Diameter of drying zone

This is the diameter around a source where the soil has dried out. The equation originates from the book 'Kabel und Leitungen für Starkstrom' by L. Heinhold, 5th edition 1999 (in German).

Symbol
$D_{dry}$
Unit
m
Formulae
$\frac{4 L_{psc} g_{dry}}{g_{dry}^{2} - 1}$