Testing the concentric-disk tension infiltrometer for field measurement of soil hydraulic conductivity

The concentric-disk tension infiltrometer (CDTI) may be used to simultaneously measure the confined (Kc) and the unconfined (Ku) soil hydraulic conductivity, but it has received little testing. Comparison between Kc and Ku can be affected by the calculation approach applied to analyze unconfined steady-state infiltration rates. The objectives of this investigation were to: i) establish the effect of the calculation approach on the estimates of Ku; and ii) compare the Kc and Ku values measured by the CDTI. A clay soil, a structureless sandy loam soil, and a sandy loam soil with a relatively high gravel content were sampled using a disk of radius equal to 107.5 mm, comprising an inner disk with a radius of 50 mm. Six to 15 multipotential experiments with applied pressure heads, h0, of −120, −60 and −30 mm, were carried out, depending on the soil. Three different approaches, i.e. the simultaneous equations (SE), piecewise exponential (PE) and regression (RE) methods, were applied to calculate Ku. The SE and PE methods yielded equivalent Ku results. For the clay soil and the lowest pressure head, the RE method yielded significantly lower Ku values as compared with the other methods. The discrepancies were attributed to the influence of the structural pore system in the clay soil that could not be represented by the RE method. The ratio between the mean values of Kc and Ku varied from 0.96 to 1.45, depending on the considered soil/pressure head combination, and the ratio between the associated coefficients of variation ranged between 0.97 and 2.36. In most cases, Kc was higher and more variable than Ku. Differences between mean Ku and Kc results were statistically significant (P = 0.05) in the structureless sandy loam soil for all h0 values and in the clay soil for the lowest h0 value. A statistically significant relationship between ln(Ku) and ln(Kc) was detected in all soils but the regression line differed from the identity line in the sandy loam soil with a high gravel content. Possible factors determining the observed discrepancies were suggested to include: i) overestimation of steady-state, one-dimensional flow rate from the inner disk, and ii) lower ability of the relatively small inner disk to sample a site-representative surface macroporosity as compared with the relatively large disk used for a three-dimensional measurement. It was concluded that differences between Kc and Ku were small and probably negligible for many practical applications but Kc was less reliable than Ku. An improved quality of the Kc data is expected with a larger inner disk and a longer infiltration run, as compared with the ones considered in this investigation. However, there are practical limits in the possible improvements in terms of both disk size and test duration.