Seasonal variability of stable isotopes in precipitation and spring water around Mt Kimpo, Kumamoto, southwestern Japan*

Stable isotopes in precipitation and spring water were observed in a low mountain region, because seasonal variability in isotope altitude and amount effects based on their simultaneous observations in low latitude area have not been studied so much. Seasonal variability in spatial averages of monthly δ18O and d-excess in precipitation ranged from −11.96 to −5.16 ‰ and from 8.0 to 24.2 ‰, respectively, while those in spring water ranged from −7.64 to −7.18 ‰ and from 11.0 to 13.6 ‰. The seasonal variability in spatial average of monthly δ18O in precipitation was much wider than in spring water. Monthly isotopic composition in spring water was mostly plotted along the local meteoric water lines for precipitation in warm months, suggesting that precipitation in warm months is the main source of spring water. Annual isotope altitude effects in precipitation (−0.12 ‰/100 m) and spring water (−0.13 ‰/100 m) were almost the same, and monthly isotope amount effects for cold months (−7.9 to −0.9 ‰/100 mm) were relatively higher than those for the warm months (−12.0 to −9.6 ‰/100 mm). The isotope amount effect at each sampling sites appeared using only the data in the warm months without extreme weather.