Constant head in a boundary of the model... how to do it?

Hi!
I want to implement a boundary condition in a boundary of my model. This boundary condition is a constant head, so I'am using Hydraulic-head BC, but I'm not sure about the nodes which I have to assign this BC ... Do I just have to assign this BC to the adjacent nodes to this limit in the first slice or in all the slices?  ???
... or just in the nodes that are located just below the constant head? if this is the case, how can I identify quickly those nodes?

I read you! thanks!  :P :P