John R. Nimmo 1991 <p><i>Luckner et al.</i> [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 (<i>θ</i>) related to suction (<i>h</i>) and hydraulic conductivity (<i>K</i>) related to <i>θ</i> or h. One essential but problematic element in the LVN models is the concept of residual water content (<i>θ_r</i>; in LVN, <i>θ_w,_r</i>). Most studies using <i>θ_r</i> determine its value as a fitted parameter and make the assumption that liquid flow processes are negligible at <i>θ</i> values less than <i>θ_r</i>. While the LVN paper contributes a valuable discussion of the nature of <i>θ_r</i>, it leaves several problems unresolved, including fundamental difficulties in associating a definite physical condition with <i>θ_r</i>, practical inadequacies of the models at low <i>θ</i> values, and difficulties in designating a main wetting curve.</p> application/pdf 10.1029/91WR00165 en American Geophysical Union 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. article