乗鞍岳東斜面における地下水流動系と降雨流出過程

抄録

In order to investigate the groundwater flow system and rainfall-runoff processes in a volcanic area, hydrometric and tracer approaches were applied in two watersheds (R. Mae and R. Ko-ohno) at the east slope of Mt. Norikura, Nagano prefecture, central Japan. Concentrations of major chemical constituent and δD and δ¹⁸O in spring, rain and stream waters were analyzed. Groundwater recharge lines were calculated on the δD-elevation and δ¹⁸O-elevation diagrams, which suggested existence of a long groundwater flow along the new volcano lava as a piston flow. The relationship between the changing ratio of isotopic composition and the dilution ratio of solute concentration in rain and stream waters shows that the runoff water consists of event and groundwater components. The result of the hydrograph separation analysis shows the proportion of the event water component to the total discharge ranged from 36 to 48 % in R. Mae watershed and from 60 to 63 % in R. Ko-ohno watershed during rainfall event, and ranged from 59 to 60 % and 75 to 83 % in the peak discharge, respectively. The ratio of event water component to the total runoff ranged from 10 to 16 % in R. Mae watershed and was 17 % in R. Ko-ohno watershed, whereas the ratio of groundwater component ranged from 7 to 8 % in R. Mae watershed and 2 to 4 % in R. Ko-ohno watershed. The discharge of event water component in the falling stage of runoff was much higher than that in the rising stage. This suggests that the event water would move quickly as a preferential flow through the fissures or pores of lava.