Comment on “Paradoxes and realities in unsaturated flow theory” by W. G. Gray and S. M. Hassanizadeh

Physic .al concepts that explain phenomena that are wellknown in the context of "beaker physics" sometimes have very different consequences in the context of the "glass bead/adsorbed film physics" of mineral softs. The negative pressure paradox of Gray and Hassanizadeh [1991] is an important example. When a beaker of tap water at room temperature is placed beneath a bell jar, evacuation of the jar promptly induces effervescence of dissolved air from the water. That gives way to boiling at greatly reduced pressure as air is pumped from the system, leaving only water and its vapor. That familiar real-life demonstration seems to imply that, indeed, liquid water in unsaturated soils could never survive, or flow, at the negative pressures that soil. physicists infer from indirect measurements, including psyc.hrometric determina-.tions of relative humidity. Years ago, in my contributions to a paper [Bolt and Miller, 1958] cited by Gray and Hassanizadeh [1991], I promoted that viewpoint. I used arguments based on published data on depression of freezing points in unsaturated soils, another proposed indirect method, seemingly beset by ambiguities that I tried to explain.