Abstract
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.