Effects of under-vacuum and vessel soil sample saturation on soil hydraulic conductivity estimated by the core method in the case of paddy soils

In paddy fields most of the percolation flux is regulated by the saturated soil hydraulic conductivity (Ks) of the hardpan. The core method performed on large undisturbed soil samples generally allows a good estimation of Ks. However, only little attention is paid in the literature to the saturation process adopted to prepare soil samples for analysis. Two methods are available: vessel saturation (AtmSat), and saturation under vacuum (VacSat). In autumn 2015, ten large undisturbed soil samples were collected from the hardpan layers of three paddy fields located in Lombardy (Italy). In particular, five soil profiles were opened and two samples were collected from the hardpan of each profile. Ks was determined through the core method by adopting the two saturation methods. The first outcome of the research is that the saturation method strongly influences the obtained Ks value. In order to evaluate the accuracy of the estimated Ks values and to determine which saturation method is advisable in the case of paddy fields, the percolation flux estimated by the Darcy law (applied considering measured Ks) for the three paddy fields was compared to the flux obtained as the residual in the water balance equation for the same fields (where all the other terms were measured during the agricultural season 2015). Results show that VacSat provides better results with respect to AtmSat; however, vacuum must be applied slowly and the final Ks value is reached after a long convergence time.