Can we define "drainage density" for a soil column (point-scale)?

From what I read all over the web and articles, drainage density is a parameter that is defined for a catchment.

I want to use a hydrological model to simulate the percolation from the bottom of a soil column, therefore my spatial scale is point-scale.

This model both works in point-scale mode and distributed mode. In the point-scale, I still need to choose a value for the drainage density (DD).

As described in this paper :

$$ DD = dfrac{1}{2 HL} ,$$

where $HL$ is the length of hillslope.

Any idea how to calculate $HL$ for a soil column?

Can I say $HL$ is equal to the depth of the soil column (surface to the seepage face)?

Or it doesn’t make sense to define DD in the point-scale?

In this model, DD affects the ratio of $dfrac{lateral flow}{percolation}$. As for the lateral flow in the soil column, beside the topographical slope and soil textures, is there any other parameters that is important to consider?

Please let me know if I did not explain my question clearly.