A Study on Hydrological Cycle in the Headwater Basins Using Isotope

通过水文观测、水样采集和水质化学分析，研究了水源流域降雨径流过程与水的同位素形成过程的关系。对户木县日光市流纹岩下的山地水源盆地和茨城县朱欧镇花岗岩下的丘陵水源盆地进行了密集的观测和采样。径流特征和化学示踪证据表明，地下水流经基岩对流纹岩下的水源盆地的径流和泉水有很大的影响。降雨量大于100 mm的暴雨的洪峰径流量比雨量小于100 mm的暴雨的洪峰径流量大10倍。降雨量为100 mm可能是影响山上水文存储的关键因素。Ju-oh镇水源流域的溪水和泉水中氚的平均浓度约为8T.U，与关东地区5T.U的降雨量相比，该值相对较高。这表明，花岗岩下的小盆地在溪水和泉水中的平均停留时间约为20年。在花岗岩风化厚度较大(一般在5m以上)的花岗岩地区，非饱和地下水流占主导地位，即使在小型水源流域，地下水流的滞留时间也较长，这对温湿地区陡峭起伏的水源流域降雨径流过程的推广具有重要意义。

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.