Porosity in MODFLOW and MT3D

[speedy]

Hello everybody

I noted GMS has values for porosity in both MODFLOW and MT3D. In MODFLOW menu (MODFLOW options) one can enter values for porosity, although porosity is not used in flow computations as far as I know. Also in MT3D menu it is essential to enter porosity value.

In the model I have (which is a SEAWAT model) I entered values for porosity in the MT3D options, but I noticed there are different values -by default- in the MODFLOW options. Which values does the model consider in this case? and why is there an option for porosity in MODFLOW?

 

Thanks

[khaled_haider]

Regarding the porosity in MODFLOW, I have noticed that the values of Velocity vectors_Mag (which is produced using the option CCF-> Velocity vectors) are proportional to porosity. Small values of porosity result in small values of Velocity vectors magnitude.

[Michael Kennard]

Also, the MODFLOW porosity is used by MODPATH.

[speedy]

Ok, then why GMS allows dupplication of entry for porosity? Why not to have it in one place (i.e. MODFLOW options) and not again in MT3D since it is essential to have MODFLOW model in all cases? Why also does GMS allows two different values to be entered for the same paramter?

[speedy]

11 hours ago, speedy said:

Which values does the model consider in this case?

 

 

You did not answer the above-question

[Michal]

Hey Speedy,

this is a good question. From Seawat User Guide (Guo, Langenvin 2002) it seems that value specified in the MT3DMS BTN Package file is the one used by Seawat.

[speedy]

1 hour ago, Michal said:

Hey Speedy,

this is a good question. From Seawat User Guide (Guo, Langenvin 2002) it seems that value specified in the MT3DMS BTN Package file is the one used by Seawat.

Hi Michal

After around a month of confusion (it turned me crazy) and not getting good match of the SEWAT model and analytical solution I accidentally discovered this problem. What happened is I entered the correct values of porosity using the MT3D menu not aware of the fact that the MODFLOW model assigns default value of porosity that are different from the values I entered in MT3D.

While I am not sure what value the model uses in the end, but what I know for sure is the results were wrong. After I entered the correct porosity values in MODFLOW model then I got meaningful output. I hope GMS developer take into consideration this problem for future update.

Edited November 9, 2017 by speedy

[Michal]

This indeed looks like some GMS bug that is probably specific to certain usage. I suggest you contact the support and make sure they can replicate your problem.

When you look into the Salt Lake example from GMS tutorials, you will find out that in MODFLOW>Global>Porosity dialog is not editable and correct values of porosity are entered in MT3DMS>Basic Transport Package>Porosity.

In the Coastal Aquifer example the situation is different. The material properties for this example are enter using GMS Materials option in the LPF Package. The MT3DMS>Basic Transport Package>Porosity button is not present in the dialog at all. MODFLOW>Global>Porosity dialog is not editable and GMS notifies the user that materials are used in stead of arrays. Porosity is therefore specified only in the Materials dialog from which it is written to the pertinent file (BTN in this case).

[speedy]

3 hours ago, Michal said:

 

When you look into the Salt Lake example from GMS tutorials, you will find out that in MODFLOW>Global>Porosity dialog is not editable and correct values of porosity are entered in MT3DMS>Basic Transport Package>Porosity.

 

This is true. In my case the porosity values were not editable in MODFLOW, but values were different than the one I entered in MT3D. This made all results wrong. SO I had to change the porosity values in MODFLOW. I selected all cells >right click- properties> then changed porosity values in MODFLOW. Only then the model produced correct results.

[khaled_haider]

I'm sorry,  my last sentence was not correct . the truth is that small values of porosity result in large values of Velocity vectors magnitude.The equation for calculating ground water velocity is: V= KI/n. (Darcy’s law).

V stands for "groundwater velocity,"

K equals the "horizontal hydraulic conductivity,"

I is the "horizontal hydraulic gradient,"

and n is the "effective porosity."

 

 

[Michal]

11 hours ago, khaled_haider said:

I'm sorry,  my last sentence was not correct . the truth is that small values of porosity result in large values of Velocity vectors magnitude.The equation for calculating ground water velocity is: V= KI/n. (Darcy’s law).

Hi Khaled,

you are right about this. However please note, that this has nothing to do with the MODFLOW model results. There is no porosity variable involved in the governing equation of the MODFLOW mathematical model. So the values of porosity entered via the Cell Properties are used either for dataset postprocessing to calculate true flow velocity or for writing inputs for MODPATH as mentioned earlier. Now the question here is if it is also transalted from the Cell Properties to MT3DMS BTN file or not.

 

16 hours ago, speedy said:

This is true. In my case the porosity values were not editable in MODFLOW, but values were different than the one I entered in MT3D. This made all results wrong. SO I had to change the porosity values in MODFLOW. I selected all cells >right click- properties> then changed porosity values in MODFLOW. Only then the model produced correct results.

Speedy,

if this is so then it should be considered a serious bug in GMS. You can try changing the porosity in the Cell Properties dialog, save the model and look into the MT3DMS\case.btn file. Change it into something obvious like 0.666666. You should see if the array is written to the .btn file instead of the correct values specified in the MT3DMS dialog in GMS. If not sure check MT3DMS manual chapter 6.4 (Zheng and Wang 1999) for the BTN file format specs to help you locate the porosity array A11 in your file.