Experimental Analysis of Soil Thermal Diffusivity and Thermo-Moisture Conductivity

Soil thermal and hydraulic processes regulate heat transfer, moisture redistribution, and soil–plant interactions in agroecosystems. The study investigated soil thermal diffusivity (K) and thermo-moisture conductivity (δ) under different soil moisture and bulk density conditions in meadow-gray soils (World Reference Base for Soil Resources (WRB): Gleyic Calcisols) and mountain gray-brown soils (WRB: Kastanozems) of Azerbaijan. Laboratory experiments under controlled temperature-gradient conditions demonstrated nonlinear, soil-specific responses. Modeled critical moisture contents for thermal diffusivity were 25.19% in mountain gray-brown soil and 19.52% in meadow-gray soil, with thermal diffusivity declining beyond these levels. Thermo-moisture conductivity also varied nonlinearly: the empirical model for mountain gray-brown soil indicated a maximum at 22.69% moisture, whereas meadow-gray soil showed its highest measured value at approximately 14%. Increasing bulk density generally increased thermal diffusivity but reduced thermo-moisture conductivity. Quadratic empirical models quantified the combined effects of moisture and bulk density. For mountain gray-brown soil, model performance was high for thermal diffusivity (adjusted R2 = 0.913, RMSE = 1.207) and thermo-moisture conductivity (adjusted R2 = 0.922, RMSE = 0.053). A conceptual thermo-hydraulic formulation supported interpretation of coupled heat–moisture transfer. These findings provide an experimental basis for improved irrigation, soil-moisture, and compaction management in agricultural soils.