Developments in the Physics of Infiltration

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Abstract
Developments in the physics of infiltration, particularly the many theoretical contributions of John Philip, have been based on solutions of Richards' equation for flow in unsaturated soils. Solutions for constant surface soil‐water pressure and constant surface flux into soils initially at a uniform water content have given the physical basis for infiltration that generally takes place in conditions described by more complex initial and boundary conditions. The Green and Ampt step function and Gardner's exponential function models for the relationship between hydraulic conductivity and soil‐water pressure head are often employed to facilitate solutions. However, Richards' equation makes assumptions in the physics of the flow that do not take account of many complicating factors that may vitiate theories based on it. These factors include the influence of the air phase on the soil‐water movement, the effects of soil heterogeneity, soil swelling, soil aggregation and soil instability, deviations from Darcy's law, thermal effects and the hysteresis in soil‐water relationships. These factors need to be addressed in order to provide a better physical understanding of infiltration behavior in the field, first by understanding simple systems, as with much of John Philip's work, and then by progressing to more complex situations.

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