Presentation: oral Water Repellent Soils: Stable Or Unstable Wetting Fronts? Sols hydrophobes: front d'humidité stable ou instable?

Only very recently, it has become evident that soil hydrophobicity is wide-spread in the world. Generally, soil hydrophobicity is caused by the presence of organic compounds, like humic and fulvic acids and fatty waxes, which are able to coat individual soil particles and aggregates. These organic compounds might become water repellent below a certain critical soil water content due to molecular conformational changes. Above the critical soil moisture content a soil behaves wettable and below it hydrophobic. Due to the presence of hydrophobic coatings, soil water retention functions show typical hysteresis loops in the range below the critical soil water content. Generally, it can be stated that wetting branches below the critical soil moisture content are very steeply curved with low water entry values, while wetting branches above the critical soil water contents are generally gently curved with much higher water entry values. Due to these typical water retention branches, perturbations at an infiltrating wetting front might grow into vertically directed preferential flow paths in initially dry soil (i.e. under conditions below the critical soil moisture content), causing an accelerated downward transport of water and solutes in this case. In the opposite situation, with initial soil water contents exceeding the critical soil water value, perturbations at a