Hillslope hydrologists have long assumed that the downslope movement of water and solutes can best be described by surface topography since gravitational potential largely dominates hydraulic gradients in steep terrain. Hence with the increased availability of Digital Terrain Maps (DTMs), surface topography is driving many popular hydrological models and is being used to estimate flow pathways in hydrological and geochemical models. This method may suffice at the catchment scale, but at the hillslope scale, flow pathways are not always determined by surface topography.
It is at this critical scale (100–10,000 m2) that water flux and the chemical composition of soil water and groundwater can be measured as they move downslope. The complex interactions between water and solutes along hillslope subsurface flow paths have not been well documented. New evidence suggests that for steep hillslopes with thin soils, the fundamental control on hillslope flow paths is the bedrock surface.