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julian.weir

MODFLOW-USG: Recharge Mapping

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julian.weir    0

Hi all.

I have a problem mapping recharge to MODFLOW-USG within GMS (version 10.2.4).  My MODFLOW-USG model has multiple layers.  Layer 1 pinches out to expose layer 2 at the surface, and similar for deeper layers (that's how GMS created the 3D grid from horizons and TINS etc).  When I map recharge to MODFLOW (steady state rates for now, but will use transient rates later), GMS maps the recharge fine to the surface where layer 1 is exposed, but will not map anything to the surface where layer 2 (and below) is exposed.  So only part of the recharge is mapped to MODFLOW.

I have discussed this with GMS support, and they have suggested that I add a thin upper layer (a new layer 1) to the model over the whole model domain, and set this layer to inactive. Recharge would then map to this new layer 1, and pass through this inactive layer (if the right settings are selected) to the first active cell below.  However, I also have a SFR2 coverage that needs to map to the uppermost layer, and this will not map to inactive cells.  This also creates errors with river bed inverts passing below the bottom elevation of layer 1 cells.  I therefore don't think this is a suitable solution.

GMS support have been very helpful, but they are very busy and response times are long.  Therefore, I was wondering if anybody else has had this issue arise, and if they have found an alternative work around.  Is it a GMS mapping problem or a MODFLOW-USG limitation?  I would have thought that having underlying layers outcrop to the surface is not uncommon, and so it would be unusual for MODFLOW-USG to not support this.

Any help would be appreciated. Thanks.

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Can you make the extension of the thin Layer 1 basically the same material/properties as Layer 2 such that you don't have to inactivate it.  For example, make your outcrop Layer 2 one foot thinner, and extend Layer 1 in that area (one foot thick)....then make all properties the same as Layer 2 (need to make sure that your vertical properties work such that flow doesn't get held up at the boundary between layers).  Just a thought.

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Alan Lemon    3

I think the response from support on recharge is the easiest way to work with USG so that the "highest active cell" option works.

I believe the current implementation of SFR mapping is to a single layer. So the answer to your question above is... Yes I think it is a GMS mapping limitation.

Other boundary conditions have this option: http://www.xmswiki.com/wiki/GMS:Map_to_MODFLOW#Automatic_Layer_Assignment

You could add a feature request for this feature to work with SFR.

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julian.weir    0

Thanks Alan.  I'll submit a feature request regarding the SFR2 mapping.  Regarding the RCH mapping, I'm still not sure if the lack of mapping to surface layers 2 (or higher) is a GMS limitation or a MODFLOW-USG limitation.  If it is a GMS limitation only, then I could also add that as a feature request.  But if it is a MODFLOW-USG limitation, then I would have to look more at an alternative workaround (or roll back to MODFLOW-NWT or similar).

That 'Layer Assignment' link is useful.  At the very bottom it states that with MODFLOW-USG, the specified layer option is not supported by Map > MODFLOW with RCH package.  So perhaps it is a MODFLOW-USG limitation (i.e only layer 1, and not auto assign to 1 cell).  Can you confirm please?  Thanks.

Thanks also Sean for the suggestion.  I would have thought that that is what GMS would do when generating a grid (set the upper most cells all to layer 1, regardless of the material properties).  It'll be quite a lot of work to manually assign a new top layer to the various material properties, but it could be done.  But I think I may still be stuck with a thin upper layer that then causes potential issues with SFR2 package inverts passing below the lower elevations of the cells.

Edited by julian.weir
Misspelled name.

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Just as a follow up based on your concern, the "thin upper layer" can be as thick as you want it to be to avoid that issue you mention.  You can design your layers to meet your needs.  Could take a little time, but shouldn't be too bad.  Try sketching it out on paper first to visualize how to do it.  First the way it currently looks, then with new layer divisions and material properties.  Just a little imagination and it will work.....

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julian.weir    0

Thanks Sean.  I'll look into that more and see if the extra effort is worth it, or whether I just roll back to an earlier version of MODFLOW.  There are other limitations with USG there seems too, such as no PEST-ASP module.

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Alan Lemon    3
20 hours ago, julian.weir said:

Thanks Alan.  I'll submit a feature request regarding the SFR2 mapping.  Regarding the RCH mapping, I'm still not sure if the lack of mapping to surface layers 2 (or higher) is a GMS limitation or a MODFLOW-USG limitation.  If it is a GMS limitation only, then I could also add that as a feature request.  But if it is a MODFLOW-USG limitation, then I would have to look more at an alternative workaround (or roll back to MODFLOW-NWT or similar).

That 'Layer Assignment' link is useful.  At the very bottom it states that with MODFLOW-USG, the specified layer option is not supported by Map > MODFLOW with RCH package.  So perhaps it is a MODFLOW-USG limitation (i.e only layer 1, and not auto assign to 1 cell).  Can you confirm please?  Thanks.

Thanks also Sean for the suggestion.  I would have thought that that is what GMS would do when generating a grid (set the upper most cells all to layer 1, regardless of the material properties).  It'll be quite a lot of work to manually assign a new top layer to the various material properties, but it could be done.  But I think I may still be stuck with a thin upper layer that then causes potential issues with SFR2 package inverts passing below the lower elevations of the cells.

I can confirm your second statement. No layer option with the Map->MODFLOW with RCH when using MODFLOW-USG.

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julian.weir    0

Hi Alan.  Thanks for your response on mapping SFR2 to multiple surface layers.  It will be good to have that available in the next release of GMS.  Would it be possible to look at doing the same for recharge mapping too?

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Alan Lemon    3

The recharge mapping has also been implemented in the development version of GMS. We will also have a tutorial that explains how it works with USG.

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Greg    0

Hi Julian,

I worked through this problem in another way with the help of Alan.  I think the inactive layer is an easier alternative than what we did, but if other features don't work with the inactive layer then this may help. In general what we did is develop an array to identify the top cell in the UGrid.  This was done by assigning a dummy KH/KV to the top cells which allowed us to get a list of the cell ids for the top cells.  We then saved as native text and re-wrote the recharge file using recharge option 2 and assigning the recharge rate to the top cells and then 0 to all the others.  From there I loaded the native text files back into GMS/MODFLOW and removed recharge from the conceptual model and reset the KH/KV with the appropriate value.  There are some minor steps and details about how to set up the native recharge files, but I can't remember them off hand nor is it easy to describe. 

Alan, if I got something mixed up on this please correct me.

In the future I will always have layer 1 extend across the model domain when setting up a USG model.

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