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Observations with MODFLOW-USG


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Hi all.  I have recently set up a MODFLOW-USG model, one that was previously constructed as a MODFLOW-NWT model (and worked very well).  I have found that the values of heads in some cells calculated by GMS are vastly different to the heads in the cells calculated by MODFLOW.  Has anybody found this before?  It may have to do with interpolating between unsaturated and saturated cells.  I’m not sure if it is a GMS bug or my operating error. I have contacted GMS support and they are looking into it, but they are extremely slow at responding. Any tips would be appreciated.  Thanks.

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I think the difference may be in the MODFLOW build itself.  While working with MODFLOW-USG a couple of years ago (which, if I understand correctly, would use similar code to MODFLOW-NWT), an updated version came out which changed how calculations were performed for "dry" cells (due to users who didn't agree with how the solver handled those cells).  It changed my model results and would have negated the work I had done for half a year.  Working with the GMS developers, we solved it by using the older MODFLOW executable during my model runs.  It is easy to do this by directing GMS to run the older executable from another location.  I would check the year that the output file says the NWT results were produced and then look at the USG output for the version there.  I think that the new version of USG came out in 2018.  I have had to use the previous version (2015) to run my model.

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Thanks for that idea.  I have trialled using GMS 10.4 (with MODFLOW-USG 1.5, 2019), GMS 10.3 (with MODFLOW-USG 1.4, 2017) and GMS 10.2 (with MODFLOW-USG 1.3, 2015), and all gave about the same solution, and all had the same large differences between MODFLOW head values and corresponding observations reported by GMS.  So, it still looks to be a GMS bug.  I'll see what the support team can find.

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Okay - so the way your first post was worded, it sounded like you were getting different head results in the NWT than the USG runs.  Your latest post suggests that what you are comparing are "observation" results (which is probably why the topic is titled as it is).  Can you confirm that?  Can you also confirm that by manually checking cells in the NWT and USG simulations that the heads are similar?  I know that I had issues with observations in my USG simulations (and seem to recall asking about it as well - there might even be a post on this board about it), so I managed it in other ways (select specific cells and get their results manually - not much fun, but I didn't have time to do anything else). 

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Yes, it is the observations that I cam comparing.  There are differences between the NWT and the USG solutions, but not huge, depending on how the unsaturated cells are treated.  There looks to be a consistent bug with how GMS interpolates head to the observation location.  I have also received a message from someone else who has experienced the same problem when the observation elevation is specified by either 'Z location' or by 'Use well screen'.  And as you have found, setting the option to 'By layer number' does not have this problem.  In my case I have hundreds of observations, so it will be very tedious to manually assign the layer number manually for each.  I have sent GMS support additional information along these lines, so we'll see what they come back with.  It just takes ages (many weeks) for a reply.  Thanks.

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