfills the Bagrov relation (cf. Glugla et al. 1971, Glugla et al. 1976, Bamberg et al. 1981 and Fig.3). The Bagrov relation is based on the evaluation of long-term lysimeter tests, and describes the nonlinear relationship between precipitation and evaporation in dependence on site characteristics. With the Bagrov relation, the real evaporation/potential evaporation quotient (ER/EP), and hence the real evaporation ER for sites and areas without groundwater influence can be ascertained, provided the climate quanta precipitation P and potential evaporation EP (quotient P/EP), and the effectivity parameter n of the quotient are known. The Bagrov method is also used in modified form to calculate the groundwater-influenced evaporation, adding the mean capillary water rise from the groundwater to the precipitation.

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Fig. 3: Representation of the Bagrov Equation for Selected Values of the Parameter n and the Dependence of this Parameter upon Land Use and Soil Type (after Glugla et al. 1995)

With growing precipitation P, the value of real evaporation ER approaches that of potential evaporation EP i.e., the ER/EP quotient approaches the value of 1. At reduced precipitation P (P/EP approaches the value 0), the real evaporation value approaches that of precipitation P. The intensity with which these boundary conditions are reached is determined by the storage qualities of the evaporating area (effectivity parameter n).

The storage qualities of the site are particularly determined by the use form (increasing storage effectivity in the following order: sealed area, vegetation-free surface, agricultural, horticultural/ silvicultural use) as well as soil type (increasing storage effectivity with higher binding capacity of the soil).

The measure for the storage effectivity of unsealed soil is the usable field capacity as a difference of the humidity values of the soil for field capacity (beginning of water seepage into the ground) and for the permanent wilting point (permanent drought damage to plants). Other land-use factors, such as hectare yield and types and ages of trees, modify the parameter value n. The parameter n has been quantified by evaluation of observation results from numerous domestic and foreign lysimeter stations and water-balance tests in river-catchment areas.

For sites and areas in surface-near groundwater, increased evaporation compared with non-groundwater-uninfluenced conditions occurs in the evaporation-influenced soil zone due to capillary rise of groundwater, depending on the depth to water table and soil qualities. Runoff is reduced. If real evaporation exceeds precipitation, water consumption occurs and the values for runoff become negative (e.g. river and lake lowlands).

Water areas have a higher potential evaporation than land areas, because of higher heat supply (lower reflectivity of the irradiation). For the sake of approximation, the actual water evaporation is stated as equal to this increased potential evaporation.

Selective seepage, e.g. via groundwater charging facilities by the waterworks, has not been taken into account. For gardening use (allotment gardens) a uniform approximation value was added to the precipitation to take irrigation into account.

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