Flux Mass Balance

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tmalmir
Posts: 7
Joined: Tue Apr 10, 2018 6:41 pm
Location: Canada

Flux Mass Balance

Post by tmalmir » Wed Jul 18, 2018 4:36 pm

Dear Jirka,

I am conducting a simulation for a lysimeter using precipitation and potential evaporation as input data from 2009-2015. I assume the wall of lysimeter to be void (no-flux boundary). I applied free drainage to the bottom boundary of the lysimeter and to the bottom boundary of the domaine, whereas the top boundary is atmospheric boundary (see attached file). When I compare the input and output fluxes to the system (flux mass balance), I obtain very surprising results: the sum of what is flushed out of the lysimeter and crosses the bottom boundary make only a fraction of the summation of the flux crossing the top (atmospheric) boundary. I wonder if I mass balances using sums of fluxes through 9 years is not the way to do it with Hydrus. I would appreciate your guidance in this matter.

Regards
TM
Attachments
problem with flux mass balance.PNG
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Jirka
Posts: 4716
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Flux Mass Balance

Post by Jirka » Wed Jul 18, 2018 11:10 pm

I would need to see the project to figure out what is wrong. Check the mass balance error WatBalR. If it is less than 1% that the simulation should be correct and there must be some explanation. J.

tmalmir
Posts: 7
Joined: Tue Apr 10, 2018 6:41 pm
Location: Canada

Re: Flux Mass Balance

Post by tmalmir » Sun Jul 22, 2018 3:56 pm

Dear Jirka,

I am simulating several lysimeters installed in several experimental landfill covers. One of the main goals is to assess infiltration rates. The domain is the experimental cover. The top and bottom boundaries of the domain are atmospheric and free drainage, respectively. The bottom boundary of each lysimeter is free drainage, whereas the walls are void (no mesh), therefore flux=0. The material inside and outside of the lysimeter are the same. Several simulations test for different heights of lysimeter walls.


Input data in Hydrus 2D are : time steps, soil hydraulic parameters, 9 years precipitation and potential evaporation, initial water content and so on. As far as water flow conditions are concerned, we opted to no solute or heat transport and no root water uptake. I check for fluxes across 4 meshlines: 1. at the bottom boundary of domain; 2. at the bottom boundary of lysimeter; 3. at the top boundary of lysimeter; and 4. at the top boundary of domain.
I checked the error of mass balance and it is actually less than 1%. So, this is covered.
So, as far as my understanding goes, after simulating for 9 consecutive years, I should have the total flux crossing the top of the domain (mesh line 4) equal to the sum of the fluxes crossing the bottom boundary (mesh line 1) and the bottom of the lysimeter (mesh line 2), both free drainage.
Unfortunately, we obtain the latter (flux 1+flux 2) as 30% of the former (flux 4).
I would appreciate your guidance and input in this matter.

Regards
TM

Jirka
Posts: 4716
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Flux Mass Balance

Post by Jirka » Sun Jul 22, 2018 7:02 pm

I do not see how I can provide any further advice based on the information that you provided. If the mass balance error is less than 1% than the simulation will likely be correct and there must be some explanation for differences in the fluxes, such as changes in storage, root water uptake, etc. I do not have such information and thus cannot address that. J.

tmalmir
Posts: 7
Joined: Tue Apr 10, 2018 6:41 pm
Location: Canada

Re: Flux Mass Balance

Post by tmalmir » Mon Aug 27, 2018 6:00 pm

Dear Jirka,

Thank you for your reply.
May I ask if it is possible to calculate the accumulated water content inside the lysimeter by Hydrus that I use?

Many thanks
TM

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