Evidence for the onset and persistence with depth of preferential flow in unsaturated fractured porous media

Two distinct types of fracture flow can occur in unsaturated fractured porous media: non-preferential flow, whereby the fractures and matrix wet up in equilibrium, and preferential flow, whereby water in the fractures bypasses the matrix. This has important implications for how infiltration, recharge, groundwater flooding and contaminant transport are modelled. Taking the UK Chalk as a case study, we explore evidence for the occurrence of unsaturated preferential fracture flow, considering separately its initiation in the near surface, and persistence at 20-30 m depth. We postulate a link between the apparent hysteretic response of the Chalk soil moisture characteristic, which was observed and simulated, and the initiation of preferential recharge. Focusing on observed water table responses to an extreme rainfall event on 20th July 2007, we show by inverse modelling of recharge that preferential flow persisted to a depth of 20 m below ground level, but not to 30 m, resulting in markedly different water table responses at two boreholes. These findings both lend support to our conceptualisation. However, since the observations from which preferential flow can be inferred are few and are indirect, quantification of the controls on the onset and depth persistence of preferential flow remains a significant challenge.