imposing changing boundary condition based on a point inside the geometry

Hi,

I have a 3D model solving Richards equation, (my goal is to catch the behaviour of a floodplain which is inundated periodically)
How is it possible to impose a boundary condition( for upper boundary of the geometry)
that is able to impose hydraulic head boundary condition for the flooded regions; which these flooded regions are changing during time based on F5 that is a point inside the geometry.
And the other parts of the domain which are not flooded regions: imposing "Inlet boundary condition"?

I mean how is it possible to impose boundary condition which is related to the spatial changing during time? Is there any tutorial like this situation?
Attached also my model.Please let me know if my question is not clear.

Thanks in advance for your answers.
Sepideh