RE: [MODFLOW] Re: MODFLOW rewetting problems...

[Simon Woodward]

I have a transient model where the water table is rising and falling through different model layers through time (on a seasonal basis). I was having trouble with rewetting, and it was suggested I use the NWT solver in MODFLOW 2005. In NWT dry cells remain active. This works very well, and I have no problems now with drying-rewetting. Although you do need to choose your solver parameters carefully to get fast convergence and low mass balance errors.

 

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From: mod...@googlegroups.com [mailto:mod...@googlegroups.com] On Behalf Of Gustav
Sent: Wednesday, 8 August 2012 05:07
To: mod...@googlegroups.com
Subject: [MODFLOW] Re: MODFLOW rewetting problems...

Hi!
I have had a problem similair to yours, the cell won't activate/re-wett when water comes from above (in my case recharge). I never got it to work, but I read something about changing some parameter to a negative value. If i find where i read this I'll post the link here. Not much help I'm sorry to say, hope someone else comes with a solution.

WIth regards
Gustav


Den måndagen den 6:e augusti 2012 kl. 17:31:53 UTC+2 skrev Khbayliss:

I am simulating a injection and abstraction dipole as part of a ground source energy project. 

The majority of the top layer is dry in steady state, and during pumping the cells which should be re-wetting due to the injection mound are struggling to, causing non-convergence. 

I am using PCG2 solver and have altered the wetting threshold and factor, but they are not helping. I have also tried the following: 

Make inactive any cells that you know should never become wet.

•	Increase the Wetting threshold value. This makes it more difficult for a cell to be wetted, and therefore helps stop MODFLOW from repeatedly turning a cell on and then off again. However, the solution may become less accurate since cells that should become wet might stay dry.

•	Modify the Wetting factor value. This will increase or decrease the new head in cells which are wetted.

•	Change the Wetting method that controls cell wetting.

•	Change the Wetting head option that calculates the new head in the wetted cell.

•	Try using the SIP or PCG solver, and modifying the solver parameters.

•

For steady-state solutions, start with good initial head estimates. This will provide good indications of which cells should be wet and dry, and therefore conversions of cells between wet and dry will be minimised Can anyone suggest anything else which may help? I have been trying for days.  Thanks, Kitty

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[Simon Woodward]

The NWT options I use are:

 

a.       Model|MODFLOW2005|Options

                                                   i.      NWT General

1.      OPTIONS = SPECIFIED by user

2.      HEADTOL = 5e-3

3.      FLUXTOL = 5e-1

4.      MAXITEROUT = 2000

5.      THICKFACT = 1e-7

6.      LINMETH = 2

7.      IPRNWT = 1

8.      IBOTAV = 0

9.      DBDTHETA = 0.8

10.  DBDKAPPA = 0

11.  DBDGAMMA = 0.001

12.  MOMFACT = 0.001

13.  BACKFLAG = 0

14.  MAXBACKITER = 10

15.  BACKTOL = 1.05

16.  BACKREDUCE = 0.9

                                                 ii.      NWT Methods

1.      IACL = 2

2.      NORDER = 1

3.      LEVEL = 1

4.      NORTH = 4

5.      IREDSYS = 1 (MODFLOW-NWT 1.0.3 onwards)

6.      RRCTOLS = 0.0

7.      IDROPTOL = 1

8.      EPSRN = 1e-3

9.      HCLOSEXMD = 1e-4

10.  MXITERXMD = 250

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From: mod...@googlegroups.com [mailto:mod...@googlegroups.com] On Behalf Of Simon Woodward
Sent: Wednesday, 8 August 2012 13:57
To: mod...@googlegroups.com
Subject: RE: [MODFLOW] Re: MODFLOW rewetting problems...