Regardless of the components included in your model, the first real step in your model development is to define the model area. On a catchment scale, the model boundary is typically a topographic divide, a groundwater divide or some combination of the two. In general, there are no constraints on the definition of the model boundaries. However, the model boundaries should be chosen carefully, keeping in mind the boundary conditions that will be used for both the surface water and groundwater components.
All other spatial data defined in the data tree, such as topography, is interpolated during pre-processing to the Model Domain and Grid.
You can define your model domain and the grid using either a DHI grid file (dfs2 format) or a GIS shape file (.shp format).
If you define your model domain using a dfs2 grid file, then you must define the cell values as follows:
· Grid cells outside of the model domain must be assigned a delete value - by default -1.0e-35.
· Grid cells inside the model domain must be assigned a value of 1.
· Grid cells on the model boundary must be assigned a value of 2.
This distinction between interior grid cells and boundary cells is to facilitate the definition of boundary conditions. For example, drainage flow can be routed to external boundaries but not to internal boundaries.
Since the model domain is defined as part of the dfs2 file format, if you want to change the extent of your model domain, you must edit the .dfs2 file. However, if you want to change the grid spacing, then it is probably easier to create a new file.
The Model Domain and Grid does not have to have the same dimensions (size and spacing) as other specified dfs2 files (e.g. Topography). However, if the other dfs2 input files are coincident, that is if the rows and columns align with one another, then an average of the cell values is used. If the dfs2 files are not coincident, then the Bilinear Interpolation (V1 p. 161) method is used to determine the cell value.
Note: The dfs2 files for integer grid codes must be coincident with the model grid. For more information on this see Integer Grid Codes (V1 p. 159).
Using an polygon shape (shp) file
It is much easier to define your Model Domain and Grid via a GIS polygon shape (.shp) file. In this case, the definition of integer code values is taken care of internally. Once you have defined the polygon file to use, then you specify the spatial extent and origin location of the model domain and grid.
An important advantage of using a polygon for the model domain, is that the number of rows and columns can be easily adjusted. See Using MIKE SHE with ArcGIS (V1 p. 151) for more information.
Creating dfs2 files
There is a Create button next to the Browse button that opens a dialogue where you can define a dfs2 grid file. This utility automatically creates the grid file with the appropriate Item Type.
In this dialogue, you can specify the overall dfs2 grid dimensions and origin. After you have created the file, then you can open and edit the file in the Grid Editor using the Edit button.
Geographic projections
MIKE SHE supports all available geographic projections. If you have defined the domain using a dfs2 file, then the geographic projection is defined in the dfs2 file. If you use polygon shape file, then you must defined the projection in the Model Domain and Grid (V1 p. 224) dialogue. See Using MIKE SHE with ArcGIS (V1 p. 151) for more information.
Note: All dfs2 and polygon shape files must use the same geographic projection. Any inconsistencies in the projections will results in an error during the pre-processing.
