User's guide of Proceessing Modflow 5.0

98 Processing Modflow
where hn is the head at the neighboring cell that causes the dry cell to wet and WETFCT is a
user-specified constant called the wetting factor. You may select between eq. 3.28 and 3.29 in
the Wetting Capability dialog box (Fig. 3.27). This dialog box appears after selecting
MODFLOW< Wetting Capability... from the Models menu.
3.6.1 MODFLOW
Fig. 3.27 The Wetting Capability dialog box
The dialog box also allows you to specify the iteration interval for attempting to wet cells
IWETIT. Wetting is attempted every IWETIT iterations. When using the PCG2 solver (Hill,
1990), this applies to outer iterations and not inner iterations. The reason for adjusting IWETIT
is that the wetting of cells sometimes produces erroneous head changes in neighboring cells
during the succeeding iteration, which may cause erroneous conversions of those cells. These
erroneous conversions can be prevented by waiting a few iterations until heads have had a chance
to adjust before testing for additional conversions. When setting IWETIT greater than one, there
is some risk that cells may be prevented from correctly converting from dry to wet. If the solution
for a time step is obtained in less than IWETIT iterations, then there will be no check during that
time step to see if cells should be converted from dry to wet. The potential for this problem to
occur is greater in transient simulations, which frequently require only a few iterations for a time
step.
The method of wetting and drying cells used in the BCF2 Package can cause problems with
the convergence of the iterative solvers used in MODFLOW. Convergence problems can occur
in MODFLOW even without the wetting capability but problems are more likely to occur when
the wetting capability is used. Symptoms of a problem are slow convergence or divergence
combined with the frequent wetting and drying of the same cells. It is normal for the same cell
to convert between wet and dry several times during the convergence process but frequent
conversions are an indication of problems. As a matter of fact, situations exist where the real
solution oscillates such as in the case of a well causing a drawdown which makes the well cells
fall dry. This in turn switches off the well and leads to a rise of the water table and wetting of the