11:00 AM - 11:15 AM
[AHW18-08] Discharge processes of the mountain-block groundwater during rainstorms in the foothills of an alpine region
Keywords:rainfall-runoff process, hydrograph separation, quick flow, mountain region
Understanding water discharge processes in mountainous areas is crucial for disaster prevention and water resource management. Recently, there's been a reassessment of the importance of bedrock groundwater discharge during rainstorms and water storage functions within mountain bodies. However, the process of groundwater discharge from the mountain body during heavy rainfall remains largely unknown. Therefore, the objective of this study is to clarify the role of mountain-block groundwater in water discharge processes at the foot of alpine mountains. Hydrological observations were conducted on springs at the foot of Myojidake (2931 m above sea level) in the Kamikochi region of the Northern Japanese Alps. Waters were sampled using an automatic water sampler and analyzed for oxygen/hydrogen stable isotope ratios (δ18O, δ2H) and major dissolved anion concentrations (Cl-, NO3-, SO42-). Using the observed water discharge data, a hydrograph separation using the method defined by Hewlett & Hibbert (1967) was performed.
The water temperature of the mountain-block spring was almost constant throughout the year, with a short-term increase only during heavy rainfall. This small seasonal change in the water temperature indicates that groundwater stored in deeper subsurface areas within the mountain body discharges. There was no tendency for the quick flow fraction to increase in the event of a change in spring water temperature. This suggests that quick flow is different from the conventional view that quick flow is a water component with a short residence time. The δ value of the mountain-block spring did not change largely during the rising limb of the hydrograph, which was similar to that in warm seasons. On the other hand, the trend of tracer values during the recession varied with the magnitude of precipitation, with δ and NO3- values being distinctly higher for precipitation events greater than 100 mm. These results suggest that although mountain-block groundwater is the main component of discharge water during rainstorms, the water component with short residence time may contribute to discharge later than the mountain-block groundwater discharge. This analysis results would infer that mountain-block groundwater contributes to the discharge as a quick flow during rainfall events. This suggests that mountain-block groundwater plays an important role in the water discharge mechanism during rainstorms in mountainous areas and that it is necessary to understand the mountain hydrological cycle system in consideration of mountain-block groundwater.
The water temperature of the mountain-block spring was almost constant throughout the year, with a short-term increase only during heavy rainfall. This small seasonal change in the water temperature indicates that groundwater stored in deeper subsurface areas within the mountain body discharges. There was no tendency for the quick flow fraction to increase in the event of a change in spring water temperature. This suggests that quick flow is different from the conventional view that quick flow is a water component with a short residence time. The δ value of the mountain-block spring did not change largely during the rising limb of the hydrograph, which was similar to that in warm seasons. On the other hand, the trend of tracer values during the recession varied with the magnitude of precipitation, with δ and NO3- values being distinctly higher for precipitation events greater than 100 mm. These results suggest that although mountain-block groundwater is the main component of discharge water during rainstorms, the water component with short residence time may contribute to discharge later than the mountain-block groundwater discharge. This analysis results would infer that mountain-block groundwater contributes to the discharge as a quick flow during rainfall events. This suggests that mountain-block groundwater plays an important role in the water discharge mechanism during rainstorms in mountainous areas and that it is necessary to understand the mountain hydrological cycle system in consideration of mountain-block groundwater.