ABSTRACT

ABSTRACT: It is important to understand river and groundwater interactions. The authors attempted to clarify these interactions by a survey of the hydrogeological structure of the study area. The alluvial fan sediments mainly consist of debris flow deposits. The gravel composition of surface outcrops includes granite on both banks of the R. Koishiwara-gawa, but these were not present on the left bank of R. Sata-gawa. The epiclastic sediments of the Aso-4 is divided into Aquifer I and II. Within the present annual water balance from 2003 to 2007, the recharge to groundwater was larger than the discharge from groundwater. It was estimated that the annual total surface flow was 400,000,000 m3/day, and that the ratio of abstraction to the whole discharge was quite small. It was observed that groundwater closely interacts with river water, and that the recharge from the irrigation water amounts to about 40% of that from rainfall.