Negative solute flux within sensitivity analysis

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Meirav
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Joined: Tue Jan 12, 2021 9:09 am
Location: Israel

Negative solute flux within sensitivity analysis

Post by Meirav » Tue Jan 12, 2021 10:55 am

Hello,
I am simulating benzene transport in unsaturated sandy soil with an upper atmospheric BC and a lower constant pressure head of h=0 (aquifer position) and solute upper concentration BC and lower zero concentration gradient BC.
The model simulates 10 years of no solute in the system for water content etc. stabilization and then application of the solute at the upper (ctop) atmospheric boundary for additional 50 years at solubility concentration. I now run a Morris global sensitivity analysis to see which parameters are the most important in the model. I check how the change in parameters values affects the cbot and sum(cvbot) at the end of the model run. In many cases I received only negative sum(cvbot) (cvbot as well was only negative) and I don't see how can I have there a negative flux without some initial positive flux (though peaks in concentration are at the same time as peaks in negative darcy flux (q) they also at the same time of negative solute flux which gets up to zero? ). Also in some cases (only a few) I receive cbot>0 though all cvbot and sum(cvbot) values are =0. I don't see how these results make sense. Do you have a possible explanation? I am not sure sensitivity analysis results have to make sense really, but still it seems weird to have concentration without flux or with only negative flux.
Kind regards,
Meirav

Jirka
Posts: 5216
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Negative solute flux within sensitivity analysis

Post by Jirka » Tue Jan 12, 2021 7:07 pm

You need to make sure that your boundary conditions are physical meaningful. The constant head BC (position of GWT) allows for flow in both direction (downward recharge or upward discharge) depending on the development of the pressure head profile (and atmospheric BC at the top). The zero-concentration gradient BC makes sense only at the outflow boundary since it enforces the same concentrations at the last two nodes (and it thus represents a convective solute flux (qc) and neglects dispersive flux). It has little sense at the inflow boundary since it does not consider inflow concentration. J.

Meirav
Posts: 7
Joined: Tue Jan 12, 2021 9:09 am
Location: Israel

Re: Negative solute flux within sensitivity analysis

Post by Meirav » Wed Jan 13, 2021 2:24 pm

Thank you very much for the fast reply!
So concentration flux BC will be best at the GWL?
I tried changing the solute bottom BC to both concentration flux BC and concentration BC in one of the runs and still I see only negative solute flux (see next message), though I do get negative water flux.
I don't understand the negative solute flux without any initial positive solute flux.
Many thanks,

Meirav
Last edited by Meirav on Thu Jan 14, 2021 5:47 am, edited 1 time in total.

Meirav
Posts: 7
Joined: Tue Jan 12, 2021 9:09 am
Location: Israel

Re: Negative solute flux within sensitivity analysis

Post by Meirav » Thu Jan 14, 2021 5:45 am

Here are the concentration and concentration flux BC model results as well as the water flux results
concentrationBC.png
Concentration BC
concentrationBC.png (30.83 KiB) Viewed 2673 times
Attachments
waterflux2.png
water flux
waterflux2.png (78.83 KiB) Viewed 2673 times
concentrationfluxBC.png
Concentration flux BC
concentrationfluxBC.png (40.63 KiB) Viewed 2673 times

Jirka
Posts: 5216
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Negative solute flux within sensitivity analysis

Post by Jirka » Thu Jan 14, 2021 9:55 pm

I think you should try to understand when the water/solute fluxes are negative and when positive, and what different solute BCs actually do. It is all explained in the manual. J.

Meirav
Posts: 7
Joined: Tue Jan 12, 2021 9:09 am
Location: Israel

Re: Negative solute flux within sensitivity analysis

Post by Meirav » Sun Jan 17, 2021 2:46 pm

Yes thank you for that. I cahnged the BC to third type BC.
The problem is that no matter what BC I use I only get negative solute flux (no positive solute flux). The peaks in solue flux (negative) takes place when peaks in water flux occurs. It just doesn't make sense.

Jirka
Posts: 5216
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Negative solute flux within sensitivity analysis

Post by Jirka » Sun Jan 17, 2021 8:17 pm

Is it clear that there is different convention for water and solute fluxes, right? The water fluxes are positive upward and negative downward, while solute fluxes are positive into the domain and negative out of the domain. J.

Meirav
Posts: 7
Joined: Tue Jan 12, 2021 9:09 am
Location: Israel

Re: Negative solute flux within sensitivity analysis

Post by Meirav » Tue Jan 19, 2021 9:42 am

Yes of course.
That's the problem. I get negative fluxes for both the solutes and the water. When I lower alpha though and the water content increases I get also small positive solute fluxes but then I also have positive water fluxes.
Is there maybe an option in the program to change solute fluxes sign? maybe I did it by mistake. Because if it wasn't for the sign the results were perfect.

Meirav
Posts: 7
Joined: Tue Jan 12, 2021 9:09 am
Location: Israel

Re: Negative solute flux within sensitivity analysis

Post by Meirav » Sat Jan 23, 2021 3:49 pm

Dear Jirka,
So I looked over all the Hydrus SOLUTE1 examples files, and actually there as well, all cvBot and Sum(cvBot) values are negative. So I guess solute fluxes in the bottom of the profiles are always negative downward and positive upward. As oppose to solute fluxes at the top of the profile. Right?
Thanks,
Meirav

Jirka
Posts: 5216
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Negative solute flux within sensitivity analysis

Post by Jirka » Sat Jan 23, 2021 5:22 pm

What do you think is the manual (or help) for? I would expect that you would seek such information there. It’s rather disappointing to see that it has not occurred to you to do so. J.

Meirav
Posts: 7
Joined: Tue Jan 12, 2021 9:09 am
Location: Israel

Re: Negative solute flux within sensitivity analysis

Post by Meirav » Sat Jan 23, 2021 6:16 pm

Dear Jirka,
I am very sorry. Of course I read the manual, the help and the files over and over again. I just misunderstood it. In the help it says "Lower boundary solute flux [M/L2/T] (inflow/outflow: +/-)" and in the file "All solute fluxes and cumulative solute fluxes are positive into the region" I just looked at it as though inflow in all cases will be positive downward and outflow negative upward and it did not occur to me to actually change the signs at the lower boundary.
Thank you again for all the help,
Meirav

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