Note on the Dynamics of Capillary Flow

Abstract

Theory of the movement of moisture in soil.---Attention is called to experimental data which show that there usually exist in the soil appreciable moisture gradients which indicate that it cannot be regarded as a bundle of capillary tubes and that therefore Washburn's theory of capillary flow does not rigorously apply in this case. However by using Poiseuille's equation modified by the assumption that the variation of moisture content does not appreciably change the kinematical resisting force, together with empirical equations derived from soil moisture studies connecting the capillary pressure and vapor pressure with the moisture content, an equation for horizontal flow is derived, $V=k(\mathrm{I}\ensuremath{-}{e}^{\ensuremath{-}\ensuremath{\beta}t})$, which is in qualitative agreement with experimental results. A more extended treatment of the subject is given in an article by the author in Soil Science.