User's guide of Proceessing Modflow 5.0

110 Processing Modflow
3.6.2 MOC3D
MOC3D < Subgrid...
Within the finite-difference grid used to solve the flow equation in MODFLOW, the user may
specify a window or subgrid over which MOC3D will solve the solute-transport equation. This
feature can significantly enhance the overall efficiency of the model by avoiding calculation
effort where it is not needed. However, MOC3D requires that within the area of the transport
subgrid, row and column discretization must be uniformly spaced (that is, )x and )y must be
constant, although they need not be equal to each other). The spatial discretization or rows and
columns beyond the boundaries of the subgrid can be nonuniform, as allowed by MODFLOW,
to permit calculations of head over a much larger area than the area of interest for transport
simulation. Vertical discretization, defined by the cell thickness, can be variable in all three
dimensions. However, large variability may adversely affect numerical accuracy. For details,
refer to Konikow et al. (1996) for the model assumptions that have been incorporated into the
MOC3D model.
The subgrid is defined in the Subgrid for Transport (MOC3D) dialog box (Fig. 3.34).
MOC3D assumes that the concentration outside of the subgrid is the same within each layer, so
only one concentration value is specified for each layer within or adjacent to the subgrid by using
the C’ Outside of Subgrid table of this diglog box. The values of other layers, which are not
within or adjacent to the subgrid, are ignored.
3.6.2 MOC3D
Fig. 3.34 The Subgrid for Transport (MOC3D) dialog box