Large imbalance errors using Phreatic vs Free surface

I have noticed very large flow imbalance errors occurring in my model when using the Phreatic method of simulating confined/unconfined conditions. Changing the top slice to "Free" improves the imbalance but degrades the model's ability to represent large vertical head gradients observed in the calibration data. I would rather use Phreatic mode for this reason as it enables the best calibration to heads and outflows.

Are there any known issues with imbalances while using the Phreatic mode? Are there any work arounds?

I have tried refinement around wells and this doesn't help. The imbalance errors are greatest when well BC abstraction is occurring.

It is a 5 layer regional scale transient flow model.

Thanks in advance...

Blair,

I don't think that there are general issues with imbalance, but the solution of course gets more difficult in the phreatic case due to potentially large parameter contrasts. I'd recommend to turn on the error norm visualization (in the context menu of the User Data section in the Data panel) to see whether there are regions in the model with a large remaining error. Having idenified those, you might have a better insight into potential solutions.

Good luck!
Peter

Phreatic mode doesn't work very well in cases where the water table must pass through several layers of the model, such as may happen in mountainous terrain.

Pete