Tomohiro YAMAMOTO

The Study of Temperature Dependence of Measurements from a Dielectric Soil Moisture Sensor

Minjiao LU

Since Topp et al. (1980) reported the TDR method, it has been used as a standard in Japan and abroad. However, there have been a number of reports of an unnatural relationship between soil temperature and soil moisture content measured by the TDR method. However, the factors causing the temperature dependence have not been clarified. Under natural conditions, there are many factors that cause soil moisture fluctuations, and it is unknown to what extent soil temperature itself plays a role. Lu et al. (2015) showed that the effect of soil temperature on soil moisture content is almost proportional to soil temperature itself and TDR soil moisture measurements when soil moisture content fluctuations due to evaporation and rainfall are excluded. In this study, a laboratory experiment was conducted to verify whether the temperature dependence is reproduced under conditions without external influences. In the experiment, glass beads were filled into cylindrical acrylic containers as simulated soil, the volumetric moisture content was adjusted, and the temperature of the sealed experimental apparatus was controlled. Experiments were conducted with four different moisture content patterns for the simulated soil: 3%, 5%, 10%, and 20%. The results showed that the temperature dependence of TDR soil moisture content measurements for all moisture content patterns was confirmed in the laboratory experiments, indicating that soil temperature itself affected the TDR soil moisture content measurements. The temperature dependence of TDR soil moisture content measurements tended to increase with increasing soil moisture content over a similar temperature range. In addition, a negative correlation was observed between soil moisture content measurements and soil temperature for the 10% volumetric moisture content experiment, and a positive correlation was observed for the other experiments. These results indicate that the effect of soil temperature on soil moisture content may be more complex than the proportional relationship reported by Lu et al. (2015). In addition, a comparison of the correction results using three different temperature calibration methods for the experimentally obtained measurements revealed that the model proposed by Lu et al. (2015), which allows for easy adjustment of the temperature correction factor α, had the best calibration performance, and that the temperature correction factor α is dependent on soil moisture content.