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Modflow flooded/dry cells

GMS User

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I've a 2 layer model.

The 1st layer is dry (sand1, HK1). As starting heads I set -888.

The 2nd layer is wet. It's un unconfined aquifer (sand2 HK2). The starting heads is set to -1 m a s l.

I run the model in steady state condition and I get:

layer 1: dry cells (ok)

layer 2: wet cells (?)

I was thinking that as the water table arises it will set the layer 1 cells from dry to wet

How can set modflow parameters to have the two layers acting as only one layer?

By the way, If a set a pumping well (injection) in layer 1 only, where does the water goes?

Thank you.


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If you want the cells in layer 1 to become wet when they start out dry then you need to use the wetting options in the LPF (or BCF/HUF) package. See the MODFLOW documentation here for information on the WET flag inputs.

If you put an injection well in layer 1 and you don't have the rewetting enabled then the flow goes no where because the cell is inactive. If rewetting is on and the cell becomes wet then the flow goes into that cell.

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Thank you for your quickly reponse, but I get in trouble again with the wet/flag and the model checker.

  • I check the 'Cell wetting parametes' with:



wetting equation= 2nd

When I press the Wet/Dry button, the new form is titled 'Wetting flag and threshold'. What does it mean?

If I set 0 the flag is off and I don't let the cell changing from wet to dry. Is it correct?

How can set a threshold?

I set 1 for all cells.

  • I set starting heads for unconfined (set convertible) layer 1 (dry) to -888 and when I run the model checker I get this error: ' starting head lower than bottom elevation of layer 1' (which is a flat plane with bottom 1.85 asl m).

In a my previus post titled 'Modflow model checker error', I was suggested to use -999.

I also tried -999 but without success.

How can I start a simulation with layer 1 dry (allowing to became wet)?
Edited by GMS User
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  • 2 weeks later...

Here is the explanation of the WETDRY flag.

WETDRY—is a combination of the wetting threshold and a flag to indicate which neighboring cells can cause a cell to become wet.

If WETDRY < 0, only the cell below a dry cell can cause the cell to become wet.

If WETDRY > 0, the cell below a dry cell and the four horizontally adjacent cells can cause a cell to become wet.

If WETDRY is 0, the cell cannot be wetted.

The absolute value of WETDRY is the wetting threshold. When the sum of BOT and the absolute value of WETDRY at a dry cell is equaled or exceeded by the head at an adjacent cell, the cell is wetted. Read only if

LAYTYP is not 0 and LAYWET is not 0.

Even though the model checker will tell you that you have an error in your simulation you can still run MODFLOW. Normally, specifying the heads below the bottom elevation would be an error, but in this case it is not.

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