Modeling the soil water retention curve for conditions of variable porosity

Temporal changes in the pore space of unsaturated soils may result from such processes as mechanical compaction and tillage of agricultural soils. Moreover, soil pore space and soil water content are mutually interactive. For example, porosity and pore-space geometry control the unsaturated soil hydraulic properties such as the water retention curve, while conversely soil water and hydraulic stress often affects soil pore size. The experiments discussed in this paper focus on both of these two aspects and additionally consider how a previously applied mechanical stress can influence the effects of subsequent hydraulic stresses on the soil hydraulic properties. We show the effects of uniaxial volume changes during drying on the water retention curves of three sandy and silty soils. Results indicate that previously loaded soils differ in how drying affects volume changes. Uniaxial volume changes of all soils could be described well with a model similar to van Genuchten's soil water retention function. Four models were developed to describe soil water retention as a function of variable void ratio and then tested against measured data. The variably porosity models generally give much better descriptions of the observed retention data than the conventional van Genuchten function. Two models which express the van Genuchten shape parameters and n in terms of two adjustable coefficients were found to be particularly attractive. The models presented in this study permit one to consider changes in the void ratio induced both by externally applied stress and by internal (i.e., hydraulic) stress.