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/yr).
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.