3.5 Unsaturated Water Transmission Parameters Obtained from Infiltration

Infiltration is the movement of water into soil. In their discussion of the various infiltration measurements, the authors encounter two soil properties that integrate the behavior of unsaturated water transmission. The first is the sorptivity, and the second is the macroscopic capillary length. To describe quantitatively soil water movement, the authors usually use Richards’ equation. The disk permeameter or tension infiltrometer allows measurement of infiltration with a constant negative pressure head at the soil surface. Compared with one-dimensional infiltration, in the three-dimensional axisymmetric flow, the importance of absorption relative to gravity is greater, depending on source geometry, the time required to reach a steady infiltration rate is reduced, and the steady state infiltration rate is greater. The authors describe three-dimensional disk permeameter methods under two headings, transient methods using early time observations, and steady-state methods using the infiltration rate at longer times.