HILLSLOPE INFILTRATION AND LATERAL DOWNSLOPE UNSATURATED FLOW

Lateral downslope vadose flow occurs in hillslopes with deep homogeneous soils. When and why this happens has been partially answered by past experimental, numerical, and quasi-analytical studies' What has been overlooked, however, is that lateral downslope flow in unlayered soils is largely a drainage phenomenon driven by the changing character of the boundary condition at the soil surface. Numerical simulations show that when rainfall ceases, the near-surface head gradient in a hillslope becomes nearly parallel to the slope as a simple consequence of the surfaces becoming a no-flow boundary, neglecting evaporation. During rainfall, infiltration is nearly vertical for most soils except those with high constant anisotropy.