The breakdown of water repellency and solute transport through a hydrophobic soil

Unsaturated infiltration into the Ramiha silt loam, an Andic Dystrochrept, follows the classic pattern. A rapid drop-off from a high flow rate, seemingly induced by capillary attraction, appears followed by an apparent steady-flow maintained by gravity at around 0.5 μm s−1. Beyond 100 min, however, the infiltration rate climbs nearly linearly to exceed 4 μm s−1 as the soilʹs water repellency breaks down. This is only evident after a period that might exceed the observerʹs attention span. The hydrophobicity in this case could be due to one, or a combination, of the many unusual characteristics of this soil—its low bulk density (0.8 Mg m−3), its strongly aggregated nature, the presence of mycorrhizal fungi, its high organic matter content (16.5%), or the presence of allophanic clay (4%).