Weird “unable to converge” problem

Today I tried to solve the converge problem which prevent my model from simulating groundwater flow in more than 1 time step or stress period. I deactivated all modules one by one, then tried to adjust time parameters. None of these work.

Then I adjusted layer type. When I set layer 2--11 as type 0: confined, and layer 1 as type 0: confined or type 2: confined/unconfined with constant transmissivity, the simulation can work until stress period 51, time step 9. Otherwise the model cannot converge and simulation stops.

It was found later that both type 0 and type 2 can be assigned to layer 1 and 2, but type 3 and 1 will make converge problems. If type 2 is assigned to other layers, the problem also occurs.

Another model is tested. It has 30x30x3 cells and similar configuration with my working model. Strange enough, this model works well. I doubt the transmissivity settings may lead to the problem of my working model.

Another file gives possible causes of the problem. Some are very possible to present in this simulation. I will need to see to them. If there are questions unsolvable without acceptable changes of the model, I may consider using conceptual models instead of spatial explicit model.

The result of many times of simulation, variable control and trial error: the dry cells generated during simulation is the key reason of convergency difficulties. MODFLOW is not suitable to deal with non-saturate flow. My counter measure thus is to use more carefully designed parameters in the model, do a single-period, single-time-step simulation and identify those cells destined to become dry and exclude them (be assigning them as inactive cells) in consequential multi-period simulations. Besides, there are some ``holes'' in the DEM, if they are in my concerned area, I should define them as inactive cells and pay attention to their adjacent cells to ensure they have correct elevation data.