Hydraulic Properties and the Water-Conducting Porosity as Affected by Subsurface Compaction using Tension Infiltrometers

Changes in soil physical properties due to compaction are a major concern in agricultural production and modeling of soil water movement and plant growth. The objectives of this study were to measure water infiltration under different compaction levels and to characterize the effects of compaction on the soil's porosity and its associated water-conducting properties. On a silt loam soil, relative to a control, four levels of subsurface compaction were induced: loosening and light, medium, and heavy compaction. Infiltration characteristics were measured in situ using tension infiltrometers. Near-saturated conductivities were calculated using Wooding's equation and further analyzed to derive the hydraulically effective macro- and mesoporosities and the flow-weighted mean pore radii. The data were also used for parameterization of the van Genuchten-Mualem model by inverse parameter estimation. A high susceptibility to the applied compaction was found: the saturated hydraulic conductivity of the heavily compacted soil was 81% less than that of the loosened soil. This soil property may be used as a proxy for compaction-induced changes in hydraulic characteristics. Increasing compaction also decreased the number of hydraulically effective macropores, reduced the flow-weighted mean pore radius, and the alpha(VG) parameter of the van Genuchten-Mualem model. Our findings give indirect evidence for two main effects of the applied compaction. First, the reduced saturated hydraulic conductivity might be due to distortion of structural flow paths, reducing the connectivity and hydraulic effectiveness of many macropores. Second, compaction rearranged the pore space, resulting in more water-conducting mesopores.