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very small scale root solute uptake

Posted: Fri Sep 03, 2021 9:33 pm
by rachel.strickman

I am trying to model transpiration driven root solute movement at a small (mm) scale. I currently have a 2-d, circular domain with a root in the center expressed as a tiny hole (0.02 cm). The flow boundary conditions on this tiny opening are time-variable, to express the daily variations in plant transpiration. This seems to work OK for water flow. However, for the solutes, I would like to add more complexity and michaelis-menten kinetics of uptake. This way, I could look for zones of depletion or accumulation of solutes over the day-night cycle.

Basically, I am trying to find a way to specify root solute uptake (which HYDRUS will do) but over a very tiny area, to simulate an individual root surface. My idea was to define the circular central hole as the soil surface, then set root distribution to be over a very shallow depth, perhaps 0.1 cm, and root density to be very high. I was going to specify only active uptake in this rooting area, and no root water uptake. The solute boundary condition of the central aperture could be set to "no movement" so that the solute could only be taken up by the plant, not washed out with the water flow.

I wondered if there was a better way to do this?

Thank you very much indeed for any advice,


Re: very small scale root solute uptake

Posted: Sat Sep 04, 2021 2:28 pm
by Jirka
The first approach using a BC on a circle: Depending on the BC that you apply on the circle, e.g., either atmospheric or time-variable flux BC, there is either a zero solute flux (no solute outflow across the atmospheric BC) or solute flux equal to qc, where q is the boundary water flux and c is the solute concentration in the domain close to the boundary. To adjust the outflow concentration (e.g., Michaelis-Menton kinetics) would require direct changes in the code.

The second approach using active root solute uptake: This may work if you use an atmospheric BC on the circle. This BC does not allow solute outflow. Note that you cannot prevent solute from leaving the domain on the flux/head boundary with the flux directed out of the domain.


Re: very small scale root solute uptake

Posted: Wed Sep 08, 2021 4:52 pm
by rachel.strickman
Hello Jiri,

Thank you for this advice. So it seems that I can do one or the other: time variable water movement, OR root solute uptake only, but not both? After all, if the boundary condition is set to atmospheric, then I won't be able to express the transpiration as the time-variable flow boundary condition, correct?

Re: very small scale root solute uptake

Posted: Fri Sep 10, 2021 5:19 pm
by Jirka
You can actually use the time-variable BC, but if you want to have no solute flux across this BC, you need to ask the code to treat the boundary as an atmospheric boundary (Special BC Options). Anway, transpiration is completely independent of the atmospheric BC. J.