Moving Away from the Lamppost of the Most Important Curve in Soil Physics.

The soil water characteristic is indeed the most important curve in soil physics. It somehow reflects the complex pore structure of soil and, by coupling the two fundamental state variables of soil water, it is a prerequisite to use the Richards equation for modeling unsaturated water dynamics.
In fact, depending on the structural properties of a porous medium there might be a well defined energetic minimum describing the soil water characteristic as a well defined curve. However, there is ample experimental evidence that even with highly sophisticated experimental set ups we will not be in the position to actually measure this curve. This is true for the lab but even more so in the field. This provokes some crucial questions on how to proceed with this curve: Do we really need more accurate measurements for a curve that in principle is not really measurable? Or do we need an alternative approach to describe the relation between water content and water potential in a more probabilistic way? The latter may account for the fact that any measured pair of these two state variables depend on the history. Such an approach would also allow for including hysteresis which is a well accepted phenomenon but typically, it is not at all accounted for when modeling soil water dynamics. In this respect, to adhere to the existence and validity of a fixed curve could obstruct further developments in the field of soil physics.  In this presentation some experimental evidence is reviewed to finally conclude that we are at a point where we should accept that the ‚most important curve‘ is a concept that should actually be revisited and eventually be replaced.