Problem with dry cells......

[Hugo]

Hello Everyone,

I'm trying to run a transient flow model in Visual Modflow. However, I
do have problem with dry cells; if the model runs with rewetting
option no activated model results will show several dry cells. Few dry
cells are found at the beginning of the simulation but the number of
dry cells increases at the end of the simulation. I have tried to
activate the Re-wetting option but model does not converge. I have
change rewetting setting but they did not work.

Does someone have this problem that could tell me how it was solved?

Do you think that need to uses Modflow 2005 engine?

Please, any comments are welcome.

Thanks

Hugo Perea

[Richard B. Winston]

Switching to MODFLOW-2005 is unlikely to help. Try changing WETDRY to a
a larger value.

The following is copied from the help for the MODFLOW GUI

Stability Problems in MODFLOW

Use of the wetting capability (first implemented in BCF2 and retained in
BCF3, BCF5, LPF and HUF) can result in non-unique solutions because the
head in a cell must be higher than some wetting threshold. If a cell
starts off wet, it can remain active even if the head drops below the
wetting threshold. However, if it starts out dry, it may not be wetted
because the head in the neighboring cells may be too low.

Use of the wetting capability can cause serious problems with
convergence. You can try to avoid this by several methods.

1. If you know a cell should never become wet, make it an inactive cell
rather than a variable head cell.

2. You can adjust the value of the wetting threshold in WETDRY. (Higher
is more stable but may be less accurate.)

3. You can decide which neighbors will be checked to decide if a cell
should be wetted using WETDRY. Often it is better to allow only the cell
beneath the dry cell to rewet it.

4. You can use IHDWET to determine which equation is used to specify the
head in newly wetted cells.

5. You can vary the wetting factor WETFCT.

6. In steady-state conditions you can adjust initial conditions to
values that are close to your best guess of the final conditions to
improve stability.

7. You can choose a different solver. The SIP, PCG1, and PCG2 solvers
will work with the wetting capability. The SOR solver doesn't work well
with the wetting capability. Note that cells can not change between wet
and dry during the inner iterations of the PCG1 and PCG2 solvers. The
PCG1 solver is no longer included in the USGS version of MODFLOW and is
not supported by the MODFLOW GUI.

8. When using the PCG2 solver, you can set RELAX in the range of 0.97 to
0.99 to avoid zero divide and non-diagonally dominant matrix errors.
(However, this is an infrequent cause of instability. If such an error
occurs, PCG2 prints an error message in the output file and aborts the
simulation.)

9. When using the PCG2 solver, you can set DAMP to a value between 0 and 1.

10. Unrealistically high conductances on boundary cells can contribute
to instability. Check the conductances in the Drain, Drain Return,
River, Reservoir, Lake, Stream, Streamflow Routing and General-Head
Boundary packages. In the Evapotranspiration check the EVT Flux
Stress[i] and EVT Extinction Depth which together control the
conductance of evapotranspiration cells.

11. Run a steady-state model as transient so that cells go dry in a more
orderly fashion. You would obtain the steady-state solution by running
the transient simulation for enough time steps to cause the storage
budget term to approach 0.

The two most important variables that affect stability are the wetting
threshold and which neighboring cells are checked to determine if a cell
should be wetted. Both of these are controlled through WETDRY. It is
often useful to look at the output file and identify cells that convert
repeatedly from wet to dry. Try raising the wetting threshold for those
cells. It may also be worthwhile looking at the boundary conditions
associated with dry cells.

Sometimes cells will go dry in a way that will completely block flow to
a sink or from a source. After that happens, the results are unlikely to
be correct. It's always a good idea to look at the flow pattern around
cells that have gone dry to see whether the results are reasonable.