The boundary between preferential flow and Richards-type flow is a priori set at a volumetric soil water content theta* at which soil water diffusivity D (theta*) = eta (= 10(-6) m(2) s(-1)), where eta is the kinematic viscosity. First we estimated with a hydrostatic approach from soil water retention curves the boundary, theta(K), between the structural pore domain, in which preferential flow occurs, and the matrix pore domain, in which Richards-type flow occurs. We then compared theta(K) with theta* that was derived from the respective soil hydrological property functions of same soil sample. Second, from in situ investigations we determined 96 values of theta(G) as the terminal soil water contents that established themselves when the corresponding water-content waves of preferential flow have practically ceased. We compared the frequency distribution of theta(G) with the one of theta* that was calculated from the respective soil hydrological property functions of 32 soil samples that were determined with pressure plate apparatuses in the laboratory. There is support of the notion that theta(K) approximately = theta(G) approximately = theta*, thus indicating the potential of theta* to explain more generally what constitutes preferential flow. However, the support is assessed as working hypothesis on which to base further research rather than a procedure to a clear-cut identification of preferential flow and associated flow paths.