Jump to content
GMS, SMS, and WMS User Forum
Sean Czarniecki

MODFLOW-USG & CLN cells staying wet

Recommended Posts

I've been given a MODFLOW-USG model (including use of the CLN package) whereby the upper layer is Convertible Upstream.  The model won't converge if the layer is just set at Convertible.  Fine.  While I'm fairly new to USG (still reviewing the calculations/equations at this point), I can accept that the model needs that layer to be Convertible Upstream.  My real issue is that cells in that layer which would normally be "dry" (head below the bottom of the cell), stay active and report back the head (and use in contouring) which is below the bottom of the cell.  If the layer were confined, I could understand it, but I'm guessing that there is something that I don't know about "Convertible Upstream" which is keeping these cells active, even though they should be dry.  Anyone have insight into this?  I don't really see it as a problem (the model results seem correct and water just passes through the cell to get to the lower layer), but rather something I'm curious about, as I'm not used to seeing "dry" cells stay active before except when modeling the layer as confined.  Thanks!

Share this post


Link to post
Share on other sites

It may be that because they are convertible cells they stay active. Although they never do get wet in your simulation, as convertible cells they have the potential to dry and rewet, so they may have to report back as active just because of that. Not sure if that's the reason why, but it could be.

I found that info in the MODFLOW-USG manual on page 20 if you want to look more into it.

Share this post


Link to post
Share on other sites

Thanks - I read that, but it wasn't as clear as I would have liked it to be, as other versions of MODFLOW have convertible cells, but once they get dry, they stay dry.  The text in the manual doesn't make it clear (to me) that it applies to more than just the CLN cells (and neighboring GWF cells).  I guess it does!  Thanks again for the response!

Share this post


Link to post
Share on other sites

I think the behaviour you are describing relates to the Upstream Weighting scheme of MF-NWT, that is implemented in MF-USG as Upstream Convertible. See this report, chapter treatment of dry cells.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×