Surface Runoff, Percolation, Total Runoff and Evaporation from Precipitation 2017

Methodology

In the middle of the ‘90s’, 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 Institute of Hydrology. The approx. 25 required basic data or input parameters could be provided by the Urban and Environmental Information System (ISU) for each of the approx. 25,000 single sections. This model has been improved (ABIMO 3.2) and applied again with updated data.

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 percolation. Fig. 2 conveys an impression of the complexity of this procedure.

Fig. 2: Flow chart of the ABIMO model

Fig. 2a: Contents of symbols of the flow chart ABIMO model

Total Runoff

Total runoff s calculated from the difference between long-term annual mean precipitation values and real evaporation. Real evaporation as it is actually encountered, as an 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 greater 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 fulfils 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 lysimetre tests, and describes the nonlinear relationship between precipitation and evaporation in dependence on site characteristics. With the Bagrov relation, the climate quanta precipitation P and potential evaporation EP (P/EP ratio), and the effectivity parameter n, and hence the real-evaporation/ potential-evaporation ratio (ER/EP) and the real evaporation ER for sites and areas without groundwater influence can be ascertained. The Bagrov method is also used in modified form to calculate the groundwater-influenced evaporation, by adding the mean capillary water rise from the groundwater to the precipitation.

Fig. 3: Representation of the Bagrov equation for select values of the parameter n, and dependence of this parameter upon land use and soil type

With increased precipitation P, the value of real evaporation ER approaches that of potential evaporation EP i.e., the ER/EP ratio approaches the value of 1. With reduced precipitation P (P/EP approaches the value of 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: impervious 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 pervious soil is the usable field capacity the difference between the humidity values of the soil for field capacity (beginning of water percolation 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 lysimetre stations, and water-balance investigations in river-catchment areas.

For sites and areas with near-surface groundwater, increased evaporation compared with non-groundwater-influenced conditions occurs in the evaporation-influenced soil zone, due to the capillary rise of the groundwater, depending on the depth to the 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 percolation, e.g. via groundwater charging facilities by the waterworks, has not been taken into account. For gardening use (allotment gardens, weekend cottages, parks, cemeteries, tree nurseries/ horticulture and partly in residential use or public facilities/special use), a uniform approximation value was added to the precipitation to take irrigation into account (50-100 mm/a).

After the mean 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. The evaporation of the blocks and block segments is then calculated from the difference between the corrected precipitation (corrected precipitation = precipitation multiplied by the global factor 1.09 for Berlin) and the total runoff.

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.

Tab. 2: Effectivity parameter n and infiltration factor Fi for various surface imperviousness classes

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 typical uses and their different typical properties; the results are shown in Tab. 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.

Since the 2012 edition, version 3.2 of the ABIMO program has been used for the current 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 to the wastewater system.

Consideration of the influence of planted roofs on the water balance data

Thanks to the extensive spatial data on planted roof areas available for the first time with the Environmental Atlas Map 06.11 Green Roofs (edition 2017), it was possible to calculate the effects of the green roofs on the water balance for the first time in this current edition. As the original model does not provide for the consideration of green roofs, a method had to be developed, which allows these effects to be budgeted nonetheless. To do this, it was first necessary to determine reliable values on evaporation behaviour from the literature. The literature search identified different annual runoff coefficients for intensively and extensively planted roofs (cf. for example, Rüngeler 1998, SenStadtWohn 2017). In the method selected for the database used (Environmental Atlas Map Green Roofs 06.11), based on the spectral reflection properties of the remote sensing data. a differentiation is only made between extensively and intensively planted roofs. Other important properties, for example, the height of the vegetation or the substrate build-up cannot be collected in this way and are therefore also not available for the evaluation with regard to the water balance.

Therefore, for the further calculation, a uniform annual runoff coefficient of 0.5 was assumed for all green roofs, that is to say, 50 % of the precipitation falling on them evaporates.

A small part of the precipitation falling on normal, unplanted roofs also evaporates. This evaporation is calculated for blocks and block segments using ABIMO 3.2. Accordingly, unplanted building roofs produce between 75.5 mm/a and 83.6 mm/a evaporation, regardless of the degree of connection to the sewage system and the type of impervious surface. This corresponds to 12.3 % and 13.4 % of the corrected precipitation. The additional evaporation of a planted roof was first calculated using the following formula:

Evaporation_{GreenRoofAdditional} = Evaporation_GreenRoof – Evaporation_NormalRoof

The total additional evaporation of all planted roofs of a block or block segment was then calculaed and deducted from the total runoff, surface runoff and percolation parameters. The evaporation with green roof is calculated from the evaporation and the additional evaporation. These calculations were performed subsequently, outside the ABIMO 3.2 program (cf. Goedecke/Gerstenberg 2019).

Final Result

As a result of these calculations, updated long-term mean values for total runoff, evaporation, surface runoff and percolation incl. consideration of the green roofs are available for each of the approx. 25,000 separate areas. These values have been shown classified in mm/a in these maps; the totals in cu. m./a have also been calculated and budgeted. It must be taken into account that the values shown are mean values covering the blocks represented as uniform areas; in fact, however, they have non-homogeneous structures. The runoffs of impervious and pervious areas have been standardized to mean 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.