"Free & movable" VS "Phreatic"

Hi! When defining the first slice, I'm very confused of when to use "Free & movable" and when to use "Phreatic". Could you tell me the difference between them and the rule of using them?

In my model, the first layer is an unconfined layer. The groundwater depth from ground surface is 2- 200 m because my model area includes plain and mountain. So in my model, which should I use to define the first slice, "Free & movable" or "Phreatic"?

Looking forward to your reply, thanks!

You can use either. Each has advantages and disadvantages.

Read the section in FEFLOW (6.1) help titled "Free surface"

Thanks for your reply, Blair!

I have read the help file titled "Free Surface Definition". But I'm still confused. So could you tell me the advantages and disadvantages of using "Free & movable" or "Phreatic" for the first slice?

Thanks!

This is from the FEFLOW help:

[b]Free and Movable Surface[/b]
Advantages: The model only contains the saturated zone. No additional assumptions are required for the capillary fringe and the unsaturated zone.
Disadvantages: In cases where the water level does not stay within one stratigrahic layer, parameter interpolation can occur during the simulation in elements where the water table cuts through slices of the initial stratigraphy.

[b]Phreatic[/b]
Advantages: This method does not suffer from parameter interpolation problems in case of steep hydraulic gradients and a water table cutting through slices.
Disadvantages: Linear conductivity reduction for partially saturated elements may lead to high contrasts in actual hydraulic conductivity between dry and saturated layers. This complicates the numerical solution. In cases where recharge is applied to a dry top layer, oscillations may occur.

Thanks for your reply, Blair!

Then I still have a question. That is for free and movable surface, how is parameter interpolation achieved during the simulation in elements where the water table cuts through slices of the initial stratigraphy? Or what law is this parameter interpolation based on?

Looking forward to your reply, thanks!