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 (Cv), and thermal diffusivity (k). Consequently, volumetric soil water content (θ) can be estimated from its linear relationship with Cv. The accuracy of the DPHP-FBG method in the estimation of Cv, 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 Cv and θ decrease with the extended heating duration. When the heating lasts 20 s, the measured Cv 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.

中文翻译：

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使用双探针热脉冲估计的原位土壤水分和热特性


土壤水分和热特性的时空分布的原位监测对于研究包气带的水和能量传输非常重要。基于光纤布拉格光栅技术的单探头热脉冲法（SPHP-FBG）因其能够实现准分布式实时监测的能力，成为野外监测的研究热点。然而，SPHP-FBG 方法只能获得热导率。本研究开发了一种基于 FBG （DPHP-FBG） 的双探针热脉冲方法。DPHP-FBG 方法可以测量热导率 （λ）、体积热容 （Cv） 和热扩散率 （k）。因此，土壤体积含水量 （θ） 可以根据它与 Cv 的线性关系来估计。在不同加热时间和不同土壤水分条件下测试了 DPHP-FBG 方法估计 Cv、λ 和 θ 的准确性。此外，还进行了蒙特卡罗仿真，以研究 FBG 测量误差对精度的影响。最后，进行了现场测试，以验证所开发的 DPHP-FBG 系统的有效性。结果表明，DPHP-FBG 方法可以准确地估计土壤水分和热特性，而无需针对土壤进行校准。Cv 和 θ 的平均误差随着加热持续时间的延长而减小。当加热持续 20 s 时，在各种水分条件下，测得的 Cv 和 θ 的平均误差分别为 0.02 MJ m-3 K-1 和 0.01 m3/m3。在田间试验中，可以实时获得土壤水分和热特性的时空分布。因此，所提出的 DPHP-FBG 监测系统有可能进行大规模的原位耦合热量和土壤水分测量。