Aspects of Quasilinear Infiltration from Surface Sources, Especially the Case α = 0

Three recent papers on steady two‐dimensional infiltration from spatially variable surface sources may lead to confusion about the quasilinear analysis. The papers assert that putting α = 0 in quasilinear solutions for the stream function gives solutions for saturated flows. In fact, putting α = 0 yields the precisely opposite case of purely capillary‐driven unsaturated flows. The argument of the papers is that for α = 0 the quasilinear equation for the stream function is reduced to Laplace's equation, which is obeyed by the stream function for saturated flows. The kinematics of all two‐dimensional solenoidal irrotational flows is, however, such that their stream functions obey Laplace's equation, and the physical meaning, and indeed the physical existence, of any flow can be determined only from supplementary dynamic information about the velocity potential.