MIM model in flow or transport

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Raffaella
Posts: 2
Joined: Tue Nov 24, 2020 4:10 pm
Location: Spain

MIM model in flow or transport

Post by Raffaella » Tue Nov 24, 2020 4:47 pm

Hello Jirka or anyone can help me.
I have a doubt about the MIM model.

I performed a tracer experiment in two columns, one containing only soil and another containing soil overlapped by a 10 cm woodchips layer.
Both systems run under unsaturated conditions: each day 50 ml of a synthetic solution were added all at once by flooding the column upper parts. Tracer was added as a single pulse during only 1 irrigation event.

I got a nice tracer breakthrough curves for both columns. While tracer in the column with only soil could be fitted by applying the single porosity model (Van Genuchten -Mualem), the column tracer in the woodchip column could only be fitted by using the MIM model, which it could make sense.

The MIM model can be applied in the "Soil hydraulic model" window or in the "Solute transport" window. I am not sure in which tab put the value of the MIM model. We have tried both and we got a nice fit anyway.
The column received every day 50 ml on the top which is open to the atmosphere. Therefore, I simulated irrigation as precipitation and evaporation on the top to fit the water volumes flowing through the outlet that are slightly different each day. Outlet drainage stopped before the next irrigation event took place and the whole water contents in the column did not changes.

Any suggestions will be very welcome.
Thank you very much.
Best regards,
Raffaella

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

Re: MIM model in flow or transport

Post by Jirka » Tue Nov 24, 2020 8:45 pm

Hi Raffaella,

We provide the detailed concepts behind these two models in:

Šimůnek, J., and M. Th. van Genuchten, Modeling nonequilibrium flow and transport processes using HYDRUS, Vadose Zone Journal, Special Issue “Vadose Zone Modeling”, 7(2), 782-797, doi: 10.2136/VZJ2007.0074, 2008.

The main difference is that the dual-porosity model, which you can select in the “Soil Hydraulic model” window can have time-variable immobile water contents (i.e., aggregates with immobile water can dry out and be rewetted, and the max immobile water content is specified with soil hydraulic parameters), while in the mobile-immobile water model (for which you can specify the immobile water content in “Solute Transport Parameter” window, the immobile water content is constant (invariable in time). If you assume steady-state water flow, then both models are identical.

J.

Raffaella
Posts: 2
Joined: Tue Nov 24, 2020 4:10 pm
Location: Spain

Re: MIM model in flow or transport

Post by Raffaella » Wed Nov 25, 2020 5:54 pm

Dear Jirka,
thank you very much for your answer.
I got the point and I think that in the case of the woodchips layer, which get wet and dry, it will be more appropriate to select the dual porosity model in the flow simulation, even if daily changes in the hydraulic flow of the entire system are small.

If I correctly understood, in this case the water moves by water content gradient. Whereas, when the dual porosity is applied in the transport simulation the solute moves through concentration gradient.

We will read the publication you suggested.
I really appreciate your help, this forum is so useful to us!

Best regards,
Raffaella

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

Re: MIM model in flow or transport

Post by Jirka » Wed Nov 25, 2020 6:11 pm

Yes, water between two domains moves by either the water content or pressure head gradient (depending on which dual-porosity model you choose). The solute transport then moves by the concentration gradient in both models (dual-porosity or MIM). However, it additionally also moves by convection with water in the dual-porosity model. J.

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