Experimental Investigation of Preferential Flow in a Near-saturated Intact Soil Sample

Hydraulic conductivity in an intact sample of coarse sandy loam from the Cambisol series containing a naturally developed, vertically connected macropore was investigated during a recurrent ponding infiltration (RPI) experiment performed over a period of 30 hours, in combination with neutron tomography imaging. The RPI experiment consisted of two consecutive ponded infiltration runs, each followed by free gravitational draining of the sample. Three-dimensional neutron tomography (NT) imaging of the dry sample was acquired before the infiltration began. The dynamics of the advancement of the wetting front was investigated using a sequence of neutron radiography (NR) images. Analysis of these images showed that the water front moved preferentially through the macropore at an approximate speed of 2 mm/s, significantly faster than the 0.3 mm/s wetting advancement in the surrounding soil matrix. After outflow started temporal changes in the local water content distribution were evaluated quantitatively by subtracting the NT image of the dry sample from the particular tomography images generated during infiltration runs. The neutron tomography data quantitatively showed the transfer of air from the soil matrix to the macropore. Accumulation of air bubbles in the macropore then affected the hydraulic conductivity of the sample reducing it to 50% of the initial value.