Role of Groundwater Originated from Volcanic Rock in Rainfall-Runoff Process of Spring in a Forested Headwater Catchment

In order to investigate the role of bedrock groundwater in rainfall-runoff process, hydrometric, tracer and subsurface water potential observations are applied in a small headwater basin situated in Shiranui, Kumamoto prefecture, south-west Japan. The area is underlain by Tertiary volcanic rock; Andesite. Concentrations of major chemical constituent and stable isotope ratio of oxygen-18 were analyzed on spring water, rain water, soil water and bedrock groungwater at a rainstorm event. We focuses on an event with total precipitation of 54.8 mm observed on 30th August 2004. A delayed secondary runoff was observed after approximately a half day from the rainfall-peak. Hydrograph separation analysis using oxygen-18 and Silica as tracers showed rain and soil water were dominant components in the spring water during the first runoff peak, whereas the bedrock groundwater and soil water were dominant components during the delayed runoff peak. Subsurface water potential variation showed that a vertical percolation was dominant at the first runoff peak and lateral subsurface flow was dominant at the delayed runoff peak. Also, saturated zone was observed just above the bedrock surface near the spring outlet at the first runoff peak. Thus, a mixing of rain water, soil water and bedrock groungwater might occur in this portion of the slope at the first runoff peak. During the secondary runoff increasing, water table fell down into the bedrock, and discharge of bedrock groundwater became predominant.