Relating Water Retention to Transient Soil Bulk Density

Abstract:

Soil bulk density (ρ_b) changes over time due to tillage, root growth, rainfall, freeze-thaw cycles and a range of other processes and disturbances. In turn, transient ρ_b influences many surface soil hydraulic properties, such as the water retention curve, but associated changes are usually ignored by vadose zone and soil-water simulation models. In this study, we introduce an approach to relate the van Genuchten water retention model and transient ρ_b. The four parameters used in the van Genuchten model are expressed by ρ_b-related empirical equations. To determine the empirical expressions of the four parameters, we applied a curve fitting procedure to measured water retention curves with two different ρ_b. These fitted equations were next used to predict the water retention curve of the soils under various ρ_b conditions. Six soils with a range of textures were used to evaluate the accuracy of the proposed approach. The predicted water retention curves agree well with measured data. The coefficient of determination varied from 0.967 to 0.999 and RMSE varied from 0.005 to 0.025 m³ m^-3 between predicted and measured water retention curves. Therefore, the proposed approach could accurately predict soil water retention curves with transient ρ_b and has the potential to be used in soil-water simulation models to account for dynamic soil surface properties.