Effects of frozen soil and snow cover on cold-season soil water dynamics in Tokachi, Japan

Despite the potential impact of winter soil water movements in cold regions, relatively few field studies have investigated cold-season hydrological processes that occur before spring-onset of snowmelt infiltration. The contribution of soil water fluxes in winter to the annual water balance was evaluated over 5 years of field observations at an agricultural field in Tokachi, Hokkaido, Japan. In two of the winters, soil frost reached a maximum depth of 0.2 m ('frozen' winters), whereas soil frost was mostly absent during the remaining three winters ('unfrozen' winters). Significant infiltration of winter snowmelt water, to a depth exceeding 1.0 m, occurred during both frozen and unfrozen winters. Such infiltration ranged between 126 and 255 mm, representing 28-51% of total annual soil water fluxes. During frozen winters, a substantial quantity of water (ca 40 mm) was drawn from deeper layers into the 0-0.2 m topsoil layer when this froze. Under such conditions, the progression and regression of the freezing front, regulated by the thickness of snow cover, controlled the quantity of soil water flux below the frozen layer. During unfrozen winters, 13-62 mm of water infiltrated to a depth of 0.2 m, before the spring snowmelt. These results indicate the importance of correctly evaluating winter soil water movement in cold regions.