The depth of soil frost is decreasing in cold regions around the world as a result of climate warming. To evaluate the potential impacts of the reduction in frost depth on the hydrologic cycle, it is necessary to understand snowmelt infiltration processes in frozen soils. A field study was conducted at an agricultural site characterized by volcanic ash soil in Tokachi, Hokkaido, Japan, where frost depths have decreased significantly in the last 20 yr. Soil temperature, water content, matric potential, snow cover, and meteorological parameters were monitored to quantify snowmelt infiltration flux for four winters that had different snow and soil conditions. When snowmelt began, the soil frost was 0.1 to 0.2 m thick in two winters and was absent in two other winters, providing a unique opportunity to compare snowmelt infiltration under frozen and unfrozen conditions. Most of the snowmelt water infiltrated into the soil under both frozen and unfrozen conditions, indicating that the frozen soil layer did not impede infiltration. The lack of flow impedance in the frozen soil was partly due to relatively high air temperature and an absence of freeze-back events during the snowmelt period. Furthermore, the temperature of the frozen soil layer was close to 0°C when the melt started, meaning that very little meltwater refroze in the soil before the temperature reached 0°C. The thick (>1 m) snow cover insulated the soil surface, allowing the frozen soil layer to warm up with the upward conduction of heat from the unfrozen layer below. These results indicate the importance of the interaction between snow cover and soil, which can be significantly affected by climate change.

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