Influence of alpine vegetation on water storage and discharge functions in an alpine headwater of Northern Japan Alps

As mountainous areas provide abundant water resources to lower elevations, and alpine zones are major recharge areas for water resources, it is important to understand water storage and discharge processes in these zones. Regarding water storage, sedimentary structures (e.g., talus and moraines) in alpine zones function as aquifers. However, the functions of vegetation, thought to contribute to water recharge and storage in forested watersheds, have rarely been investigated. Accordingly, we evaluated the influence of alpine vegetation on water storage processes in alpine zones. Two intensive field surveys were conducted on August 17 and October 5, 2019, in the alpine headwaters of Mt. Norikura in the Northern Japan Alps. Chemical analyses were conducted of rainwater, snowmelt water, and runoff water from bare and vegetated catchments. From the results, a two-component separation was conducted to calculate the contributions of precipitation and groundwater components to runoff water. Our results implied that runoff water from vegetated catchments was in contact with the regolith for longer, with the contribution of groundwater being higher in this runoff water. Moreover, the groundwater component contribution tended to increase as the ratio of vegetation area to bare area in each catchment increased, suggesting a higher water storage function for vegetated areas. In other words, the subsurface water flow should be slower in vegetated areas due to the presence of vegetated soils compared to bare areas where coarse-grained sediments are dominant. Accordingly, the alpine vegetated area has a higher water storage function than the alpine bare area.