One of the most important steps in defining a MODFLOW model is to define the layer elevations. The layer elevations are stored in the Global Options/Basic package. The layer elevations include a top elevation for layer 1 and a bottom elevation for all layers. For all layers except for the top layers, the top elevation for the layer is assumed to be equal to the bottom elevation of the layer above.
As described above, MODFLOW models can be defined using one of two approaches: (1) by editing the input values on a cell-by-cell basis directly on the grid, or (2) by creating a high level conceptual model using the Map module. Both approaches can be used to define the layer elevation arrays. With the grid approach, a constant value can be assigned to the entire array at once or to a set of selected cells. With the conceptual model approach, a set of polygons can be used to define zones of elevations within each layer. While these two approaches are simple to use, in most cases they result in an overly simplistic stair step definition of the layer elevations. In most cases, it is more appropriate to use the 2D geostatistical tools in GMS to smoothly interpolate layer elevations.
The following steps are used to interpolate elevation data:
Solids can also be used to define layer data for MODFLOW. The Solids -> MODFLOW and Solids -> HUF commands in the Solid module set up the layer elevations using the currently defined solids.
Related Links:
Building a MODFLOW Model
MODFLOW Interface