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

Macleay region of New South Wales (NSW), Australia The stable water isotopic composition (the water isotopic ratios δ 18 O and δ 2 H and d -excess) of monthly and event precipitation samples are used to identify the role of synoptic climate and the ‘Tinderbox drought’ on precipitation isotopic composition in the region. Comparison of the stable water isotopic composition of rainfall and karst springs and rivers provide new information on rainfall recharge thresholds. Monthly precipitation samples (2017–2020) exhibited an increase in the gradient of the local meteoric water line over time as the regional climate moved from drought to wet conditions. Comparison of the event precipitation isotope composition and synoptic climate classifications revealed that offshore low-pressure systems had a unique and more negative δ 18 O and δ 2 H rain composition to all other synoptic weather patterns. These weather systems typically have high rainfall amounts that are more likely to lead to groundwater recharge. Springs and spring-fed rivers in the Macleay karst regions had a median water isotope composition that was more negative than the annual weighted mean of precipitation, indicative of a bias towards higher magnitude rainfall events recharging these groundwater systems. The rainfall recharge threshold to generate groundwater recharge in the region is estimated to be between 11 and 40 mm of precipitation per day. • Offshore low pressure weather systems have a unique water isotope composition. • Local meteoric water line changes in gradient between drought and wet. • Spring and river water isotope composition is more negative than rainfall mean. • Multiple methods constrain the rainfall recharge threshold to be 11–40 mm.