About GHB and 5 layers too...

[Lucía M. Ariza]

Hello everyone,

I’ve seen this thread today about assigning General Head Boundaries (GHB) in a five-layer MODFLOW model built in GMS, and I am dealing with a  similar situation, so I would like to ask for some guidance.

I am working with a 5-layer model in GMS that contains two aquifers:

• an upper, unconfined , and

• a lower aquifer that is partially confined by an impermeable layer.

I have been able to successfully calibrate the upper aquifer using GHBs along the model boundaries, and the simulated heads match the observed values reasonably well. However, I am struggling with the lower aquifer.

Observed heads in the lower aquifer are typically 1–2 m lower than in the upper aquifer. When I try to impose GHBs for the lower aquifer in order to reproduce this difference, the model often fails to converge. My impression is that the problem may be related to the fact that the GHBs for the lower aquifer spatially coincide (in plan view) with the GHBs already imposed for the upper aquifer, even though they are assigned to different layers.

I am wondering:

• Is there any recommended workflow or guideline for applying GHBs in multilayer systems with two hydraulically distinct aquifers?

• Is it generally advisable (or not) to place GHBs for different aquifers along the same model boundaries?

• Are there best practices to impose a small but consistent head difference between aquifers without causing numerical instability or convergence problems?

Any advice, references, or practical recommendations would be greatly appreciated.

Thank you in advance.

Best regards,

Lucía