Estimating soil water retention curves from soil thermal conductivity measurements

The soil water retention curve represents the relationship between soil water content (theta) and matric potential (psi). The van Genuchten (vG) model is commonly used to characterize the shape of a theta(psi) curve. Based on the similarities between theta(psi) curves and soil thermal conductivity (lambda) versus theta curves, Lu and Dong proposed a unified conceptual lambda(theta) model (LD model) for estimating lambda(theta) curves from theta(psi) curves. Their work makes it possible to relate the shapes of lambda(theta) curves to theta(psi) curves. In this study, we present an empirical approach to estimate the vG model parameter m from the LD model shape parameter p based on a model calibration with theta(psi) and lambda(theta) datasets obtained from 10 soils. The saturated water content theta s and the vG model parameter alpha are estimated from selected soil properties (i.e., bulk density, particle density, particle size distribution and organic carbon content), and the residual water content theta r is estimated from the LD model parameter theta f. For model evaluation, the theta(psi) curves of six soils were estimated from measured lambda(theta) values and selected soil properties, and were compared to direct theta(psi) measurements. The proposed method performed well with root mean square errors of estimated theta values ranging from 0.015 to 0.052 cm3 cm-3 and bias ranging from -0.009 to 0.040 cm3 cm-3. We conclude that the proposed method accurately estimates theta(psi) curves from lambda(theta) curves and selected soil properties.