Abstract: In recent years, the active heated fiber optic(AHFO) method has been widely applied in the in-situ soil water content due to its ability to be used in long distance, good durability and distributed monitoring. However, the soil temperature in some areas in China varies greatly with seasons. The soil temperature reaches dozens of degrees below zero in winter, causing the water to freeze. Therefore, it is of great significance to study the influence of different ambient temperatures on the in-situ monitoring results of soil water content using AHFO method. In this paper, the self-developed internally heating alundum tube FBG sensor was used to monitor three loess samples with different water contents at a temperature of ?20 to 40 ℃. The results show that when the water content of the loess is not higher than 8.8%, the maximum error of water content measured at different temperatures is less than 1.5% of the range. Since the measurement accuracy is high, the effect of ambient temperature on the monitoring results of water content of soils can be ignored in the actual measurement. When the water content of loess is higher than 8.8%, the maximum error of the measurement of the water content in the positive temperature zone accounts for about 1.34% of the range, and the measurement accuracy is still high. In the negative temperature zone, since free water and capillary water undergo phase transition when the temperature is lower than 0 ℃, the heat transfer mechanism and sample structure change. As a result, the original calibration formula is no longer applicable. Therefore, it is necessary to recalibrate the relevant parameters in water content measurement. The research results provide important reference value for further improvement and application of this technology.

Key words: active heated fiber optical method, soil, in-situ water content, ambient temperature, error analysis