Moisture structures of laterally expanding fingering flows in sandy soils

Water sometimes infiltrates into a sandy soil along preferential flow paths called fingers, rather than homogeneously. In order to understand the physical fundamentals of fingering flow, we carried out two-dimensional infiltration tests under continuous rainfall. Each finger was composed of two moisture structure zones, a finger core and a finger-swelling zone surrounding the finger core. Each finger can be classified as a low-swell finger (LSF) or a high-swell finger (HSF) based on the swelling velocity. In LSF, water moves laterally across a constant potential boundary where sorptivity is a dominant factor in making the finger swell. In HSF, on the other hand, water moves laterally through a constant flux boundary where pressure gradient is a driving force. In the latter, accordingly, the corresponding water flux can be estimated by the hydraulic conductivity multiplied by the water pressure gradient. Based on these observations, we propose a growth model for a finger moving downward along with subsequent lateral expansion, which makes it possible to estimate the area of soil wetted by fingers. The estimated area is in a fairly good agreement with that observed.