Accepted conventional infiltration theory states that application of water to dry soil over time follows two phases: first the sorptive phase where capillarity rapidly fills the pore system, and the steady state phase where the pore system conducts water at the rate of the soil saturated hydraulic conductivity.  In most cases this is also what we observe in the field:  water enters the soil quickly and it then slows down to reach a steady rate.  This infiltration theory assumes that the wetting angle of the capillaries is 0°; i.e. the capillary has a water film.  If the wetting angle is larger than 0°, the capillary potential that drives water intake during the sorptive phase will also be much smaller, resulting in slow sorptivity.  On hydrophobic soils the wetting angle can approach 90° or, in extreme cases become convex.  

Applying water to very dry soil, perhaps after a long drought, or soil surfaces that were exposed to bushfires or soils with water repellent organic matter, may not have a capillary water film that allows immediate rapid water intake.  Under field condition this can occasionally be observed when water is applied to very dry soil: it initially ponds on the surface before it then quickly infiltrates.  This initial ponding may be the consequence of a larger then 0° wetting angle.  Length of initial ponding will affect onset of runoff and erosion.

This project aims to verify if the sorptive phase can be preceded by a ponded phase and under what conditions it can occur.  The experimental plan would assess the impact of soil type and texture, soil organic matter and antecedent soil water content on the formation of an initial potential ponded phase.  The research would initially be conducted under laboratory conditions using intact soil cores (St Lucia or Gatton Research Labs) and possibly followed up for in situ field conditions.

The results will have implications on how to prepare and avoid runoff and erosion from very dry soils during heavy rainfall event.

Project members

Dr Gunnar Kirchhof

Principal Research Fellow & Senior Lecturer
School of Agriculture and Food Sciences