Posted Tue, 11 May 2021 08:32:57 GMT by Ben Weber

I'm relatively new to FeFlow and I thought that this is an easy problem which I could solve after a bit of research but I can't figure it out. So what I'm trying to do is pretty simple, basically I just want the groundwater table of my 3D (8 layers each 25 m) model to be at 20 m below the surface. I've tried a couple of things to achieve this but none of them yielded the results I am looking for.

First of all I went to Problem settings > Problem class > Free surface and chose 'unconfined aquifer' (unsure about constraint vs unconstraint head?). When I display the 'Pressure' of my model it transitions smoothly from 0 at the very top to 1961 kPa (200 m depth) at the very bottom. So far so good. Now for my purposes it should be 0 at 20 m depth and then 1765 kPa at 200 m depth. I know that pressure is just calculated from the hydraulic head input but I read in the tutorial files that you can use the pressure as input as well and FeFlow automatically calculates the corresponding hydraulic head. Sooo I went to the top slice and put 0 kPa, then to the second slice (25 m) and put 49 kPa because there would be a 5 m water column at 25 m depth and then finally to the bottom slice and put the 1765 kPa. Now that looked perfect when I displayed the pressure but once I switch to the hydraulic head view it's really messy (see attachment). This is the closest I got to what I'm looking for. How do I fix this?

I used the hydraulic head IC and set it to 180 m at the bottom slice but then the pressure at the bottom is at 3726 kPa (=380 m water column) which is way too high. It would be nice to set it up in a way that I can just change the hydraulic head BC at the left/right borders a little bit to generate flow but to have the water table at 20 m below ground is more important.

(Also what EXACTLY does the hydraulic head define? Say I set the hydraulic head for a node in 10 m depth to 5 m what does that mean? I understand it as 'There is the pressure of 5 m water column at this point' which would mean that the water table is at 5 m below surface at that point. Is this right? The tutorials don't really go into much detail.)

There should be an easy solution to this and I hope someone can point me in the right direction.
Posted Wed, 12 May 2021 12:40:19 GMT by Igor Pavlovskii Dalhousie University Post-Doctoral Fellow
I assume you want to setup Initial Conditions for the run (not boundary conditions):
just set up hydraulic head at all nodes (not just specific slice) to whatever [b]absolute[/b] elevation your groundwater table should be in the model.
Based on your post I think -20 m. I suspect your model top is actually at 0 m and bottom is at -200 m. You can change these through Edit => 3D layer configuration
Posted Wed, 12 May 2021 14:03:36 GMT by Ben Weber
Yes initial conditions primarily. When I set up the hydraulic head to -20 m for all nodes it looks fine. Except that the pressure above the water level is negativ when I set it up via hydraulic head IC. Is this negligible? Because when I change the pressure above manually to 0 it the hydraulic head looks like the pictures above.
Posted Wed, 12 May 2021 15:29:40 GMT by Peter Schätzl Grundwassermodellierer
You might have to look up the concept of "suction pressure" - it is perfectly OK to have negative pressure above the water table.
Posted Thu, 13 May 2021 11:32:36 GMT by Ben Weber
Thanks a lot for the answers guys, your help is greatly appreciated :)

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