Modelling subsurface drainage pathways in an artificial catchment

The relationship between rainfall and runoff in catchments is a nonlinear function largely controlled by hydrologically connected hillslope elements. In this study, we investigate material and geometrical hillslope properties determining connection of hillslope elements to form subsurface drainage pathways in an artificial catchment (Chicken Creek) and we examine the impact of the drainage pathway connectivity on the catchment runoff. We developed a three-dimensional model based on percolation theory to simulate the drainage pathways in the catchment and to quantify the contribution from subsurface flow paths to total catchment outflow. The simulations show that the degree of drainage pathway connectivity depends on small-scale variability of soil depth, soil hydraulic properties and topography. Additionally, connectivity was very sensitive to changes of soil water content in the catchment with dramatic change of subsurface flow for small changes in soil water content. This threshold-like behaviour of flow paths connectivity accounted for the sudden availability of a large amount of drainable water, postulating a nonlinear rainfall–runoff relationship as confirmed by first observations in the Chicken Creek catchment. The model reproduced subsurface flow patterns, which were indicated by soil moisture measurements and visual observations in the catchment.