boundary conditions (BC) at the lower boundary

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Mitra
Posts: 9
Joined: Thu Dec 06, 2018 3:44 pm
Location: Switzerland

boundary conditions (BC) at the lower boundary

Post by Mitra » Mon Feb 04, 2019 1:03 pm

Dear Jirka,

I have a question regarding the lower boundary conditions for solute and water transport in hydrus1D.

Firstly, for solute, I was wondering if I can use the flux boundary condition instead of zero gradient, and what are the specific differences between the two. from what i understood in the manual, flux boundary condition should automatically reduce to zero gradient at the outlet. However, when I change the bottom boundary from zero gradient to flux boundary, I can clearly see some differences between the two scenarios in the sign of solute flux.

Secondly, for water, I was wondering if you would help me to better understand the difference between the ''seepage face'' and ''free drainage''. Does ''seepage face'' imply usage of pump or an impermeable layer at the outlet of fluid? Also in your manual seepage face is recommended for lysimeter simulations. I am wondering if ''free drainage'' has applications for modelling lysimeters, if so, what would be the required physical conditions.

Also, at some points during my simulation I am observing negatively signed cumulative solute mass at the bottom changing signs to positive values. I am having a hard time understanding what is the physical meaning of such a change in sign.

Jirka
Posts: 4712
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: boundary conditions (BC) at the lower boundary

Post by Jirka » Mon Feb 04, 2019 10:11 pm

Note that we discuss most of these topics in our Tutorial Book (downloadable from the HYDRUS website). J.

Rassam, D., J. Šimůnek, D. Mallants, and M. Th. van Genuchten, The HYDRUS-1D Software Package for Simulating the Movement of Water, Heat, and Multiple Solutes in Variably Saturated Media: Tutorial, Version 1.00, CSIRO Land and Water, Adelaide, Australia, 183 pp., ISBN 978-1-4863-1001-2, 2018.
https://www.pc-progress.com/Downloads/P ... 0_2018.pdf

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