MODFLOW-LGR (Mehl and Hill, 2005, 2007, 2010b, 2013) allows Local Grid Refinement to be used in a MODFLOW model. With local grid refinement, a child model can be embedded inside a parent model. The packages used in the child and parent model can be different. However, ModelMuse does not support using a different main flow package in the parent and child models. In addition, any parameters that are defined will apply to both parent and child models.
In the version 1 of MODFLOW-LGR (Mehl and Hill, 2005), certain nodes are shared between the child and parent model. MODFLOW-LGR version 1 treats the cells at the edge of the child model as specified head cells with the specified heads being calculated by the parent model. In the parent model, the shared cells are treated as specified flow cells with the flows being calculated by the child model.
In version 2 of MODFLOW-LGR (Mehl and Hill, 2013), there are "ghost nodes" just outside the edge of the child grid that are used to coordinate the transfer of fluid between the parent and child grids.
To arrive at a solution, both versions of MODFLOW-LGR iterate between solving the parent and child models.
The MODFLOW-LGR version 1 (Mehl and Hill, 2005) documentation points out that certain input variables need to be adjusted in the shared nodes in order for the conceptual model to be properly represented in the numerical model. (Such adjustments are not required in MODFLOW-LGR version 2.) For example, if the recharge rate of the parent cell is applied to the child cells without any adjustments, some of the recharge will be applied twice: once in the parent model and then again in the child model. ModelMuse makes the required adjustments automatically. For most boundary conditions, the flow rate or conductance of the parent cell is reduced so that only the unshared area of the parent cell is used for the flow rate or conductance. In the SFR package, the adjustment is done slightly differently. The entire length of a reach in a shared parent cell is allocated to the appropriate reaches of the child model and the reach in the parent cell is placed in a segment that will be deactivated by MODFLOW-LGR after the first iteration.
Sometimes the user may wish to apply different values of some input value in the parent and child models. The MODFLOW-LGR functions are provided for that purpose. For example, it might not be appropriate to use the same conductance of a boundary in both the parent and child models (Mehl and Hill, 2010a).
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