Using numerical modelling to evaluate the capillary fringe groundwater ridging hypothesis of streamflow generation

The controls on pre-event water contributions to streamflow are still poorly understood, despite a number of proposed processes. One of the most common is the capillary fringe induced groundwater ridging mechanism, identified in many environments as a control on rapid mobilization of groundwater into the channel during events. Nevertheless, despite widespread acceptance, there is little evidence for such a phenomenon outside of particular environments and test cases for which it has been quantified. We use a flow and transport modelling tool to test a number of hypotheses concerning the capillary-fringe groundwater ridging mechanism. The original Abdul and Gilham (Abdul, A.S., Gillham, R.W., 1989. Field studies of the effects of the capillary fringe on streamflow generation. Journal of Hydrology 112, 1-18) laboratory experiment (that is still regarded by those working in the field as the main proof-of-concept) is replicated numerically within a 2D finite element code. An indication of the 'spaces' of applicability of the process in the context of the laboratory experiment is determined (soil type, antecedent moisture, riparian volume, slope, rainfall intensity). We show that in only a limited number of cases, high proportions of pre-event water are sustained from this process.