Investigating the hydrological cycle of a subtropical mountainous island using water isotopes

Water resources help maintain the delicate balance on which tropical island ecosystems and populations depend. A clear understanding of the water cycle on tropical islands is therefore essential to ensure that life remains sustainable in such biodiverse and vulnerable places. Water isotopes are a powerful tool in this effort, although the scarcity of long-term and high-frequency data makes interpretation challenging. Here, we present a new dataset based on a weekly collection of the H and O isotopic composition of rainfall on the island of Oʻahu, Hawaiʻi, beginning in July 2019 and continuing. The data show considerable differences in isotopic ratios produced by different weather systems, as well as significant spatial variability, with some sites characterized by higher isotopic ratios than others. The amount effect is also not consistently observed at all sites. The deuterium excess shows a pronounced seasonal cycle, which is attributed to the different characteristics of the air masses responsible for precipitation in the winter and summer months. A comparison with a long-term historical record illustrates the strong interannual variability and the need to establish a long-term precipitation isotope monitoring network for Hawaiʻi.