In a snowy watershed, the response of streamwater chemistry to snowmelt depends not only on meltwater chemistry but on the hydrochemical processes of the subsurface water. In this study, the δ
18O values and solute concentrations of the precipitation, meltwater, subsurface water and surface water at the Moshiri experimental watershed located in the northern part of Hokkaido, Japan were obtained, and were discussed here in order to clarify the subsurface runoff processes and the formational mechanisms of the subsurface and surface water chemistry during the snowmelt season. A groundwater with a constant δ
18O value (-12.3‰) and constant Cl
- concentration (200 μeq/L) was widely found in the watershed and in the shallow saturated water zone. Hydrograph separations with high precision were conducted using δ
18O early in the snowmelt season (Mar. 23-Apr. 14), and using a Cl
- concentration during the active snowmelt season (Apr. 21-May 1). The contributions of the old water to the streamwater discharge were 82% (Mar. 23-Apr. 14) and 76% (Apr. 21-May 1). The contributions of old water were very high throughout the snowmelt season, which means that the groundwater reservoir must be significantly large. Subsequently, the temporal and spatial variations of the SiO
2 concentrations of the subsurface water and surface water were observed to clarify the chemistry of the subsurface water, and revealed that there were two geographic sources (shallow groundwater and deep groundwater) in the ground. The SiO
2 concentration of the deep groundwater was higher than that of the shallow groundwater. The sources of two spring waters changed from the deep groundwater to the shallow groundwater, and the SiO
2 concentrations of these waters decreased while the discharges of these waters increased. The SiO
2 concentration of old water was estimated using the results of isotopic and chloride hydrograph separation. Then, the result of the relationship between the amount of old water discharge and the SiO
2 concentration of old water showed that the source of old water during the low outflow period was deep groundwater, and the source during the low outflow period was shallow groundwater. As the discharge of the old water reached over 0.08m
3/s, the estimated SiO
2 concentration of the old water became constant. These results indicated that a large amount of the old water flowing out to the river in the active snowmelt season was discharged from shallow groundwater. By observing the snowmelt runoff processes in a nival watershed, it was clarified that the old water is composed of a combination of two sources of groundwater and that the shallow groundwater chemistry greatly affects the streamwater chemistry during the large stormflow period.
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