In SPRING there are a number of options for specifying the thickness of an aquifer. The model checking is able to determine the thickness from the following data types (see figure below).

• Explicit definition of the thickness (MAEC or MAEK, iterative thickness not possible)

• Difference between the upper boundary (OBER) and lower boundary (UNTE) of the aquifer (2D only)

• Difference between the surface elevation (GELA) and the lower boundary of the aquifer (UNTE or UNTK), while taking into consideration an impermeable layer (UNDU). Calculated as: (GELA – UNDU) – UNTE/UNTK)

• Difference between the initial potential heads (EICH) and the lower boundary of the aquifer (UNTE or UNTK)

Calculating of the thickness M from existing data

The thickness values are assigned to the relevant element or node. As the GELA, UNDU and EICH attributes are node-wise, they are first interpolated to element values (if the values have been assigned to each node of an element) before the thickness values are assigned to the elements. This interpolation could lead to a loss of accuracy.

The specification of an upper boundary for the aquifer (horizontal models only) is recommended in cases where there is a natural or artificial (e.g. a building) restriction on the aquifer. These boundaries can also be assigned to only parts of the modelled area in contrast to the entire model domain. If the aquifer is bounded/covered by a relatively impermeable layer, the thickness of the impermeable layer and the surface elevation should be assigned. This allows leakage effects to be taken into account. For 3D models, an upper limit can be defined by creating an additional layer with very low hydraulic conductivity.

The definition of a measured groundwater surface (attribute EICH) is only mandatory for model calibration. However, it is advisable to continue using the EICH attribute even if the model has already been calibrated, as the resulting initial thicknesses usually provide the best starting values when iterating the thicknesses. If both upper boundaries (OBER) and initial potential heads (EICH) are specified, both thicknesses are calculated and the smaller value between the two is used.

The thicknesses calculated in this way by the model checking module are used in horizontal models as for the iteration of the free/phreatic surface.

Analogue to the calculation of the initial thicknesses, a calculation of the maximum possible thickness is carried out by the model checking module to determine (for each element) whether confined or unconfined aquifer conditions prevail. The maximum thicknesses are determined as the difference between:

• The upper (OBER) and lower (UNTE) boundaries of the aquifer

• The surface elevation (GELA) and the impermeable layer (UNDU), with the lower boundary of the aquifer (UNTE or UNTK) subtracted from the result. calculated as: (GELA – UNDU) – UNTE/UNTK

• The surface elevation (GELA) and the lower boundary of the aquifer (UNTE or UNTK)

The calculations to determine the free/phreatic surface is carried out based on these initial and maximum possible thicknesses.