Influence of the pressure head sequence on the soil hydraulic conductivity determined with tension infiltrometer

An increasing and a decreasing sequence of pressure head, h0, values were applied with the tension infiltrometer (TI) to determine the corresponding hydraulic conductivity, K0. The pressure head sequence is expected to influence the K0 results given the hysteretic nature of the hydraulic conductivity relationship. The objective of this study was to evaluate the influence of the selected pressure head sequence on the hydraulic conductivity of a sandy loam soil measured by a multipotential TI experiment. Twenty experiments were carried out by applying h0 values varying between -150 and +5 mm (site A). The h0 values ranged from -150 to -10 mm in another 20 spots (site B). Both wetting and drying values of K0 corresponding to h0 = -150, -75, and -30 mm were calculated for each experiment using the measured steady-state flow rates. At both sites, higher K0 results were obtained with the descending h0 sequence than with the ascending one. The deviations between the two sequences were more noticeable in site A (deviations by a factor ranging from 2.1 to 3.3, depending on h0) than in site B (deviations by a factor ranging from 1.0 to 2.2), and the values decreased as h0 increased. For most of the considered type of site/pressure head combinations, the differences between the K0 results were statistically significant (P = 0.05). In all cases, the coefficients of variation of the K0 data obtained with the two sequences differed at most by a factor of 1.2, suggesting that the applied h0 sequence did not affect appreciably the relative variability of the K0 results. It was concluded that the dependence of the K0 estimates on both the pressure head sequence (ascending or descending) and the highest value of h0 used within a descending sequence experiment may be neglected for a rough hydraulic characterization of the selected area. However, both factors should be maintained constant in order to obtain truly comparable K0 data from different experiments.