# iMOD User Manual version 5.2 (html)

#### 13.4Warnings, limitations and known issues

##### 13.4.1Clipping and scaling

When RUNTYPE = MT3DMS, scaling to other resolutions than the resolution on which the groundwater fluxes are available is not possible. That would require scaling of the flow field, which is not implemented.

When RUNTYPE = MT3DMS, clipping (= choosing a modelling extent which is different (smaller) from the extent of the files in the DBASE), is possible, but mass cannot move over the model boundary in this case. A contaminant plume that reaches a lateral model boundary will accumulate next to the boundary. This is a known issue that will be resolved shortly. A workaround for now is to choose the model domain large enough so this does not happen or this does not affect results in the areas of interest.

When clipping and/or scaling, it is strongly recommended to choose the model domain such that the model boundaries are aligned with the grids in the DBASE. Unexpected behaviour may result from having model boundaries cut through grid cells in DBASE grids.

For the CHD package, iMOD-WQ follows standard MODFLOW-2000 conventions, which is different from the MODFLOW version of iMOD (iMODFLOW):

• • In iMODFLOW, a CHD is invoked for a certain cell by setting IBOUND < 0 for that cell, in combination with a value $\neq$ NODATA in the CHD package. If the CHD value = NODATA, the starting head value is used as constant head value.

• • In iMOD-WQ, a CHD is invoked for a certain cell by setting IBOUND > 0 for that cell, in combination with a value $\neq$ NODATA in the CHD package. If the CHD value = NODATA, no CHD boundary condition is imposed. If IBOUND < 0, the starting head is used as constant head, also when the CHD value $\neq$ NODATA.

##### 13.4.3Automatic allocation of boundary conditions to layers

Except for the WEL package, iMOD-WQ does not support automatic allocation of boundary conditions to model layers, in contrast to iMODFLOW (via assigning layer number = 0).

##### 13.4.4Transport calculations for unconfined model layers

iMOD-WQ does not yet support MT3DMS and RT3D calculations based on fluxes from a flow model that has unconfined layers. The computation of the necessary THKSAT variable for such calculations has not yet been implemented.

##### 13.4.5Double Precision

iMOD-WQ is not (yet) able to read double precision IDF’s. Therefore, when using iMOD to produce IDF’s that serve as input for iMOD-WQ, make sure that these are single precision (i.e., when creating an IDF from scratch in the GUI, make sure that the “double precision” checkbox is deselected).