Effect of changing bulk density during water desorption measurement on soil hydraulic properties

Bulk density changes during alternative wetting and drying processes in soils, as well as during measurement of soil hydraulic properties, especially the soil water characteristic curve. However bulk density is usually assumed to be constant. The objective of this study was to investigate bulk density changes during water desorption measurement and the effects of these changes on soil hydraulic properties. Soil water characteristic curves were measured by a pressure plate technique for five soils with textures ranging from silt loam to clay loam. Bulk densities of the soils were determined at all equilibrium pressure values during the water desorption measurements. Measured water characteristic curves for the soils were described by the Brooks and Corey model. Measured water characteristic curves with changing bulk density were compared with those derived from using four characteristic bulk densities-assigned, packing, averaged, and optimal-for five soils. Results showed that there were measurable changes in bulk density during the pressure plate tests. For the four characteristic bulk densities, the differences between fitted Brooks and Corey soil water characteristic curves and those derived from measured bulk density data at each pressure were the smallest for the optimal bulk density. The optimal bulk densities were associated with a pressure of about 0.7 m. For each soil, estimated hydraulic conductivities and water diffusivities were smaller for larger characteristic bulk density values. The hydraulic conductivities and water diffusivities estimated using measured bulk densities were smaller than those derived from any of the four characteristic bulk densities at volumetric water contents less than 30%. The consideration of bulk density change during measurement of soil water characteristic curves and its effect on soil hydraulic properties will be important for bridging the knowledge gap between water flow in hypothetical soils and water flow in real soils.