The actively heated fiber-optic (AHFO) technology has become emerged as a research focus due to its advantages of distributed, real-time measurement and good durability. These attributes have led to the gradual application of AHFO technology to the water content measurement of in situ soil. However, all existing in situ applications of AHFO technology fail to consider the effect of soil–sensor contact quality on water content measurements, limiting potential for the wider application of AHFO technology. To address this issue, the authors propose a method for determining the soil–sensor thermal contact resistance based on the principle of an infinite cylindrical heat source. This is then used to establish an AHFO water content measurement technology that considers the thermal contact resistance. The reliability and validity of the new measurement technology are explored through a laboratory test and a field case study, and the spatial-temporal evolution of the soil water content in the case is revealed. The results demonstrate that method for determining the soil–sensor thermal contact resistance is highly effective and applicable to all types of soils. This method requires only the moisture content, dry density, and thermal response of the in situ soil to be obtained. In the field case, the measurement error of soil water content between the AHFO method, which takes into account the thermal contact resistance, and the neutron scattering method is only 0.011. The water content of in situ soil exhibits a seasonal variation, with an increase in spring and autumn and a decrease in summer and winter. Furthermore, the response of shallow soils to precipitation and evaporation is significant. These findings contribute to the enhancement of the accuracy of the AHFO technology in the measurement of the water content of in situ soils, thereby facilitating the dissemination and utilization of this technology.
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