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Dejan

modflow errors

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Hi,

I am trying to create a quite simple model using seven layers in a shape of triangle. Input data are:

recharge from rainfall, constant head of grundwater on the west and river boundaries in the east and bottom.

layers have in general constant hydraulic conductivity around 1 * 10-3 m/s, but in some places there are clay lenses with lower hydraulic conductivity.

When I have started to create a model a had only one layer with several different hydraulic conductivites, constant head and two rivers. Everything worked fine.

Now when I have created modflow model with seven layers, with same parameters as before, the modflow does not want to converge. I tried to use different parameters and it converged only when I set the recharge to 0.

Do you have any idea why is that so.

And I would also like to know, if is it possible to check somewhere which input data makes errors and prevents modflow to converge.

I hope I made question clear enough.

thanks for the help

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There is a Check Simulation menu option under the MODFLOW menu which will do a lot of that for you. You can also look at your output file - go to the end and you can often see what kind of issue was giving you problems. I would bet that the Check Simulation will give you the answer, as you can often forget things when you add layers (like inputting a valid starting head in the new layers). The checker will point out some of those things.

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Check simulation gives no errors. Everything is in the way it should be. And this is output information: (see attachment).

like I said before. I think there is something wrong with recharge...when I set it to zero, the model converge, otherwise it stops, but all parameters are the same like when I have used only one layer. And now the layers should be OK, since the check simulation does not report any errors.

post-16724-0-86905500-1364466930_thumb.j

Edited by Dejan

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You can try another solver package to see if that works. You can look at your vertical conductance/anisotropy to make sure that makes sense (check each layer). If you are using cell rewetting, that would be the likely issue. Make sure your starting heads are close to what you are looking for. Confirm that your recharge is in the right units (is it too high?). You could try changing your solver convergence criteria to see if that gets you there.

So many options.... :)

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Ok, with direct solver package it works (but that is the only one which works). I don not use cell rewetting, so this is not the issue. Starting heads are similar to expected. Recharge rates are in right units.

Can you please tell me, where can I change solver convergance critera?

Can I maybe find some literature about solver packages to see what are the differences? I would like to know how they work and why one works and the other doesn`t.

In GMS user manual I just read that most of the models are based on one layer, due to the different reasons. Do you think is it better to construct only model with one layer. Max thickness of my aquifer is 40m.

You are right....so many options - makes me totally confused <_<

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When you pick your solver, it then becomes a menu option under MODFLOW. Click on that and you will see the convergence criteria available for that solver. You can often make it a little less stringent, which allows an iteration to converge. The solver package literature is in multiple MODFLOW references. Click on Help and you should be able to get to them.

I can't tell you how many layers is best for your model. The fewer layers you have certainly makes things easier, but might not be as accurate as you need.

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Hello!

I have a new question.

For now I have quit idea for model with several layer, now I have only one layer. I have created a model with constant head and two rivers. I have also a good observation wells network on the area. I have created polygons for hydraulic conductivity and run PEST for optimization the hydraulic conductivity.

The results are quite ok. The problem appears when I want to run the model in forward mode and I got dry cells in one area. If I use cells rewetting model does not converge. Can I do something about that?

I have also one question about river conductance. Does it affect any way to the head? I played a bit with different data but I did`t notice the differnce only at the comupted flow (and that flow represent flow from/into the aquifer, right?). Please correct me if I am wrong.

How can I set conductance correct? I have mesured only river flow, but I have no idea what is the exchange between river and aquifer.

thanks,

regards Dejan

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Cell rewetting often causes a situation where the model doesn't converge - there may be a cell that is on the edge of the wet/dry zone that is wet during one iteration, but dry during the next. It goes back and forth and never settles on wet or dry. The model can't converge. You can play with the various wetting parameters to allow convergence, but that also sometimes leaves more cells dry than you would expect.

River conductance can certainly affect head, depending on how the river is being used. Without field data (piezometers, seepage meters), often the best you can do is attempt to calibrate the conductance through nearby monitoring well results. Usually, the best thing to do is pick a conductance to start with. Run your model. Perform sensitivity with the conductance (which it sounds like you did), but make some extreme changes to see how much the results are impacted if there is not a good connection between the river and the aquifer (or if there is no resistance in the connection). You can then discuss those results in your write up.

