Determining water content and bulk density: The heat-pulse method outperforms the thermo-TDR method in high-salinity soils

Abstract

Heat-pulse (HP) and thermo-time domain reflectometry (thermo-TDR) methods have been used to determine soil thermal properties, water content (θ) and bulk density (ρb) simultaneously. Their performances on salt-affected soils, however, remain unknown. This study investigated the effect of salinity on HP signals and thermo-TDR measured electromagnetic waveforms, and the derived θ and thermal property values of packed soil columns with various textures, saturations and bulk electrical conductivities (σa). The thermo-TDR and HP-based methods for estimating ρb values were also evaluated. The results showed that: (1) at σa values lower than 1.0 dS m−1, the TDR method provided reliable θ with relative errors within 5%; salt effects became apparent at σa values greater than 1.0 dS m−1 due to the distortion of TDR waveforms; the TDR method failed to estimate θ at σa > 2.71 dS m−1 because the 2nd reflection point on the waveform was undetectable; (2) salinity had negligible effects on soil thermal property values in the studied range (σa < 7.59 dS m−1), and the HP-based approach was able to derive θ and ρb values from thermal property measurements, with root mean square errors within 0.02 m3 m−3 for θ and within 0.12 Mg m−3 for ρb. Thus, the HP-based approach outperformed the thermo-TDR approach for determining θ and ρb values in soils with σa > 1.0 dS m−1.