Baker (2026) Identification of synoptic climate and drought controls on rainfall stable water isotopic composition in the Macleay karst region of eastern Australia

Identification

• Journal: Journal of Hydrology Regional Studies
• Year: 2026
• Date: 2026-03-07
• Authors: Andy Baker
• DOI: 10.1016/j.ejrh.2026.103313

Research Groups

• Earth and Sustainability Science Research Centre, School of Biological, Earth and Environmental Sciences, UNSW Sydney, Australia
• Kempsey Speleological Society, West Kempsey, Australia

Short Summary

This study investigates the stable water isotopic composition of precipitation, karst springs, and rivers in the Macleay region of eastern Australia to understand the influence of synoptic climate and drought on rainfall isotopes and to estimate groundwater recharge thresholds. It found that offshore low-pressure systems deliver isotopically depleted rainfall, which preferentially recharges groundwater, with a daily recharge threshold estimated between 11 mm and 40 mm.

Objective

• To identify the role of synoptic climate and the 'Tinderbox drought' on precipitation stable water isotopic composition in the Macleay karst region of eastern Australia.
• To provide new information on rainfall recharge thresholds by comparing the stable water isotopic composition of rainfall and karst springs and rivers.

Study Configuration

• Spatial Scale: Macleay region of New South Wales, Australia, including specific sampling sites at Millbank (monthly precipitation), Yarravel (event precipitation), and various karst springs, rivers, and a groundwater bore in the Upper and Lower Macleay.
• Temporal Scale: Monthly precipitation samples collected from October 2017 to October 2020. Event-based precipitation samples collected from December 2020 to February 2024. Surface and groundwater hydrology samples collected since 2017 to September 2023.

Methodology and Data

• Models used: None explicitly for hydrological simulation; statistical analyses (Pearson's product correlation, Wilcoxon Rank Sum Tests) and synoptic climate classification were used.
• Data sources:
  • Monthly composite precipitation samples (Millbank).
  • Event-based precipitation samples (Yarravel).
  • Surface and groundwater hydrology samples from karst springs, rivers, and a groundwater bore in the Upper and Lower Macleay.
  • Daily and monthly precipitation data from Australian Bureau of Meteorology (BOM) stations (Kempsey Airport AWS, Bellbrook).
  • Mean sea level pressure maps from the BOM Analysis Chart Archive for synoptic classification.
  • Stable water isotope analysis (δ18O, δ2H, D-excess) using a Los Gatos® cavity ring down laser spectrometer.

Main Results

• The local meteoric water line (LMWL) gradient for monthly samples (δ2H = 6.9 δ18O + 11.2‰) and event samples (δ2H = 7.8 δ18O + 17‰) varied over time, with a lower gradient observed during the 'Tinderbox drought' (2017–2019), indicative of increased sub-cloud evaporation.
• Offshore low-pressure systems (including East Coast Lows) were associated with a unique and significantly more negative δ18O and δ2H precipitation composition compared to other synoptic weather patterns (troughs, offshore highs, frontal systems).
• Groundwater-fed rivers and springs in the Macleay karst exhibited a median water isotope composition (e.g., Upper Macleay: δ18O −5.3‰; Lower Macleay: δ18O −5.8‰) that was more negative than the annual weighted mean of precipitation (δ18O −3.3‰), suggesting a bias towards higher magnitude rainfall events for groundwater recharge.
• A weak but statistically significant 'amount effect' (Pearson's correlation r = -0.40, n = 71, p < 0.01) was observed between event precipitation amount and water isotope composition.
• The rainfall recharge threshold to generate groundwater recharge in the region is estimated to be between 11 mm and 40 mm of precipitation per day, consistent with physical measurements in caves.

Contributions

• Provides the first isotope hydrology data for the Macleay karst region, filling a significant 700 km spatial gap in precipitation stable water isotope coverage along the Australian east coast.
• Identifies offshore low-pressure systems as a primary driver of isotopically depleted rainfall and a major contributor to groundwater recharge in eastern Australia.
• Demonstrates the temporal variability of the local meteoric water line in response to drought and wet conditions, highlighting the role of sub-cloud evaporation.
• Establishes a rainfall recharge threshold for karst groundwater systems in the region using stable water isotopes, validated by independent physical measurements.
• Proposes the highly-depleted isotopic fingerprint of offshore low-pressure systems as a potential natural tracer for rainfall from these synoptic weather patterns across the southeast Australian coastline.

Funding

• Australian Research Council grant FL240100057

Citation

@article{Baker2026Identification,
  author = {Baker, Andy},
  title = {Identification of synoptic climate and drought controls on rainfall stable water isotopic composition in the Macleay karst region of eastern Australia},
  journal = {Journal of Hydrology Regional Studies},
  year = {2026},
  doi = {10.1016/j.ejrh.2026.103313},
  url = {https://doi.org/10.1016/j.ejrh.2026.103313}
}