Connectivity due to preferential flow controls water flow and solute transport at the hillslope scale

Understanding the major controls on water flow and solute transport at the hillslope scale remains a major topic of research despite numerous hillslope experiments at different sites around the world. For example, the influence of lateral preferential flow due to pipes or macropores in the subsurface flow is still unresolved. Experiments show the often paradoxical finding of fast entry of event water and fast mobilization of applied tracers with a concomitant displacement of largely “old” water from the slope base into the stream or riparian zone. Detailed investigations of the actual structures in the soil that promote and conduct lateral water transfer often show relatively short length scales. Thus, many key questions exist, including: How do the preferential flow systems connect across the hillslope during rainfall events? How does this preferential connection affect the mixing of new and old water? We use a new virtual experiment approach to identify the controls of water flow and solute transport at the hillslope scale for two well studied hillslopes in New Zealand and Japan. Previous detailed information on soil pipe and soil matrix properties at these sites provides the necessary dialog for our virtual experiments. Virtual experiments combine the knowledge of experimentalist and modeler to study the influence of pipe density, geometry and pipe length on water flow, nutrient transport, and event and pre-event water contribution. The approach is able to show how different areas within the hillslope connect and disconnect and how these episodic connections might control threshold behavior observed at these and other experimental hillslope sites around the world.