The Flow of Water and Air through Soils

In Part I of the current volume of this Journal Green and Ampt make certain assumptions regarding the factors which control the rate of flow of water and air through soils, the basis of which is that the channels in a soil may be regarded as a bundle of capillary tubes, and that the velocity of flow of a liquid will consequently conform to Poiseuille’s formula. Such an assumption cannot however be maintained. Poiseuille’s formula was deduced from the ascertained rate of flow of liquids through straight tubes of approximately uniform diameter; the latter being 1 mm. or so. The formula is not however really accurate even under these limited conditions, as was shown by Thorpe and Roger, who found it necessary to add the correction – ρV/8πlt. But the passages through the soil are not straight or of uniform diameter for any measurable distance, in fact they are wholly irregular, which Green and Ampt recognise, for they describe them as “irregular in area, length, direction and shape.” But this being the case, Poiseuille’s formula obviously cannot be expected to apply.