Utilising the watercourse network in SPRING ensures that the upstream groundwater volumes are taken into account in the downstream exchange balance. This ensures that when sections of watercourse dry up, only as much water can seep out of the watercourse as the upstream volume allows, e.g. due to seasonal factors.

The underlying formula for the simplified derivation of the water level in the water body is as follows:

(Quelle: U.S. Geological Survey: Open-File Report 88-729; Formel 5; 1989)

The approach is only valid for a ratio of b >> h.

With:

• Water level h (above water course bed)

• outflow q

• bottom gradient I

• Manning/Strickler coefficient k_St

• Wetted perimeter b

If attributes for the water system calculation are available in the model file (VINZ, VGRD, VSYS) an additional input area for the water system calculation appears when the SITRA module is selected:

Maximum change [m³/s]

When determining the groundwater-based runoff at a node, the rates that flow from the upper reaches of the watercourse to the node are taken into account. These leakage rates are totalled for each iteration from the headwaters to the node under consideration in order to evaluate how much water can seep away at the corresponding potential head (< water level). This infiltration in turn causes a change in the leakage rates and thus a change in the groundwater-based runoff. This iteration is limited by the specification of a maximum (rate) change.

Maximum number of iterations

When iterating the leakage rates, a very high number of calculation steps may be required before the maximum (rate) change is reached. In extreme cases, no convergence is achieved. For this reason, the number of iterations is limited at this point.

As soon as a restriction (max. (rate) change or max. number of iterations) is reached, the iteration of the groundwater-based discharge is cancelled.

Advanced settings in the module GEONEU