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.

Luckner 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 (θ) related to suction (h) and hydraulic conductivity (K) related to θ or h. One essential but problematic element in the LVN models is the concept of residual water content (θ_r; in LVN, θ_w,_r). Most studies using θ_r determine its value as a fitted parameter and make the assumption that liquid flow processes are negligible at θ values less than θ_r. While the LVN paper contributes a valuable discussion of the nature of θ_r, it leaves several problems unresolved, including fundamental difficulties in associating a definite physical condition with θ_r, practical inadequacies of the models at low θ values, and difficulties in designating a main wetting curve.