Near‐Saturated Hydraulic Conductivity in Soils of Contrasting Texture Measured by Tension Infiltrometers

Abstract

AbstractSoil structure is known to strongly affect water flow and solute transport, yet little information is available concerning soil hydraulic properties close to saturation. This study reports detailed measurements of near‐saturated hydraulic conductivity in undisturbed field soils. Steady‐state infiltration rates from tension infiltrometers were measured at the surface of six tilled soils of contrasting texture ranging from loamy sand to silty clay. Hydraulic conductivities (K) at supply pressure heads (h) in the range from − 100 mm to zero were obtained using Wooding's solution for infiltration from a circular source. The paired K,h data were summarized using the technique of functional normalization with three alternative models of K(h): a single exponential function (Gardner's model), a two‐line exponential model, and the Mualem‐van Genuchten model. The commonly used single exponential model was found to be inappropriate in all cases. Both the two‐line exponential and Mualem‐van Genuchten models satisfactorily described the data, with the former performing marginally better for five of the six soils. In the supply pressure head range − 100 mm to zero, K increased by three to four orders of magnitude in the finer textured soils and by about two orders of magnitude in two sandy soils. Estimates of field‐saturated hydraulic conductivity (Kfs) were largest in the fine‐textured soils, presumably because of the influence of continuous surface‐vented macropores. Spatial variability in measured K(h) was small to moderate for all soils. Predicting unsaturated K from soil water release data using Kfs as a matching point was shown to result in serious overestimations.