Areal Extent of Preferential Flow with Profile Depth in Sand and Clay Monoliths

During recent years much attention has been focused on the role of large pores in soil as a preferred route for solute movement. However, most studies have failed to relate the macropore areal extent to profile depth, which was done in this study. The areal extent of pores participating in displacement of water and contaminants with soil depth and its hydrological significance was investigated in two texturally different soils, both under ponded and transient flow conditions. Soil monoliths (1 m long, 0.3 m diameter) of a clay soil and a sandy soil were used for this purpose. Water (100 mm) stained with acid red (azophloxine) at a concentration of 3.8 g/L was applied to each monolith under the two hydro-logically different regimes. Subsequently, they were cut in 0.1-m increments and each transect was examined visually and recorded graphically for the areal extent of stained soil. A considerably larger area was stained at each depth in the sand compared with the clay soil, for example, around 83 and 22% at...