In situ soil moisture and thermal properties estimated using a dual-probe heat-pulse

In situ monitoring of the temporal and spatial distribution of soil moisture and thermal properties are important for studying the water and energy transport in the vadose zone. The single-probe heat-pulse method based on fiber Bragg grating technology (SPHP-FBG) has become a research focus in field monitoring because of its capability to realize quasi-distributed and real-time monitoring. However, the SPHP-FBG method can only obtain thermal conductivity. This study developed a dual-probe heat-pulse method based on FBG (DPHP-FBG). The DPHP-FBG method can measure thermal conductivity (λ), volumetric heat capacity (C_v), and thermal diffusivity (k). Consequently, volumetric soil water content (θ) can be estimated from its linear relationship with C_v. The accuracy of the DPHP-FBG method in the estimation of C_v, λ, and θ was tested under different heating duration and various soil moisture conditions. In addition, Monte Carlo simulation was performed to investigate the impact of FBG measurement errors on accuracy. Finally, a field test was conducted to verify the effectiveness of the developed DPHP-FBG system. The results show that the DPHP-FBG method allows accurate soil moisture and thermal properties estimation without soil-specific calibration. The mean errors of the C_v and θ decrease with the extended heating duration. When the heating lasts 20 s, the measured C_v and θ have mean errors of 0.02 MJ m⁻³ K⁻¹ and 0.01 m³/m³, respectively, for various moisture conditions. In the field test, the spatio-temporal distribution of soil moisture and thermal properties can be obtained in real time. Thereby, the proposed DPHP-FBG monitoring system is potential to conduct in situ coupled heat and soil moisture measurements at a large scale.