PREFERENTIAL FLOW MECHANISM IN A WATER REPELLENT SANDY SOIL

Dry water repellent soils are difficult to wet, forcing water and solu tes to flow via preferential paths through the unsaturated zone. A tra cer field experiment was used to investigate the mechanism of preferen tial flow and transport in a water repellent sandy soit. Water and sol utes are distributed by lateral flow within the, relatively wet, thin humose topsoil toward preferential flow paths below this zone. This fl ow in the upper layer, which is of major importance in the spatial dis tribution of water and solutes in field soils, is generally not consid ered, and is introduced here as ''distribution flow.'' The preferentia l flow paths are separated by dry soil bodies, which are highly persis tent due to their extremely water repellent character and their low hy draulic conductivity. In the wettable zone, below 45 cm depth, water a nd solutes diverge toward areas below dry soil bodies. Despite this pr ocess, the spatial variation in solute concentrations in this zone rem ained relatively high.