iMOD User Manual version 4.4 (html)

12.2BND Boundary conditions

The boundary conditions (-) consist of one IDF (or a constant value) for each modellayer specifying for each cell whether

The cell values correspond with the IBOUND values specified in the MODFLOW BAS package.


Figure 12.2: Example of the boundary conditions for a single layer (source McDonald and Harbaugh, 1988)

12.2.1BND Scaling

For different grid resolutions the boundary is scaled internally via iMOD or iMODFLOW. For downscaling (e.g. from an input of 100 x 100 meter to a finer resolution of 10 x 10 meter) all finer grid cells obtain the identical value of the original cell. No interpolation of boundary conditions is performed. That means that the image looks identical to the original dataset but then at a finer resolution. For upscaling on the other hand, a different approach is used, which can be described best by the following rules that lists the steps to come from local values (the original values of the fine grid) to the global value (the upscaled courser grid):

To summarize the above, fix boundary conditions goes before variable locations, which go before inactive locations. As a consequence, an up-scaled model is always equal or larger in size than its finer representation, as well as the total size of fix boundary conditions. To compare an up-scaled model with its finer representation might yield differences in fluxes from boundary conditions as well as differences in RCH, EVT, UZF packages.