5:15 PM - 7:15 PM
[AHW23-P01] Effect of subsurface water on snowmelt and rainfall runoff in an alpine headwater catchment, Northern Japan Alps
Keywords:Alpine, Hydrograph separation, Rainfall-runoff, Snowmelt runoff, Subsurface water
We conducted field surveys and water sampling with 1-day or 2-day interval from July through October in 2023 and 2024 in an alpine headwater catchment of Mt. Norikura, Japan to clarify the effect of subsurface water on stream runoff in snowmelt and post-snowmelt seasons in the vegetated alpine zone. We performed the observation in two neighboring watersheds, namely NR1 and NR2 watersheds with vegetation coverage of 20.7% and 51.2%.
Stream runoff was high in snowmelt season in both watersheds. The runoff decreased after snowmelt season in both watersheds, and runoff was not observed after snowmelt season in NR1 except for rainfall-runoff events. In addition, some springs dried up in NR1 and some spring locations moved to the lower altitude in NR2 during the low flow period in the post-snowmelt season. These results indicate that groundwater level seemed to decline after the snowmelt season. The d-excess value was higher in snowmelt water and lower in rainwater, and that in stream water fluctuated between that of snowmelt water and rainwater. The d-excess value in stream water in NR1 reflected that of snowmelt water in snowmelt season, and that of rainwater in post-snowmelt season. In addition, the d-excess value in stream water decreased gradually throughout the observation period in NR2. These results indicate that snowmelt water and rainwater are sources of stream water, that is, they mix in the subsurface zone and discharge to the stream. Besides, the contribution of snowmelt water to stream runoff decreased through the observation period. The results of hydrograph separation observed in 2023 show that as stream runoff decreases after snowmelt season, contribution ratio of subsurface water increases while the runoff of subsurface water decreases in NR2. Also, the contribution ratio and runoff of subsurface water are higher in snowmelt season as compared with those in post-snowmelt season. Additionally, the contribution ratio of subsurface water to stream runoff decreases, when API (Antecedent Precipitation Index) exceeds 10 mm during post-snowmelt season, while the runoff of subsurface water does not decrease. These results indicate that discharge of subsurface water could be affected by the volume of groundwater reservoir and moisture condition in the watershed, and it could vary seasonally.
Stream runoff was high in snowmelt season in both watersheds. The runoff decreased after snowmelt season in both watersheds, and runoff was not observed after snowmelt season in NR1 except for rainfall-runoff events. In addition, some springs dried up in NR1 and some spring locations moved to the lower altitude in NR2 during the low flow period in the post-snowmelt season. These results indicate that groundwater level seemed to decline after the snowmelt season. The d-excess value was higher in snowmelt water and lower in rainwater, and that in stream water fluctuated between that of snowmelt water and rainwater. The d-excess value in stream water in NR1 reflected that of snowmelt water in snowmelt season, and that of rainwater in post-snowmelt season. In addition, the d-excess value in stream water decreased gradually throughout the observation period in NR2. These results indicate that snowmelt water and rainwater are sources of stream water, that is, they mix in the subsurface zone and discharge to the stream. Besides, the contribution of snowmelt water to stream runoff decreased through the observation period. The results of hydrograph separation observed in 2023 show that as stream runoff decreases after snowmelt season, contribution ratio of subsurface water increases while the runoff of subsurface water decreases in NR2. Also, the contribution ratio and runoff of subsurface water are higher in snowmelt season as compared with those in post-snowmelt season. Additionally, the contribution ratio of subsurface water to stream runoff decreases, when API (Antecedent Precipitation Index) exceeds 10 mm during post-snowmelt season, while the runoff of subsurface water does not decrease. These results indicate that discharge of subsurface water could be affected by the volume of groundwater reservoir and moisture condition in the watershed, and it could vary seasonally.