Masaya Yasuhara

The Hachioji Experimental Basin Study — Storm runoff processes and the mechanism of its generation

Abstract The study discusses storm runoff processes and the mechanism of its generation in relation to dynamic responses of subsurface water in a small forested drainage basin located in the western suburbs of Tokyo, Japan. Intensive field observations were carried out for the period 1980–1983. In the drainage basin overland flow occurred from restricted areas on the valley floor. No significant overland flow was produced on the steep hillside slopes even during a heavy storm event. The maximum extent of the saturated area occupied only 1–4% of the total basin area. The main source of storm runoff was groundwater flow. More than 90% of the total discharge was due to groundwater flow and the surface flow component contributed less than 10%. The rapid and large amounts of groundwater discharge during a storm event could not be explained solely by the traditional concept of Darcian matrix flow. These phenomena were mainly attributed to rapid flow such as pipe flow which has velocities about as high as those of surface flows. The pulsating flow phenomenon was observed as a characteristic of pipe flow.

Escape of volcanic gas into shallow groundwater systems at Unzen Volcano (Japan) : evidence from chemical and stable carbon isotope compositions of dissolved inorganic carbon

Abstract Chemical and stable carbon isotopic analyses of dissolved inorganic carbon (DIC) were carried out for groundwater samples collected from cold springs and shallow wells in the Unzen volcanic region in 1999 and 2000. All of the data sets plotted on the carbon isotope ratio (δ13C) vs 1/DIC diagram can be explained by mixing of volcanic CO2 with DIC equilibrated with soil CO2. Groundwater DIC showing a high mixing ratio of volcanic CO2 appears to have a tendency to distribute along two major faults near the activity center of the 1990–1995 eruption. This suggests that these faults are escape routes of volcanic CO2 diffused into the volcanic edifice. The total flux of the volcanic DIC discharged from the cold springs is shown to be one to two orders of magnitude lower than the roughly estimated flux of volcanic CO2 discharged from the summit during the eruptive period.