Hydraulic properties characterization of undisturbed soil cores from upward infiltration measurements

Estimation of the alpha and n van Genuchten (1980) parameters and the saturated hydraulic conductivity, K-s, of undisturbed soil samples is critical for realistic water flow simulations in the vadose zone. Given the Latorre and Mores-Fernandez (2019) method, which allows estimating K-s, alpha and n from the inverse analysis of a single upward infiltration, the objective of this work is to advance on the theoretical understanding of this method and to apply it on undisturbed soil cores. To this end, the influence of the saturated, theta(s), and residual, theta(r), water content and initial soil tension, h(i), on the alpha, n and K-s optimization was first studied on synthetic soils. A procedure to simultaneously estimate 0(s), alpha, n and K-s, which consisted on optimizing alpha, n and K-s for a range of 0(s) values, was proposed. To this end, alpha, n and K-s were optimized leaving each theta(s) as a fixed value. The method was next applied to experimental curves measured from sieved and undisturbed soil cores of 50 mm high and diameter, sampled the last ones from three fields with different management bare vs. under plant soil; agricultural soil under conservation vs. conventional tillage; and overburden soil from mine vs. topsoil formed by the original soil. Results showed that theta(s) had an important effect on alpha, n and K-s optimization, where the minimum error within the selected theta(s) interval corresponded to the theoretical theta(s) An important effect of h(i) on the optimization was also observed. However, this influence was omitted by using a h(i) value located in the theta(r) region, where do d theta/dh approximate to 0. Under this assumption, theta(r) only affected the theta(s) optimization, which value varied with theta(s). A robust relationship (R-2 = 0.99; p < 0.0001) was found between theoretical and optimized theta(s), K-s and alpha and n calculated for the synthetic soils. Thus, the method allowed estimating four of the five van Genuchten (1980) parameters. An absolute minimum was also observed when the method was applied on experimental infiltration curves. The theta(s) measured gravimetrically on the undisturbed samples was, on average, 8.1% higher than the optimized one. Overall, the method allowed detecting significant differences (p < 0.05) in hydraulic properties between bare and under plant soils and among tillage treatments. The large variability found in the mine's soils prevented to find significant differences within this scenario.

Automated summary

Estimating the van Genuchten parameters and saturated hydraulic conductivity of undisturbed soil samples is crucial for water flow simulations. The study proposed a method to optimize the parameters and applied it to different soil cores, detecting significant differences in hydraulic properties. The relationship between theoretical and optimized values was found to be robust.