Variations in soil moisture conditions and their contributions to stream runoff during the snowmelt season

In a snowy mountain watershed, hillslope soil moisture becomes highest during the snowmelt season because of the continuous water input to the soil from the bottom of the snowpack. We call this water snowpack bottom outflow (SBO). This study evaluates the soil moisture conditions during the snowmelt season using the Soil Water Index (SWI), and examines the relationships between variations in the SWI and the snowmelt stream runoff in a mountainous basin in Moshiri, Hokkaido, Japan. We found that the SWI fluctuates greatly throughout the snowmelt season and, based on the positive relationship between the SWI and the subsurface flow components, there is clearly a positive correlation with stream discharge (Q). However, the runoff ratios of SBO do not change even if the soil becomes wet. Similar characteristics have been found in another snowy mountain watershed in Nagaoka, Niigata Prefecture, Japan, suggesting that our results from Moshiri may be common in snowy mountain watershed areas. In addition, subsurface flow components in both areas occupy more than 60 to 70% of the average Q. Therefore, subsurface flow components are quantitatively important in snowmelt stream runoff. The results of our study suggest that even if SBO is small during a particular snowmelt season, the risk of flooding increases under particularly wet soil moisture conditions.