In Situ Determination of Soil Freezing Characteristics for Estimation of Soil Moisture Characteristics using a Dielectric Tube Sensor

The soil freezing characteristic (SFC) plays a critical role in modeling the transport of water, heat, and solutes in frozen soil. Especially, the SFC can be used to estimate the soil moisture characteristic (SMC) for unfrozen soil due to the known similarity between them. In this study, we tested an existing portable dielectric tube sensor in vertical access tubes together with a set of temperature sensors for the determination of the in situ SFC and SMC. The experiment was conducted at three sites with sandy, loamy, and silt loam soils. At each site, three SFC curves were obtained at depths of 15, 25, and 35 cm. The resulting SMCs estimated from the SFCs were compared with the SMCs determined using a pressure plate apparatus. The data from the proposed method and those of the pressure plate fit the Campbell model with 0.806 ≤ R2 ≤ 0.994 and fit the van Genuchten model with 0.638 ≤ R2 ≤ 0.994. The coefficient of determination (R2 = 0.837) and RMSE (131 kPa) showed a good agreement between the characteristic curves from the two methods. Thus, we recommend the dielectric tube sensor for determining the SFC in situ and inferring the SMC across depths.