Water Drop Penetration Time Revisited

Water drop penetration time (WDPT), i.e. the time it takes for a water drop to be absorbed by the soil, is widely used as a measure of soil water repellency. Despite its popularity, little is known about the processes that govern WDPT and how WDPT is related to other soil hydraulic properties such as sorptivity. To shed some light on the physics of the WDPT, we measured apparent contact angles of sessile water drops on water repellent sand using a goniometer and compared apparent with effective contact angles of the same sand. Results showed that WDPT can be related to sorptivity by means of apparent and effective contact angles.