Subsurface water runoff in an alpine catchment covered by Pinus pumila , Northern Japan Alps, Asian monsoon region

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This study investigated subsurface water runoff characteristics in two alpine watersheds – NR1 and NR2 – with vegetation coverages of 20.7% and 51.2%, respectively, Mt. Norikura, Northern Japan Alps. We monitored stream level from July to October 2023, conducted chemical and stable isotope analyses on stream, spring, rain, and snowmelt waters, and performed hydrograph separation using SiO2 as a tracer. The total precipitation was 775.8 mm during the monitoring period, and the streamflow was ephemeral in NR1 and perennial in NR2, resulting in runoff ratios of 12.3% in NR1 and 80.5% in NR2. Additionally, stream water in NR2 showed higher ion concentrations and lower temporal variation of d-excess than those in NR1. Also, estimated groundwater runoff is higher in NR2 (458.7 mm) than in NR1 (12.8 mm). These results suggest that runoff characteristics are influenced by vegetation cover and also subsurface flow processes, including the groundwater flow across the topographic watershed boundaries.