Soil water evaporation

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Dana2020
Posts: 4
Joined: Sat May 09, 2020 4:08 am
Location: United Kingdom

Soil water evaporation

Post by Dana2020 » Wed Jun 10, 2020 10:42 am

Hello,

Hope that you are keeping well and safe.

I am using HYDRYS-1D to simulate pure water evaporation under constant temperature. I have set a surface boundary condition for the top layer and zero flux at the bottom. I am assuming that the soil is fully saturated and therefore set the pressure head to zero. however, when I do that the simulation doesn't work. however, when I set the pressure head to say 1E-08 it works. Is it okay to do that?
Sorry for that as I don't have any background on soil physics and due to the current situation I had to choose a topic related to water evaporation from porous media that I can do remotely.

Your help is highly appreciated.

Thanks

Jirka
Posts: 4961
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Soil water evaporation

Post by Jirka » Wed Jun 10, 2020 5:45 pm

The surface node should be slightly unsaturated, i.e., the pressure head should be negative.

J.

Dana2020
Posts: 4
Joined: Sat May 09, 2020 4:08 am
Location: United Kingdom

Re: Soil water evaporation

Post by Dana2020 » Thu Jun 11, 2020 6:38 am

Thank you

Dana2020
Posts: 4
Joined: Sat May 09, 2020 4:08 am
Location: United Kingdom

Re: Soil water evaporation

Post by Dana2020 » Sat Jun 27, 2020 6:44 pm

Hello again,

I have a relevant question so I thought I should post it here.
I am trying to simulate water evaporation from saturated clay. I used the van Genuchten-Mualem model with an air entry value of -2 cm as recommended. however, the simulation did not converge. I tried using finer mesh size at the top and it still didn't converge. I also tried to use smaller values for the time steps. I am wondering if it would be possible to help me on that? I used a value of -500 m for hcrita.

As for hcrita value, I understand that the value should be lower for finer soils (-500 m) and higher for coarse soils (-10 m ), and that it should be calculated using the equation provided in the help section, and that this value shall be adjusted such that the water content at this pressure is at least 0.005 higher than the residual water content. However, since I don't have any experimental data and my work is based on reported values in literature (I am currently using the values provided in the soil catalog) so I am wondering if it would be okay to adjust these values based on the soil water retention curves provided by the software?

Thanks in advance

Jirka
Posts: 4961
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Soil water evaporation

Post by Jirka » Sat Jun 27, 2020 10:06 pm

As I wrote above, the surface node should be slightly unsaturated. When you have the VG model with an air entry value of -2 cm, i.e., the surface pressure head should be below -2 to make it unsaturated.

hCritA: I do not have more to say in addition to what's written in the help. I would not change the recommended values unless you have some problems with convergence (for other default parameters).
J.

Dana2020
Posts: 4
Joined: Sat May 09, 2020 4:08 am
Location: United Kingdom

Re: Soil water evaporation

Post by Dana2020 » Sun Jun 28, 2020 9:27 am

Thank you

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