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The isotopic composition of rainfall on a subtropical mountainous island
  • Giuseppe Torri,
  • Alison D. Nugent,
  • Brian N Popp
Giuseppe Torri
University of Hawaii

Corresponding Author:gtorri@hawaii.edu

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Alison D. Nugent
University of Hawaii at Manoa
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Brian N Popp
University of Hawaii at Manoa
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Tropical islands are simultaneously some of the most biodiverse and vulnerable places on Earth. Water resources help maintain the delicate balance on which the ecosystems and the population of tropical islands rely. Hydrogen and oxygen isotope analyses are a powerful tool in the study of the water cycle on tropical islands, although the scarcity of long-term and high-frequency data makes interpretation challenging. Here, a new dataset is presented based on weekly collection of rainfall H and O isotopic composition on the island of O‘ahu, Hawai‘i, beginning from July 2019 and still ongoing. Throughout this time, a variety of weather conditions have affected the island, each producing rainfall with different isotopic ratios: precipitation from Kona lows was found to have the lowest isotopic ratios, whereas trade-wind showers had the highest. These data also show some differences between the windward and the leeward side of the island, the latter being associated with higher rainfall isotope ratios due to increased rain evaporation. At all sites, the measured deuterium excess shows a marked seasonal cycle which is attributed to different origins of the air masses that are responsible for rainfall in the winter and summer months. The local meteoric water line is then determined and compared with similar lines for O‘ahu and other Hawaiian islands. Finally, a comparison is made with data collected on Hawai‘i Island for a longer period of time, and it is shown that the isotopic composition of rainfall exhibits significant interannual variability.