GMS transient flow model

[ravichander]

Hi,

I'm trying to run a transient flow model for 28 days. I have constant head arcs (stresses) that turn on and off during various times during 28 days (for example 7 to 14 days or 14 to 21 days). When i run the flow model GMS is assigning the constant head to the grid cells at the location of arcs for the entire duration (28 days) when the constant head has been assigned for only few days. Not sure why GMS is extending the constant head stress to the entire duration. If i go into grid source/sinks and look at various stress periods, GMS autimatically uses data from previous stress periods. I have stress periods setup correctly.

Any suggestions would be greatly appreciated.

[ravichander]

I have defined the stress periods as follows

SP1- 0-7 days

SP2-7-14 days

SP3-14-21 days

SP4-21-28 days.

I have xy series for some arcs 7-14 days and 14-28 or 21-28 days for some with some constant values. But GMS is assigning the constant head to the entire duration 0-28 days. How do i overcome it?

[Sean Czarniecki]

If I'm hearing you correctly, it sounds like you are trying to set a constant head for a set time period and then turn off the constant head so that things recover. I'm sitting here thinking that you can't do that (but I could be missing something). Once you make a cell a constant/transient head boundary condition, you need to define how that head changes and don't really get to turn it off. You would have to set up a different run where you pull in the results of the first run.

Again, maybe I'm not thinking about it clearly right now....I'm sitting in a gym.....

[woodward]

Is this a MODFLOW model?

[ravichander]

Yes. This is a modflow model and Sean Czarniecki's description of the issue is correct.

Sean, Setting up a different run and pulling in the results of first run will not help since modflow solution does not depend on the initial head solution. This is a transient model, so i want to see influence of change in various constant stresses over time. Not sure if this is a GMS issue or a modflow issue.

[Sean Czarniecki]

So if you run your first stress period as your 1st run, then pull the last time step from your solution into your next run's starting heads...change the stress.....etc. This will give you what you are looking for. Not easy, but that's the work-around.

[Rob Virtue]

Another option would be to use a general head boundary instead, with heads as you are using for the constant head boundary, and use the conductance values to turn it "on" and "off".

[ravichander]

Hi Rob,

How can i use the conductance to turn on/off the constant heads? Can you please elaborate?

I tried Sean's approach and it works. But i need multiple models for that. Maybe with general head approach one model might suffice.

[Rob Virtue]

If you have a high conductance value for the general head boundary relative to the cell K, it will act as a variable head cell as there is not a significant restriction to flow. If you drop the conductance to a very low value relative to K (conductance can be varied over time just like head) flow is restricted so that the is negligable exchange to and from the GHB term. By varying the conductance, the GHB can be used like adding or removing a constant/variable head boundary, without the need to stop and start the model.