[A-HW24] 同位体水文学2024

[AHW24-01] Intra-event changes in stable isotope ratios of precipitation using automatic samplers in 2023

*大竹 樹生¹、一柳 錦平²、壁谷 直記³ (1.熊本大学大学院自然科学教育部、2.熊本大学大学院先端科学研究部、3.森林総合研究所)

キーワード：降水の安定同位体、イベント内の変化、自動降雨採水器、熊本

Monthly and daily samplings of stable isotopes (δ¹⁸O and δ²H) in precipitation are commonly conducted, however, a few studies have reported short-term changes in precipitation isotopes within individual events in Japan. In this study, intra-event changes in stable isotope ratios of precipitation were observed in Kumamoto, Japan, using two types of automatic water samplers. A total of 323 samples from March to December 2023 were collected on the rooftop of the building of Faculty of Science in Kumamoto University. δ¹⁸O and δ²H in precipitation samples were analyzed using Water Isotope Analyzer (Picarro L2120-i). In April 2023, four precipitation events were observed. During the precipitation event on 5th April, a total of 7 samples were collected and ranged from +1.48‰ to -3.73‰ and from +21.18‰ to -16.23‰ for δ¹⁸O and δ²H, respectively. High isotopic ratios were shown at the beginning of the precipitation event and gradually decreased. The slope of the relationship between δ¹⁸O and δ²H (meteoric water line) is 7.3, which is close to that observed for a long term in Kumamoto. On the 14-15th April event, a total of 12 samples were collected and ranged from +0.48‰ to -6.26‰ and from +10.26‰ to -39.84‰ for δ¹⁸O and δ²H, respectively. When the precipitation amount was less than 3 mm/hr in the first half of the event, the isotope ratio gradually decreased. In the second half of the event, precipitation amount exceeded 5 mm/hr and the isotope ratio remained low values. However, clear relationship between precipitation amount and isotope ratios (amount effect) was not observed. The slope of the meteoric water line is 5.3, which is close to the evaporation line. It is interesting that very high isotopic ratios (> 0‰) were observed only in April and slopes of the meteoric water lines were varied among the individual precipitation events. In the future, we plan to include results from observations in different months and further discussion on the factors that control isotopic changes during precipitation events.