New insights into the correlation between soil thermal conductivity and water retention in unsaturated soils

The heat transfer and water retention in soils, governed by soil thermal conductivity (λ) and soil water retention curve (SWRC), are coupled. Soil water content (θ) significantly affects λ. Several models have been developed to describe λ(θ) relationships for unsaturated soils. Ghanbarian and Daigle presented a percolation-based effective-medium approximation (P-EMA) for λ(θ) with two parameters: scaling exponent (t_s) and critical water content (θ_c). In this study, we explored the new insights into the correlation between soil thermal conductivity and water retention using the P-EMA and van Genuchten models. The θ_c was strongly correlated to selected soil hydraulic and physical properties, such as water contents at wilting point (θ_pwp), inflection point (θ_i), and hydraulic continuity (θ_hc) determined from measured SWRCs for a 23-soil calibration dataset. The established relationships were then evaluated on a seven-soil validation dataset to estimate θ_c. Results confirmed their robustness with root mean square error ranging from 0.011 to 0.015 cm³ cm⁻³, MAE ranging from 0.008 to 0.013 cm³ cm⁻³, and R² of 0.98. Further discussion investigated the underlying mechanism for the correlation between θ_c with θ_hc which dominate both heat transfer and water flow. More importantly, this study revealed the possibility to further investigate the general relationship between λ(θ) and SWRC data in the future.