MODFLOW 6 GHB question

[Nick Lyle]

MODFLOW 6 prevents me from defining a GHB with a head lower than the bottom of the cell. MODFLOW 2005 was perfectly happy to let me do this, and conceptually I don't see a problem with it. Why was this disabled?

[Boyce, Scott E]

I know that allowing the GHB head to be below the cell bottom was disabled in MODFLOW-NWT, then re-enabled for MODFLOW-OWHM (https://code.usgs.gov/modflow/mf-owhm).

While I dont use MODFLOW-6, they probably decided to follow what was done with MODFLOW-NWT. I think conceptually it is fine.

I think it might lead to problems for "convertible" layers with the Newton formulation because--at least in the MODFLOW-2005 variant of NWT--cells never really go dry because veritical conductance is held constant at the confine value. You could have a situation, where the GHB is below the bottom of a convertible layer which then pulls water out of the model from the surrounding cells, where traditional MF-2005 would just set the cell to IBOUND=0 and disable the GHB.

Its very semantic, but you might see if the GHB works for CONFINE layers or not using a newton formulation.

Hope that helps out

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Sent: Monday, October 3, 2022 8:56 AM
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Subject: [EXTERNAL] [MODFLOW] MODFLOW 6 GHB question

 

 

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MODFLOW 6 prevents me from defining a GHB with a head lower than the bottom of the cell. MODFLOW 2005 was perfectly happy to let me do this, and conceptually I don't see a problem with it. Why was this disabled?

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