Thanks for your feedback.

I have looked into the manual again and it seems I have to apply a
minimum thickness as it says that in Modflow layers must be continuous.
So minimum thickness + copying the properties from the layer below seems
to be a good solution.

I have thought about inactive cells too but I could imagine I get in
trouble in case I want to use recharge in the model later.

Dieter Plass
HydroGeo
Nuernberg
Germany