Computational Layers

The vertical discretisation in the saturated zone can be defined in two ways:

· by the geological layers, in which case there will be one calculation node in each geological layer,

· by explicitly defining the lower level of each calculation layer.

Vertical discretisation

Defined by the geological layers - Groundwater flow in a multi-layer aquifer can be described by a model in which the computational layers follow the interpreted geological layers. Each layer is characterised only by its base level specified either by a constant level or by a distributed file. The number of numerical layers will be identical with the number of geologic layers.

Explicit definition of lower levels - If you define the vertical discretisation explicitly each computational layer is defined by its lower elevation.

Bottom elevation correction

There are two corrections that may be made to the layer elevations.

Minimum layer thickness - The minimum thickness of the calculation layers must be specified to adjust the geological model or the specified levels to prevent layers with zero thickness or very thin layers. Otherwise, very thin layers may cause numerical difficulties. The default value is 0.5, which is usually sufficient. This means that if the two geological layers or specified layer elevations approach one another then the bottom layer will be pushed down to maintain the minimum layer thickness.

Adjust top SZ layer thickness to the initial water table - In principle, the UZ calculates vertical infiltration in the unsaturated zone and the SZ calculates 3D saturated flow below the water table. However, the UZ and SZ layer elevations are defined independently of one another. In MIKE SHE, the UZ flow is only calculated to the water table, or to the bottom of top SZ layer. Any UZ cells below this are ignored and outflow from the UZ model is added to the top layer of the SZ model. This option allows you to more easily keep the UZ and SZ models consistent based on the depth of the initial SZ water table.