Implications for groundwater recharge from stable isotopic composition of precipitation in Hawai'i during the 2017–2018 La Niña

Abstract

The project captured a subset of the hydrological cycle for the tropical island of O'ahu, linking precipitation to groundwater recharge and aquifer storage. We determined seasonal storm events contributed more to aquifer recharge than year‐round baseline orographic trade wind rainfall. Hydrogen and oxygen isotope values from an island‐wide rain collector network with 20 locations deployed for 16 months and sampled at 3‐month intervals were used to create the first local meteoric water line for O'ahu. Isotopic measurements were influenced by the amount effect, seasonality, storm type, and La Niña, though little elevation control was noted. Certain groundwater compositions from legacy data showed a strong similarity with collected precipitation from our stations. The majority of these significant relationships were between wet season precipitation and groundwater. A high number of moderate and heavy rainfall days during the dry season, large percentage of event‐based rainfall, and wind directions outside of the typical NE trade wind direction were characteristics of the 2017–2018 wet season. This indicates that the majority of wet season precipitation is from event‐based storms rather than typical trade wind weather. The deuterium‐excess values provided the strongest evidence of a relationship between groundwater and different precipitation sources, indicating that this may be a useful metric for determining the extent of recharge from different rain events and systems.

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

“…Previous similar studies (Scholl et al 1996, Scholl et al 2002, Dores et al 2020, Fackrell et al 2020, Tachera et al 2021 "▲" and "△" indicate multiple-shower and single-shower precipitation events, respectively, coming from the South. "▼" and "▽" indicate multiple shower and single shower precipitation events, respectively, coming from the North.…”

“…While there are some discrepancies across months from both Kau and Ho#oilo, these can be attributed to (1) the position of our experimental network versus the Rainfall Atlas site and (2) the seasonal variation discussed in "Weather During the Sampling Period" section. The Rainfall Atlas site corresponds to a location sampled by Dores et al (2020)-a comparison of data from our sites with this site, which emphasized the influences of microclimates and regional geography, is at the bottom of this section. The isotopic composition of precipitation along the transect varies based on the evaporative conditions at the different sites as they are subject to seasonal variation brought on by yearly shifts between Ho#oilo and Kau and periodic climatic cycles, such as ENSO and PDO (Figure 5).…”

“…The HIG-UH Manoa sampling location of Dores et al (2020) was located 1.5 km due north of our Makai site and 1 km due west of our Mauka 1 site (highly proximal) and collected precipitation on O'ahu from April 2017 to July 2018, thus in partial overlap with this study. As such, this station provides a good reference for the data collected in our study.…”

“…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, this work is the first to investigate event-based precipitation in detail, with a focus on the island of O#ahu. Previous studies in the tropics had success in rainfall characterization (Munksgaard et al 2019) and differentiating the event type of precipitation, such as orographic rainfall vs. low-pressure systems (Scholl et al 2009, Zwart et al 2018.…”

“…While stable isotope studies on the hydrological cycle within Hawai#i have focused on precipitation over long sampling intervals and large geographical areas, there is a need for stable isotope data at higher spatial and temporal resolutions in order to further define the characteristics of individual precipitation events and how they relate to groundwater recharge. Recent studies highlight the need for both the long-term monitoring of the isotopic composition of precipitation in Hawai'i (Tachera et al 2021) and finer resolution sampling to better understand the importance of event-based precipitation (Dores et al 2020). Here, we focus on the latter, with an experimental design to improve the spatial and temporal resolution of precipitation collection studies.…”