Role of Subsurface Water in Stream Runoff in an Alpine Headwater Catchment

We conducted field surveys and water sampling from July through October 2023. We compared stream runoff and stream water quality in two watersheds with different land cover in an alpine headwater, Mt. Norikura, Japan. We performed observation in two watersheds, namely NR1 with dominant bare soil surface with limited vegetation cover, 21% of total area, and NR2 dominantly covered by vegetation, 51% in total area. Stream runoff in the NR1 decreased to 0 m after the snowmelt season and runoff occurred only after rainfall, whereas runoff occurred constantly during the observation period in NR2. The stable isotope ratios of hydrogen and oxygen (δ²H and δ¹⁸O) in stream shows variation close to that of precipitation in NR1, whereas those are stable in NR2. These results suggest that the transit time of water in NR2 is longer than that in NR1. The contribution ratio of the groundwater component to the stream runoff during the observation period was higher in NR2 (72.2%) than in NR1 (15.5%). In NR2, the contribution ratio of the groundwater component to stream runoff tends to be lower when API (Antecedent Precipitation Index) is higher. Additionally, the contribution ratio of snowmelt water component increases       with rainfall and decreases promptly. The results indicate that groundwater plays an important role for maintaining stream runoff in NR2 with high coverage of vegetation. Even in NR1, where the contribution ratio of groundwater component to stream runoff is low, the presence of groundwater table is necessary for the discharge of water that is in the subsurface zones.