Soil water characteristic curve measurement without bulk density changes and its implications in the estimation of soil hydraulic properties

During soil water characteristic curve (SWCC) measurement, bulk density has been found to deviate from in situ values largely because soil first swells during saturation and then shrinks during dehydration. To obtain the actual in situ soil hydraulic properties, there is a need for a method to measure soil hydraulic properties at the in situ bulk density. In this study, we proposed a new method to measure soil hydraulic properties (SWCC and saturated hydraulic conductivity (Ks)) at the in situ bulk density encountered. Bulk density changes during SWCC measurements based on a centrifuge method for loam and clay were characterized, and their effects on soil hydraulic properties at constant bulk densities (expressed with a suffix of “-C”) were investigated. Over the pressure head range from 0 to 10,200 cm during conventional SWCC measurement of the centrifuge method, the bulk density increased by 0.49 and 0.40 Mg m− 3 for loam and clay, respectively. Ks-C decreased by three and four orders of magnitude for loam and clay, respectively, when the corresponding bulk density increased from 1.20 to 1.59 and from 1.35 to 1.54 Mg m− 3. The increase in bulk density during conventional SWCC measurement changed the shape of the SWCC-C. The modeled unsaturated hydraulic conductivities at constant bulk densities presented approximately one and two orders of magnitude differences when the bulk densities increased from 1.40 to 1.59 and from 1.45 to 1.54 Mg m− 3 for loam and clay, respectively. This type of investigation provides a method to obtain soil hydraulic properties at in situ bulk densities and, thus, should aid our understanding of in situ soil water and solute transport.

► ρb of loam and clay increased during conventional SWCC measurement by a centrifuget.
► The new method obtained soil hydraulic properties with constant ρb (with “-C”).
► ρb increase by conventional procedure decreased Ks-C and changed SWCC-C shape.
► Such changes led to notable errors in unsaturated hydraulic properties estimation.