Depth-dependant Evaporation

Hi

I'm trying to set up an evaporation BC at the surface of a 2D vertical model. I would like to use a depth-dependant evaporation rate, but I don't know how to edit the formula with the "formula editor".
If anyone has experience on this matter, please send me more information.

Andrés T.

Maybe someone can help me sending a .fme file of a user-defined formula. That would help me to learn the FEMATHED syntax.
I'll be grateful.

Have a look at:
http://www.wasy.de/english/produkte/feflow/howto/video_evapotranspiration.htm

Hi Peter!

Thak you for your support.
The video that you recommended is applied to an horizontal 2D problem?
I'm working on a vertical 2D flow and mass problem.

Best regards

For a vertical model, the situation is more complex: You cannot use the equation editor for a 2nd kind boundary condition in the moment. The only way I could imagine is using a row of elements with a fixed size (height) on top. Then you could use the source/sink term in material properties for recharge. You'd have to think again about the different values in the equation then, too.

http://www.wasy.de/english/produkte/feflow/howto/video_evapotranspiration.htm
the video is very useful.could you share more videos for us?
thanks
Best regards

[quote author=Peter Schätzl link=topic=278.msg635#msg635 date=1190303541]
For a vertical model, the situation is more complex: You cannot use the equation editor for a 2nd kind boundary condition in the moment. The only way I could imagine is using a row of elements with a fixed size (height) on top. Then you could use the source/sink term in material properties for recharge. You'd have to think again about the different values in the equation then, too.
[/quote]
Thanks Peter for your help. It seems to me that I didn't understand very well your previous suggestion respect to using a row of elements with a fixed size on top. Would you be more precise, please?
I'm working with an unsaturated aquifer model. How can I know the depth of the water table in every time step. In the editor i can only use "Head" or "Saturation" as known variables, but i need to use only the pressure term of total head (H=h+z) in order to use an evaporation expression as a function of the water table depth, i.e. ET=ET(Z_wt). Any information will be appreciated.

Best Regards

Hey Andrés.

To my knowledge there currently isn't an easy way to reference z_WT. It's simply not available as a variable in FeFlow. And it's not that easy of a value to find in the first place (since there might be more than one "water-table" at a given x/y location!).

We developed an IFM module to find the "water-table elevation" and export it to a file for visualisation in a GIS. I would use a similar approach: Write an IFM module to set the evaporation rate based on the water-table elevation, which I'd sear for first. Unfortunately, this doesn't help you much, since I assume you don't wanna go to such an advanced extent as using IFM models/code writing.

Can you explain how your evaporation rate should be calculated and specified in FeFlow (i.e. fnx(z_wt) and the type of BC you plan to use)?!

Chris

Thanks Chris for your support.
I'm trying to simulate a playa/sabkha groundwater flow. I want to add on an exponential function between the evaporation rate and the water table depth, i.e. ET(z_wt)=A*exp(-B*z_wt). I'm working on a 2D vertical model with unsaturated zone.
I was thinking to use the source/sink stuff in order to set up the evaporation condition. Recently I've been trying to use a pressure BC  on the top of the model (no good results yet), but i've realized that i can't use a power function to set a time-variable pressure BC.
I will continue trying.

Best regards

Andrés T.

Hey Andrés,

I assume your model would be transient and the water table somewhat "well behaved", meaning there is only one (no perched system with more than one free water table at different elevations).

Between time steps, you would have to find that water table, calculate the depth from GS to water table, and then set the boundary condition according to your equation?? The boundary condition would then be assumed to be constant for the next time step. Are parameters A and B constant or variabel over time? They would have to come from an external data source (text file, DB). Is that would you were thinking?

If your FEM isn't too too large, would you mind emailing me a version?

Thanks, Chris