Approaches for Estimating Soil Water Retention Curves at Various Bulk Densities With the Extended Van Genuchten Model

Soil bulk density (ρ_b) variations influence soil hydraulic properties, such as the water retention curve (WRC), but they are usually ignored in soil water simulation models. We extend the van Genuchten WRC model parameters to account for ρ_b variations using a series of empirical expressions. WRC measurements made on eight soils with various ρ_b, and textures are used to calibrate these ρ_b-related empirical equations. Accordingly, two approaches are developed to estimate WRCs of soils at various ρ_b. Another eight soils with a wide range of ρ_b and textures are used to evaluate the accuracy of the new approaches. Approach 1 estimates WRCs for each soil at various ρ_b using a WRC measurement made at a reference ρ_b and the soil texture fractions. This approach gives reasonable WRC estimates for the eight validation soils, with an average root-mean-square error (RMSE) of 0.025 m³/m³ and an average determination coefficient (R²) of 0.94. For Approach 2, a WRC measurement made at a reference ρ_b and one additional water content-matric potential value measured at a different ρ_b value are used, which produces WRC estimates with an average RMSE of 0.017 m³/m³ and an average R² of 0.97. The methodology used in Approach 2 is also applied to the Brooks and Corey WRC model to obtain accurate and precise WRC estimates. The proposed approaches have the potential to be incorporated into simulation models for estimating soil hydraulic properties that are affected by transient and variable ρ_b.