The time-space variations in stable isotopic compositions (δD and δ18O) of precipitation were investigated using event-based isotope data obtained at 18 sites in the Kanto Plain, Japan, from June to September, 1997. A principal component analysis applied to time series for every station showed that the first principal component, which is identical to the temporal variation of spatially averaged isotopic composition for each event, explains more than 90 % of total variance, and thus that temporal variation patterns for every station are all alike. Both temperature and precipitation amount are not a main factor controlling the temporal variation pattern. However, isotopic composition correlates highly with westerly vapor transport, and extraordinary, isotopically light rains are found when large rainfall occurs over the western Japan under dominant westerly conditions. These facts indicate that the isotopic composition of precipitation is controlled by trajectory of atmospheric vapor transport and precipitation history along it. On the other hand, the range of spatial variation within each event is small and a half of that of the temporal variation. A principal component analysis applied to spatial distribution data for every event showed that the most dominant pattern displays a decrease from the southeast to the northwest (i.e., toward inland area) and its dominance is significantly strong in convective events. It is pointed out that this spatial pattern may be introduced by a mixing of isotopically heavier, oceanic vapor and lighter vapor recycled in inland area, though its verification is further needed.
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.