What's Wrong with Soil Physics? II. Concealed Contradictions.

Repeated use of the van Genuchten (vG) soil water retention function cannot give it a firm physical basis, any more than endlessly repeating a statement can make it true.  The vG equation, which has been accorded far more physical significance than its originator ever intended, owes its “success” to the very contradictions embedded in it.  For example, while it supposedly applies to systems of infinite size, it ignores the fact that the curvature at the wet end of the water retention curve is typically eliminated by increasing the system size.  Similarly, its hydraulic conductivity formulation contains an adjustable tortuosity/connectivity factor, but its arithmetic averaging implies parallel flow, i.e. infinite connectivity.  When one of our key equations purchases flexibility at the price of losing its physical basis, we come face to face with a deeper question: is soil physics a branch of physics seeking the physical bases of soil phenomena, or is it merely about finding convenient numerical descriptions of those phenomena?