The method according to Schroeder and Wyrwich is based on the converted water balance equation:

Groundwater recharge: It will be calculated in SPRING from all input data and assigned to the attribute FLAE (column 16: "*") in the unit m³/m²/a. The time unit "year" is required in the model file.

Precipitation: Precipitation in m/year for all elements or m³/m²/TU element-wise (column 16: „*“) (attribute NIED).

Total evaporation: The average total evaporation rate is taken into account depending on the soil type and land use class of the following table (values in mm/year, N means precipitation):

Direct runoff: The still available water from the difference between precipitation and evaporation is then reduced by the direct runoff. Besides the parameters soil type and land use on farm- and grassland the relief energy or the slope is needed. The definitive slope is the maximum slope between two nodes of an element. The program computes this value automatically from the ground level (attribute ).

The direct runoff depends on the land use, for farm- and grassland it is given by:

Other land uses are independent of the soil type and the slope:

*: In contrast to Schroeder and Wyrwich the part of the direct runoff on build-up areas is set to 90% instead of 100%.

Input data