Improved thermo-time domain reflectometry method for continuous in-situ determination of soil bulk density

Quantifying the dynamics of surface soil bulk density (ρb) is important for characterizing water, heat, and gas exchanges in agricultural and environmental applications. Unfortunately, very few approaches are available for continuous in-situ monitoring of ρb. The soil heat capacity-based (C-based) thermo-time domain reflectometry (thermo-TDR) approach has been used to measure ρb in-situ, but this approach gives ρb estimates with relatively large errors. In this study, we present a new soil thermal conductivity-based (λ-based) thermo-TDR approach for continuous and automatic determination of ρb variation in-situ. An error analysis, literature data, and field experiments were used to evaluate the performance of the C-based and λ-based approaches. The error analysis undertaken on hypothetical soils indicated that the new λ-based approach was less sensitive to errors in the measurement inputs than was the C-based approach when the same relative errors occurred, except on very dry soils. Thermo-TDR measurements reported in the literature on seven soils showed that the new λ-based approach provided more accurate and precise ρb estimates, with coefficient of determination (R2) of 0.70 and root mean square error (RMSE) of 0.103 Mg m−3, than did the C-based approach which gave ρb with R2 of 0.32 and RMSE of 0.178 Mg m−3. Two field experiments were conducted to test the performance of the new λ-based thermo-TDR approach for monitoring ρb dynamics. The results showed that following tillage surface ρb increased by about 35% within 40 days. The ρb obtained by the λ-based thermo-TDR approach agreed well with independent core sampling measurements, with an average RMSE of 0.122 Mg m−3. The C-based approach failed to give acceptable ρb estimates in most cases because of probe deflection and environmental factors. We conclude that the new λ-based thermo-TDR approach is a promising method for continuous in situ measurements of ρb.