The Schroeder and Wyrwich method is based on the rearranged water balance equation:

groundwater recharge = precipitation - total evaporation - direct runoff:

• Groundwater recharge: It is calculated from all data entered in SPRING 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: Amount of precipitation entered in the unit m/year for all elements or m³/m²/TU by element (column 16: "*") (attribute NIED).

• Total evaporation: The average total evaporation is taken into account depending on the soil type and the land use class according to the following table (values in mm/year, N stands for precipitation:

• Direct runoff: The water still available from the difference between precipitation and evaporation is additionally reduced by the direct runoff. In addition to the parameters of soil type and land use on arable land and grassland, the relief energy or slope is required for this. The maximum slope between the nodes (possibly also via the diagonal) of the element is used as the decisive slope in the finite element concerned, which the programme automatically calculates from the terrain elevation at the individual nodes).

The direct runoff depends on the land utilisation of farmland and grassland:

For the other land uses, direct runoff is independent of soil type and gradient:

*: In contrast to Schroeder and Wyrwich, the proportion of direct runoff from built-up areas is set at 90% instead of 100%.

Input data