METHODOLOGY FOR DETERMINING HYDRAULIC CONDUCTIVITY WITH TENSION INFILTROMETERS

Tension infiltrometers have become a valuable tool for understanding infiltration in macropores and the soil matrix, but methodology varies. Our objective was to compare tension infiltrometer techniques and calculation procedures for determining unsaturated hydraulic conductivity, K(h), as a function of soil water pressure head (h). Field tension infiltrometer measurements were run to determine K(h) from: (i) steady-state infiltration into an excavated one-dimensional column, (ii) calculated sorptivity and measured change in soil water content for steady-state three-dimensional infiltration into dry soil, (iii) steady-state three-dimensional infiltration with two infiltrometer base sizes, and (iv) steady-state infiltration for three negative heads at the same location using two different calculation schemes. For one scheme, a nonlinear regression method was used to fit a [a constant relating In(K) and h] and K(0) from measured infiltration across three negative heads. The fitted alpha and K(h) were then used to calculate K(h) at each negative pressure head. Calculated K(h) by the nonlinear regression method from three-dimensional infiltration measurements were 105% of measured one-dimensional rates (from excavated columns), closer than any other method of calculation. More importantly, this method did not result in calculated K(h) less than zero or larger than three-dimensional infiltration rates, as some calculation procedures did. The method did not depend on determinations of sorptivity or an initial or final soil water content.