Event-Based Stable Isotope Analysis of Precipitation Along a High Resolution Transect on the South Face of O‘ahu, Hawai‘i

Abstract: While the influence of elevation and seasonal variation on isotopic composition has been studied on Maui, Hawai‘i Island, and O‘ahu (Scholl et al. 1996, Scholl et al. 2002, Scholl et al. 2007, Dores et al. 2020, Fackrell et al. 2020, Tachera et al. 2021), this work is the first to investigate event-based precipitation in detail on the island of O‘ahu. The stable isotopic composition of water has been used to track the movement of water within the hydrosphere, to investigate the type and origin of a rainfall event, and elevation of collection, among other characteristics. Here, we present a high-resolution study of the stable isotopes δ2H and δ18O of precipitation along a compact land-to-sea transect in Waikīkī, a southwest facing region on O‘ahu. The study provides a unique, in-depth investigation into the nature of individual storm events, and how they contribute to a larger seasonal climatic pattern. Monthly precipitation samples were collected at three sites along the transect from December 2017 to March 2019 and event-based samples were collected at the Makai site from October 2018 to February 2019. Storm direction, temperature, and relative humidity were recorded for each event-based sample. Results suggest that evaporative conditions at different elevations influence the isotopic composition of precipitation, either through net addition as moisture recycling, or net loss of evaporated water. The spatial distribution of these patterns from site to site illustrates the extreme heterogeneity of Hawaiian watersheds.