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Hello,

I have constructed my model in the way that it fits pretty well to the observed data. Now I am trying to do MODPATH particle track. When I use area of contamination the model works, and I got pathlines.

But if I want to create pathlines from the wells, I recive this error message:(do you know what could be the problem?)

DISCRETIZATION INPUT DATA READ FROM UNIT 19

# MF2K DISCRETIZATION FILE

#

#

# NLAY NROW NCOL NPER TIMEUNITS LENUNITS

1 LAYERS 134 ROWS 165 COLUMNS

1 STRESS PERIOD(S) IN SIMULATION

MODEL TIME UNIT IS SECONDS

MODEL LENGTH UNIT IS METERS

Confining bed flag for each layer:

0

IRCHTP= 1 IEVTTP= 1

MAXIMUM NUMBER OF PARTICLES IS 500000

4810443 ELEMENTS IN THE "A" ARRAY

DELR

READING ON UNIT 19 WITH FORMAT: (FREE)

DELC

READING ON UNIT 19 WITH FORMAT: (FREE)

STRESS PERIOD LENGTH TIME STEPS MULTIPLIER FOR DELT SS FLAG

----------------------------------------------------------------------------

1 1.000000 1 1.000 SS

STEADY-STATE SIMULATION

165 COLUMNS 134 ROWS 1 LAYERS 0 CONFINING LAYERS

IGRID (GRID TYPE CODE) IS 0

LAYCON (LAYER TYPE CODES):

1

IBOUND FOR LAYER 1

READING ON UNIT 10 WITH FORMAT: (FREE)

POROSITY = 0.3000000 FOR LAYER 1

Single Precision Binary Files determined from Head file

---------------

HEADS WERE READ FOR LAYER 1, STRESS PERIOD 1 TIME STEP 1

HEADS HAVE BEEN READ

READING FLOW PACKAGE BUDGET DATA FOR STRESS PERIOD 1 , TIME STEP 1 ...

Reading CONSTANT HEAD for Period 1 Time Step 1

Reading FLOW RIGHT FACE for Period 1 Time Step 1

Reading FLOW FRONT FACE for Period 1 Time Step 1

FLOW RATES BETWEEN CELLS HAVE BEEN READ

Reading WELLS for Period 1 Time Step 1

Reading DRAINS for Period 1 Time Step 1

Reading RIVER LEAKAGE for Period 1 Time Step 1

Reading RECHARGE for Period 1 Time Step 1

STARTING LOCATIONS NOW BEING READ FROM FILE...

STARTING LOCATIONS HAVE BEEN READ FROM FILE

NUMBER OF VALID PARTICLES = 60

4827 CELLS HAD ERRORS > 0.0010

MAXIMUM ERROR = 200.0000% IN ROW 7 COL 25 LAYER 1

CELL-BY-CELL ERROR SUMMARY: ROW COLUMN LAYER ERROR(%) ERROR(ABSOLUTE)

(ONLY FIRST 500 ERRORS ARE LISTED)

7 25 1 200.0000 0.53087E-02

7 26 1 200.0000 0.10316E-01

7 27 1-200.0000 -0.10549E-02

7 28 1-200.0000 -0.55771E-01

7 29 1-200.0000 -0.90281E-01

25 43 1 0.0037 0.46287E-06

25 44 1 -0.0233 -0.22678E-05

25 45 1 -0.0470 -0.33309E-05

RUN STOPPED because cell thickness is negative.

1 15 25 258.8919 261.6815

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Can you perform a check simulation? The last "cell thickness is negative" appears to be saying that one of your bottom elevations is higher than the cell's top elevation. I don't know how your MODFLOW run would have worked if that was the case, but stranger things have happened before.....

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I know. I preform check simulation often, just to make sure everything is OK and also this time it is. I also check specified cell if the cell thickness is negative, but it is not. strange...

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I have found the error. I had few dry cells in my model and that disturbed it. I have create those cells as inactive, ran the model and now it works.

Thanks, Dejan

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