A new thermo-time domain reflectometry approach to quantify soil ice content at temperatures near the freezing point
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Soil ice content (θi) is an important property for many studies associated with cold regions. In situ quantification of θi with thermo-time domain reflectometry (TDR) at temperatures near the freezing point has been difficult. The objective of this study is to propose and test a new thermo-TDR approach to determine θi. First, the liquid water content (θl) of a partially frozen soil is determined from a TDR waveform. Next, a pulse of heat is applied through the thermo-TDR sensor to melt the ice in the partially frozen soil. Then, a second TDR waveform is obtained after melting to determine the θl, which is equivalent to the total water content (θt ) of the partially frozen soil. Finally, θi is calculated as the difference between θt and θl. The performance of the new approach was evaluated in sand and loam soils at a variety of θ t values. The new approach estimated θt , θl, and θi accurately. The root mean square errors (RMSE) of estimation were 0.013, 0.020, and 0.023 m3 m−3 for sand, and 0.041, 0.026, and 0.031 m3 m−3 for loam. These RMSE values are smaller than those reported in earlier thermo-TDR studies. Repeating the thermo-TDR measurements at the same location on the same soil sample caused decreased accuracy of estimated values, because of radial water transfer away from the heater tube of the thermo-TDR sensor. Further research is needed to determine if it is possible to obtain accurate repeated measurements. The use of a dielectric mixing model to convert the soil apparent dielectric constant to θl improved the accuracy of this approach. In our investigation, application of a small heat intensity until the partially frozen soil temperature became larger than about 1°C was favorable. The new method was shown to be suitable for estimating ice contents in soil at temperatures between 0°C and -2°C, and it could be combined with the volumetric heat capacity or thermal conductivity thermo-TDR based methods, which measured ice content at colder temperatures. Thus, the thermo-TDR technique could measure θi at all temperatures.
This is a manuscript of an article published as Kojima, Yuki, Yuta Nakano, Chihiro Kato, Kosuke Noborio, Kohji Kamiya, and Robert Horton. "A new thermo-time domain reflectometry approach to quantify soil ice content at temperatures near the freezing point." Cold Regions Science and Technology (2020): 103060. doi: 10.1016/j.coldregions.2020.103060.