Water Potential in an Unsaturated Soil during Freezing and Thawing Processes.

Estimating the water flow in freezing and thawing soils is important in agricultural management and water balance calculation in cold regions. Soil water is induced to flow to the freezing front due to the low water potential in the frozen soil, and flows through the partially frozen soil under the potential gradient of unfrozen water. However, the unfrozen water potential is difficult to directly measure because of the low water pressure and non-equilibrium pore ice growth. A micro-chilled-mirror hygrometer that can be inserted into a soil has recently been developed. In this study, we first determined the suitability of this hygrometer for frozen soils, then used it to measure the water potential gradient in freezing and thawing soils.

The sample (Andisol) was packed into a brass cylinder and the hygrometer and thermocouples were inserted into the sample. The cylinder was placed into coolant, and the temperature of the coolant was raised and dropped. We confirmed that the hygrometer directly measures soil water potential in frozen soil below -2°C. At equilibrium, the measured potential h_RH corresponded to the calculated potential h_CC determined using the Clapeyron equation. Soil water potential was found to require time to reach equilibrium after a temperature change.

Second, the sample was packed into an acrylic column. Thirty-four thermocouples, seven TDRs, and two hygrometers were inserted into the column. The column was then frozen from the upper end by setting the top and bottom temperatures at −15 and 3 °C, and thawed by −10 and 3 °C. Under the high cooling rate, the h_RH was higher than the h_CC. As the cooling rate decreases, the h_RH reaches the h_CC. This indicates that the potential gradient in directionally freezing ground tends to be underestimated near ground surface and overestimated near the freezing front, if it derived by the h_CC.