Cells Dry Issue and How to Set Initial Head

[Htun Thiri Naing]

Hello,

I am a master student and currently working on Modflow for my master thesis.

I am having cells dry issue (picture below). Mostly cells going dry in that white area. 

My topography varies a lot. Topography is elevated to the north of the river (blue line), I would thus expect deeper ground water levels at higher elevations, vice versa for monitoring wells located at a lower topography. Assuming an initial head of ca. 14 m for the model domain is a big oversimplification of the actual situation. Head data from monitoring wells across the domain roughly vary between 50 cm up to 7 meters below ground level. 

How can I start the initial head and so cells won't go dry in those elevated areas?

Thank you!

Best regards,

Thiri

[Abioye, R. Damilola]

Hi Thiri,

It is best to start with a steady-state simulation to establish a reliable estimate of the initial head, which helps the model begin from a good starting point. Did you mean that you expected to see higher water elevations in areas with higher topography? If so, that’s a reasonable assumption. If you are seeing a different result, it may indicate that your boundary conditions need to be reviewed .  If some of your cells are going dry, consider using MODFLOW-NWT. It's well-suited for handling issues related to cell drying and rewetting. Also, could you clarify what boundary conditions you are using? If your boundary conditions are not correct, you may also be having erroneous output.

Best regards,

Riliwan Damilola ("Dami") Abioye

PhD Candidate in Geology (Hydrogeology)

School of the Earth, Ocean & Environment (SEOE)

University of South Carolina

29208 Columbia, SC 

Official: rab...@email.sc.edu, rab...@seoe.sc.edu

Personal: abioyed...@gmail.com

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[Htun Thiri Naing]

Dear Riliwan,

Yes, this is steady state simulation. 

So in those areas the elevation is between 18 and 20 m. The heads observed from the monitoring wells near those areas are between 12 and 13 m. 

My boundary conditions are wells (15 wells), infiltration ponds (recharge package), river (river package), recharge, evapotranspiration and GHB (general head boundary package).

The elevation near the wells are around 13 and 14 m and the head values from monitoring wells are between 12 and 13m. So it is a huge difference with the upland (i.e. 18 and 20 m elevations).

How can I solve this? Cells mostly dry in the first layer whose thickness is maximum 5 m. I tried in NWT too and giving the first layer as confined does not have cells dry problem but it is not realistic because this first layer is unconfined. I am using Modflow 6 and Wetting option is also active. But still cells dry ...

Best regards,

Thiri

To view this discussion visit https://groups.google.com/d/msgid/modflow/CAKP%2BzwiW%3DehAqr2idPfvBMrV-cdcUYfsL7ekV9dVDaKyLJ1w3g%40mail.gmail.com.

[Jakab Andras - Gmail]

If the observed - and simulated - water depth is over the layer thickness then it is fine to have dry cells in those areas. This is not a problem but normal behavior. You should, however, make sure you use the Newton formulation in MODFLOW 6. If for watever reason you don't want to have dry cells then you have to increase the layer thickness. As far as the initial heads are concerned, you may simply use higyer initial heads so that every cell is wet at the start of the simulation.

To view this discussion visit https://groups.google.com/d/msgid/modflow/CAKau7uwVheCdTLZ0fWGc-4f7QaWea2F6tok6r69AXWGNELCMuA%40mail.gmail.com.