Snowmelt runoff characteristics in an alpine headwater catchment, Northern Japan Alps

It is important to understand the hydrological processes in the alpine headwaters, because the areas sustain the water resources in the down streams. In Japanese high mountain areas affected by Asian monsoon, there is large amount of precipitation in summer season, and is covered by vegetation even in the area with an elevation more than 2,000 m. We focus on snowmelt runoff processes in a Japanese alpine headwater catchment with different land cover conditions.We performed an intensive field monitoring at Mt. Norikura, a stratovolcano mountain, located at the southern end of the Northern Japan Alps with the maximum elevation of 3026m, specifically two headwater catchments, namely NR1 and NR2. The dominant area of NR1 is bare, whereas NR2 is covered by forest dominantly.We observed precipitation, temperature, and runoff of stream from 13^th July to 11^th October 2023. which includes snowmelt season. In addition, we collected stream water daily, and rainwater, snowmelt water, and spring water at the intervals of approximately two weeks. The concentrations of major inorganic solutions and stable isotopic ratios of oxygen and deuterium are determined on all water samples.The d-excess value of snowmelt water was higher than that of rainwater, whereas SiO₂ concentration of groundwater/ spring water was higher than that of rainwater/snowmelt water. Therefore, we applied End Member Mixing Analysis（EMMA）to separate stream water into three components, rainwater, snowmelt water, and groundwater, using d-excess and SiO₂ as tracers, focusing on snowmelt season. The EMMA results show that the snowmelt water contribution to the stream water was estimated to be 55% in NR1, whereas that in NR2 was estimates to be 25% in the beginning of snowmelt season, then the snowmelt component decreased gradually. The groundwater contribution to the stream water in NR1 was estimated to be 15%, whereas that in NR2 was estimated to be 75%.There results show that the effect of snowmelt water to stream water varies depending on land cover condition, snow cover and vegetation. The snowmelt component contributed to the stream water, even after the snow cover disappeared. This suggests that the snowmelt water would contribute to the stream water via the shallow groundwater nearby the stream in addition to the direct discharge.