The convergence criteria such as HCLOSE and RCLOSE in the PCG2 package may be too strict or the number of iterations in the solver may be too small. Adjusting other parameters of the selected solver or changing to a different solver may also be helpful. For large models, the GMG solver can prove helpful.
If the solver has the capability of printing out additional information about the solution process such as by setting IPRPCG to 1 in the PCG2 package, it can be worthwhile to examine the solution information to gain insight into what is causing the problem.
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 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 PCG2 solver. |
| 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. |
| 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.
If the Lake package is being used, it may be worth checking whether the value of THETA is appropriate.
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