A calibration-free approach for measuring soil thermal properties and bulk density using the thermo-TDR technique

Traditional heat-pulse methods for determining soil thermal properties require frequent probe spacing calibrations due to their susceptibility to geometric errors in heat capacity ( C ) estimation. Here we present a novel calibration-free thermo-TDR technique as a methodological innovation that concurrently measures soil bulk density ( ρ b ), thermal conductivity ( λ ), and C by integrating heat-pulse theory with soil constitutive relationships within a unified inversion framework. The procedure: (i) directly calculates λ from heat-pulse signals using physical probe spacing without calibration; (ii) predicts ρ b via a λ - ρ b model that is parameterized with soil texture and TDR-derived water content ( θ TDR ); and (iii) estimates C from ρ b and θ TDR using the de Vries (1963) model. This approach eliminates the need for probe spacing calibration, which is a key limitation of traditional methods. Laboratory validation with packed soils yielded high accuracy ( RMSE : ≤0.08 Mg m −3 for ρ b ; ≤0.11 MJ m −3 K −1 for C ). Field deployments tracked dynamic ρ b and C during natural drying events (mean RMSE : 0.09 Mg m −3 for ρ b , 0.13 MJ m −3 K −1 for C ), outperforming traditional spaced probe methods that were sensitive to sensor misalignment. Eliminating probe spacing calibration makes this approach ideal for long-term monitoring and integration with automated field sensor networks, advancing studies of soil–water-energy dynamics in agronomic and vadose zone hydrology.