Measuring soil freezing characteristic curve with thermo‐time domain reflectometry

The soil freezing characteristic curve (SFCC) has applications in determining the soil water retention curve (SWRC) and modelling thermal and hydrological processes in cold regions. Accurate measurement of the SFCC in the laboratory has been constrained to the thawing process because of the interference of supercooling during the freezing process. In this study, we introduce a dynamic method based on the technique of thermo‐time domain reflectometry (thermo‐TDR) for measuring the SFCC quickly and accurately. A simple iced‐toothpick approach was used for eliminating supercooling during a freezing experiment. The performance of the thermo‐TDR‐based method was evaluated by comparing the measurements with the equilibration method on two soils of different textures. The results showed that the supercooling phenomena during the freezing process could be eliminated effectively by inserting iced‐toothpicks into the samples at soil freezing point, and both the SFCC (at a freezing rate of 2.5°C h−1) and the soil thawing characteristic curves (STCC) could be determined accurately with the thermo‐TDR‐based method. The thermo‐TDR measured SFCCs agreed well with estimates from the SWRCs, with an average root‐mean square error (RMSE) of 0.02 m3 m−3. The dynamic thermo‐TDR‐based method has the advantages of being easy to operate, time‐saving, and applicable for in situ monitoring of SFCC, and could be used to determine SWRCs indirectly. Highlights The thermo‐TDR method can measure the soil freezing characteristic curve (SFCC) accurately. Supercooling during the freezing process was eliminated by using an iced‐toothpick approach. The slow‐rate dynamic method provided reliable SFCCs as compared with the equilibration method. The thermo‐TDR measured SFCCs could be used to estimate soil water retention curves.