The boundary conditions (-) consist of one IDF (or a constant value) for each modellayer specifying for each cell whether
• Boundary value \(<0\)
Those values denote areas that fixated head. The model will not change these values and they act as a fix boundary condition;
• Boundary value \(=0\)
Those values denote areas that are excluded for the simulation. No groundwater flow will go through those areas;
• Boundary value \(>0\)
Those values denote areas that take part of the simulation, groundwater flow goes through them and the head are computed. An important constraint to those locations is that need to be connected to at least a single fixed boundary condition, e.g. a boundary condition \(<0\) or one of the other packages that are head-dependent, such as the RIV, GHB, DRN package. The latter could be risky since that boundary condition might be removed whenever the head is below the drainage base.
The cell values correspond with the IBOUND values specified in the MODFLOW BAS package.
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):
1. If the local value is equal to its NodataValue it is set to zero, it becomes inactivated;
2. If the (corrected) local value is less than 0.0 (fix boundary condition) the global value becomes equal to the local value;
3. If the global value is equal to zero and the local value is greater than zero, the global value becomes equal to the local value.
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.