Preferential flow mechanism in a water repellent sandy soil

Dry water repellent soils are difficult to wet, forcing water and solutes to flow via preferential paths through the unsaturated zone. A tracer field experiment was used to investigate the mechanism of preferential flow and transport in a water repellent sandy soil. Water and solutes are distributed by lateral flow within the, relatively wet, thin humose topsoil toward preferential flow paths below this zone. This flow in the upper layer, which is of major importance in the spatial distribution of water and solutes in field soils, is generally not considered, and is introduced here as “distribution flow.” The preferential flow paths are separated by dry soil bodies, which are highly persistent due to their extremely water repellent character and their low hydraulic conductivity. In the wettable zone, below 45 cm depth, water and solutes diverge toward areas below dry soil bodies. Despite this process, the spatial variation in solute concentrations in this zone remained relatively high.