Test of a Sensible Heat Balance Method to Determine Rates of Soil Freezing and Thawing Under Controlled Boundary Temperatures.

A sensible heat balance (SHB) method using a sequence of heat pulse probes has the possibility of determining soil ice contents with depth and time. However, feasibility of the SHB method is unknown because of difficulties in measuring soil thermal properties accurately in partially frozen soils. A previous study showed that feasibility of the SHB method relies primarily on measurement time interval and accuracy of thermal conductivity measurements. The objective of this study is to examine the SHB method for determining soil ice contents in a soil column with controlled boundary temperatures. Special attention is given to the measurement interval and thermal conductivity accuracy.

Sand and silt loam samples were packed in 30-cm long columns with water contents 0.13 m³ m^-3 and 0.28 m³ m^-3, respectively. Thermo-time domain reflectometry (thermo-TDR) probes were installed in the columns to measure soil temperature, liquid water contents, and thermal properties at several depths every 0.25 h, 0.5 h, and 3 h, respectively. The initial soil temperature was set at 5°C. Surface boundary temperatures for freezing was -15°C, and temperature at the bottom of all soil columns was maintained constant at 5 °C. Seven days following soil freezing, the surface boundary temperature was increased to 5 °C to produce soil thawing. Soil ice contents were determined at a 0.25-h interval using SHB theory. The SHB-determined ice contents were compared with ice contents determined with gravimetric sampling and TDR measurements, and were also compared with numerically simulated ice contents using the simultaneous heat and water model. The feasibility and limitation of the methods for estimating soil ice content were evaluated.