Can we define "drainage density" for a soil column (point-scale)?
Fundamentally it doesn't make sense to describe the point scale model with hillslope length and drainage density. If you are looking to model a soil column, you could use Darcy's law or Richard's equation. If you are looking to model the catchment, the point scale is not appropriate, although you could always incorporate those point-scale appropriate equations into a more complete hydrologic model if you wish (something like HydroGeoSphere does), but that is still a catchment scale model.
In either case, this comes back to 1) what is the purpose of your modelling exercise, and 2) making sure that your choice of model is appropriate for that purpose. A hydrologic model that uses hillslope length for a point scale is not an appropriate choice for what (I think) you are trying to do.