A modified multiple tension upward infiltration method to estimate the soil hydraulic properties

Determination of saturated hydraulic conductivity, K-s, and the shape parameters and n of the water retention curve, (h), is of paramount importance to characterize the water flow in the vadose zone. This work presents a modified upward infiltration method to estimate K-s, and n from numerical inverse analysis of the measured cumulative upward infiltration (CUI) at multiple constant tension lower boundary conditions. Using the HYDRUS-2D software, a theoretical analysis on a synthetic loam soil under different soil tensions (0, 0-10, 0-50 and 0-100cm), with and without an overpressure step of 10cm high from the top boundary condition at the end of the upward infiltration process, was performed to check the uniqueness and the accuracy of the solutions. Using a tension sorptivimeter device, the method was validated in a laboratory experiment on five different soils: a coarse and a fine sand, and a 1-mm sieved loam, clay loam and silt-gypseous soils. The estimated and n parameters were compared to the corresponding values measured with the TDR-pressure cell method. The theoretical analysis demonstrates that K-s and (h) can be simultaneously estimated from measured upward cumulative infiltration when high (>50cm) soil tensions are initially applied at the lower boundary. Alternatively, satisfactory results can be also obtained when medium tensions (<50cm) and the K-s calculated from the overpressure step at the end of the experiment are considered. A consistent relationship was found between the (R-2=0.86, p<0.02) and n (R-2=0.97, p<0.001) values measured with the TDR-pressure cell and the corresponding values estimated with the tension sorptivimeter. The error between the (in logarithm scale) and n values estimated with the inverse analysis and the corresponding values measured with pressure chamber were 3.1 and 6.1%, respectively. Copyright (c) 2016 John Wiley & Sons, Ltd.