A Study on Hydrological Cycle in the Headwater Basins Using Isotope

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Hydrological observation, water sampling and chemical analysis of water were performed to investigate the relationship between the rainfall-runoff process and isotopic formation process of water in the headwater basins. Intensive observation and sampling were performed in mountainous headwater basins underlain by rhyolite in Nikko city, Tochigi prefecture, and hilly headwater basins underlain by granite in Ju-oh town, Ibaraki prefecture.Runoff characteristics and chemical tracer evidences snowed that subsurface water flowing through the bedrock should affect considerably on stream flow and spring water quantitatively and qualitatively in headwater basins underlain by rhyolite. Peak runoff rate at the rainstorms with the rainfall amount of more than 100 mm was ten times more than that at the rainstorm with the rainfall of less than 100 mm. A value of 100 mm in rainfall amount might be a critical factor in relation to the hydrological storage in the mountain.The mean tritium concentration in stream and spring waters of headwater basins in Ju-oh town was approximately 8 T.U., that value was relatively high as compared with that of precipitation of 5 T.U. in the Kanto district. This suggests that the average residence time in stream and spring waters would be approximately 20 years in the small basins underlain by granite. In the granite regions with thick weathered granite (usually more than 5 m), unsaturated subsurface flow would be predominant, and this should cause the long residence time in subsurface flow even in the small headwater basins.These results should be important for generalization of the rainfall-runoff processes in the headwater basins with steep relief in warm humid regions.