Role of Soil and Bedrock Layers on Water Storage in a Vegetated Alpine Headwater under the Asian Monsoon Climate

The alpine zone has been considered to have poor water storage. However, recent studies have shown that alpine vegetated areas possess water storage function. But differences in the water storage between soil and bedrock and the processes of the runoff of stored water are still not clear. Therefore, we aimed to make clear the water storage functions of alpine vegetation areas by focusing on runoff processes of groundwater. We conducted field surveys from July to October 2023 on Mt. Norikura (3,026 m), a stratovolcano in central area of Japan. These were conducted in two adjacent catchments with differing land cover types: a bare area covered with debris and a vegetated area covered with soil and dominated by Japanese stone pine (Pinus pumila). During the study period, we monitored water levels in streams and precipitation within both areas. Additionally, biweekly field surveys were conducted to measure water temperature, pH, and electrical conductivity (EC) for precipitation, snowmelt water, stream water, and spring water, as well as to collect water samples. We analyzed them for the oxygen and hydrogen stable isotope ratios, the concentrations of major dissolved inorganic ions, SiO₂, and radon (Rn-222). The radon concentration was measured by liquid scintillation counter. The stream in the bare area dried up after the snowmelt season whereas that in the vegetated area gradually decreased and did not dry up throughout the study period. In the δ-diagram of oxygen and hydrogen stable isotope ratios, spring water from the bare area plotted along the local meteoric water line (LMWL), while spring water from the vegetated area plotted with a gentler slope than the LMWL. This result indicates that the spring water in the vegetated area is influenced by evaporation from canopy interception. In the vegetated area, the concentrations of major dissolved inorganic ions (particularly SO₄), SiO₂, and radon in the spring water were all higher than those in the bare area. This indicates that spring water undergoes ion exchange with clay minerals and SO₄ leaching from volcanic sulfide minerals, indicating a longer and deeper flow path compared to bare spring water. Additionally, the spring water in the vegetated area has no large fluctuation in water temperature, concentration of major dissolved inorganic ions, SiO₂, and radon. This indicates the contribution of groundwater from the bedrock layer. The spring water in the vegetated area, a largely negative correlation was observed between discharge and radon concentration, while only a weak correlation was found between SiO₂ and discharge. This indicates that groundwater from the bedrock layer passed through the soil layer, allowing radon to undergo gas exchange with the gas phase. These results indicate that in the bare area, the coarse-grained sediment structure allows rainfall to quickly reach the bedrock surface and flow out rapidly over the bedrock. However, in the vegetated area, the developed soil restricts rainfall runoff. This likely promotes groundwater recharge into the bedrock layer, forming an aquifer and indicates a water storage function.