In situ characterization of hydraulic conductivities of individual soil profile layers during infiltration over long time periods

Several studies have raised serious doubts about the suitability of small cores for measuring water‐movement attributes, due to their potential to provide unrealistic representation of macropore connectivity and abundance. This study explored the potential of lysimeter‐scale experiments to calculate the hydraulic conductivity, K ( ψ m ), of undisturbed soil layers in a matric potential ( ψ m ) range between 0 and −4 kPa. Four large lysimeters were collected from a Dystric Cambisol. For each lysimeter a tension infiltrometer supplied infiltrating water under suctions of 0, 0.5, 1 and 1.5 kPa. Soil water dynamics were measured in situ using arrays of tensiometers, at depths corresponding with layer boundaries. The results show clearly that infiltration and drainage rates are intimately linked to temporal ψ m dynamics, which themselves are determined by preferential flow and soil‐layer interactions. A quasi‐steady state was identified as when infiltration matched drainage, and ψ m measurements showed each layer had a stable hydraulic gradient, which then allowed in situ determination of the K ( ψ m ) relationship of individual soil layers. For this soil K ( ψ m ) is distinctly different for each soil layer, and these differences are consistent among the four lysimeters. A consistent feature is that all layers have a distinct change in the slope of the K ( ψ m ) relationship, in the ψ m range of −0.5 to −1.5 kPa, highlighting a dual‐porosity character. The whole‐column infiltration behaviour was strongly linked to the K ( ψ m ) relationship of the surface layer (0–2 cm depth), and therefore hydraulic characterization of this layer should be a critical component of a soil survey.