Stable or unstable wetting fronts in water repellent soils – effect of antecedent soil moisture content

Abstract

Dry water repellent soils are known to inhibit water in®ltration, ultimately forcing water to¯ow via preferential paths through the vadose zone. To study water¯ow and transport in a water repellent sandy soil, a bromide tracer experiment had been carried out, which started in the fall after winter wheat had been sown. Despite the uniform tracer application, soil core sampling indicated that bromide concentrations varied largely from place to place. Wetter sites in the experimental ®eld received more bromide, due to lateral transport through a thin top layer. Wetting fronts in®ltrated deeper here, leading to perturbed wetting fronts in the experimental ®eld. In contrast to what was expected, the wetting front perturbations did not grow to ®ngers. Numerical results indicate that this was attributed to the relatively high soil water contents during the experiment, which caused the soil to be wettable instead of water repellent. The water-entry capillary pressure of the secondary wetting branch exceeds the air-entry capillary pressure of the primary drainage branch in this case. In the opposite situation, with the water-entry capillary pressure of the secondary wetting branch beneath the air-entry capillary pressure of the primary drainage branch, perturbations would have grown to ®ngers. Such a situation occurs during in®ltration in initially dry, water repellent soil. The results presented illustrate the effect of antecedent moisture conditions on the formation of stable and unstable wetting fronts, and its relation to the moment of tracer application. #

Development of Unstable Flow and Reduced Hydraulic Conductivity due to Water Repellence and Restricted Drainage

“…While the effect of soil‐air compression on wetting front instability is well documented (Latifi et al, 1994; Wang et al, 1997, 1998b; Bicalho et al, 2000; Seymour 2000), the effect of restricted water movement on wetting front stability has not been widely reported. In keeping with the findings of this study, Ritsema et al (1998b) also found wetting front instability or perturbations in the wetting front developed during infiltration into previously water repellent soil at high antecedent soil moisture. They attributed these perturbations to places in the soil with lower (actual) water repellence due to either a localized reduction in potential water repellence or higher soil water content.…”

“…Water repellence and the occurrence of unstable flow is strongly influenced by antecedent soil moisture (King, 1981; Ritsema and Dekker, 1994; Dekker and Ritsema, 1996b; Ritsema et al, 1998b; Dekker and Ritsema, 2000; Wang et al, 2000b). Water repellence is usually low or completely absent under prolonged wet conditions and most severe during extended dry periods (Leighton‐Boyce et al, 2005; Keizer et al, 2008).…”

Influence of antecedent soil moisture on hydraulic conductivity in a series of texture‐contrast soils

“…In the wet treatment, instability in the wetting front developed between 4 and 10 cm depth which led to a greater proportion of the A1 horizon participating in flow in the wet treatments, than in the dry treatments ( Figure 3). The development of perturbations in the wetting front of the wet treatment were attributed to variation in soil moisture which led to slightly greater infiltration in some places than others ( Figure 3) (Ritsema et al, 1998).…”

Influence of biofilms on the water repellency of urban soil samples

“…King, 1981;Quyum et al, 2002;Ritsema et al, 1998). Further studies point to an additional influence of time (Doerr et al, 2000;Schaumann et al, 2005), of sample history (Hurraß and Schaumann, 2006;Douglas et al, 2007) and of drying temperature (Bayer and Schaumann, 2007) on wettability.…”