A streamline describes the boundary beyond which no water is transported, i.e. the mass flow rate of water between two streamlines remains constant (as long as there is no infiltration from above). From a theoretical point of view, such a calculation is only possible for 2D horizontal models.

In the streamline calculation, the mass balance is established in each node. The streamlines are determined by solving the resulting system of equations. The starting points for the streamlines are also determined from the node balances. The user defines a threshold value for the mass flow rate of water for each streamline.

The streamlines are then distributed according to the mass flow rate of water flowing over the adjacent element sides by adding up the rates until the selected threshold value is reached. The streamlines can then be traced from the points found on the element sides through the next element and then over the next edge. The starting points along the boundary or along receiving watercourses can also be determined.

Backward-calculated streamlines end at an element side to a node that has an infiltration greater than the threshold value. This can either be a boundary node at which inflows over the boundary have been calculated or any node in the area with infiltration (also from new groundwater formation).

As a rule, groundwater recharge must be taken into account in a horizontal model. It is distributed to all nodes of the mesh as a small infiltration. This gives each node an inflow and can therefore be the end point for tracing a streamline backwards. This leads to a thinning of the path lines, e.g. along the catchment area of a well, and thus represents the distribution of groundwater recharge. This effect is only clearly visible if a large number of streamlines have been generated for each well.

In addition to the flow paths of the groundwater, the streamlines calculated in this way also provide an overview of the flow rates, so that large rates can be assumed for closely spaced streamlines and small rates for isolated streamlines. A streamline image is also a good way of checking a calibration. If the K value in individual elements is very small or very large compared to the surroundings, this is shown by a sharp bend in the streamlines.

The visualisation of the streamlines is strongly influenced by the quality of the FE mesh, as large discontinuities at the element edges lead to a large lateral offset of the lines. An important effect that is not taken into account in the path line tracing is the lateral distribution at the transition between two elements with different thicknesses. To maintain the mass, the streamlines in the element with the greater thickness must be closer together than in the element with the lesser thickness.

Generating and displaying stream lines