Comment on the treatment of residual water content in “A consistent set of parametric models for the two‐phase flow of immiscible fluids in the subsurface” by L. Luckner et al.

Luckher et al. [1989] (hereinafter LVN) present a clear summary and generalization of popular formulations used for convenient representation of porous media fluid flow characteristics, including water content (0) related to suction (h) and hydraulic conductivity (K) related to 0 or h. One essential but problematic element in the LVN models is the concept of residual water content (Or; in LVN, Ow,r). Most studies using Or determine its value as a fitted parameter and make the assumption that liquid flow processes are negligible at 0 values less than O r. While the LVN paper contributes a valuable discussion of the nature of Or, it leaves several problems unresolved, including fundamental difficulties in associating a definite physical condition with Or, practical inadequacies of the models at low 0 values, and difficulties in designating a main wetting curve. The LVN paper (p. 2188) defines Or as the value of 0 at which films of wetting liquid coating the solid particles are reduced to the point where "all or parts of the connecting films become so thin, and hence so strongly adsorbed onto the solid phase, that the wetting fluid loses its capability to respond to hydraulic gradients." It is highly desirable to have such a physically based definition, but this particular definition is not well supported by observation. The possibility that liquid flow might cease at a nonzero 0 value has been investigated, but so far there is no conclusive experimental evidence that such a condition exists. Even if it does, there is no evidence that commonly cited 0 r values (sometimes as large as 0.2 or more [e.g., van Genuchten, 1980]) represent he 0 at which liquid flow ceases. On the contrary, an experimental investigation with a sandy soil (porosity 0.335) at 0 -0.088 has shown not only that liquid flow occurs under conditions near Or (determined by curve fitting to be 0.076), but that it closely obeys Darcy's law [Nimmo et al., 1987]. If the described no-liquid-flow condition does exist, it would be no surprise if Or determined from it differed substantially from Or determined from O(h) curve fits. The condition described by LVN is based on dynamic phenomena, so O(h) curves, being based on static liquid retention properties, have no necessary fundamental ink with it. Apart from definitional difficulties, it is unrealistic to represent a O(h) curve as never becoming less than Or. In reality, as h continues increasing, 0 continues decreasing until it is 0. Vapor transport eventually becomes dominant over liquid transport (see, e.g., Rose [1963]), but this fact does not require the curve to remain above any particular 0 value. Figure 1 illustrates this fact with data of Schofield