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.
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 sewage system. Areas not connected to the sewage system thus produce no surface runoff. Non-built-up sealed areas infiltrate a part of their drainage into the sub-surface, depending on the type of surface (surface-cover types). This Infiltration factor is dependent on the width, age and type of the seams. The non-seeping runoff is passed to the sewage system as surface runoff – depending on the degree of connection to the sewage system – or, if the sewage system does not receive it, seeps away at the edge of the sealed areas. Those portions of the precipitation onto roof areas not connected to the sewage system also seep away (cf. Tab. 1). The difference between total runoff and surface runoff thus corresponds to
seepage as a basic quantity 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 sealing materials. Both lysimeter tests were evaluated with different sealing 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-cover types into surface-cover classes would be desirable from a hydrological point of view.