What the stable isotope composition of precipitation reveals when it rarely rains - a decade of observations from northwestern Australia

Global climate change is reshaping precipitation regimes worldwide and intensifying aridity across many dryland regions. These changes also impact the dry subtropics of northwestern Australia. In this region, the strong spatial and temporal variability of rainfall complicates the characterisation of hydrologic processes, making it challenging to assess groundwater recharge, manage water resources effectively, and protect vulnerable ecosystems.

The region is marked by a distinct dry winter season (April-October) and a wet summer season (November-March), and it receives a far greater proportion of cyclone-driven rainfall than most other parts of Australia. These cyclonic systems contribute to pronounced seasonal and interannual variability, producing intense but short-lived flash-flooding events separated by extended droughts. During the study period (2015–2024), more than 80 % of total rainfall occurred between December and March across the five monitored weather stations. Mean annual rainfall ranged from 288 to 366 mm, while mean relative humidity remained low (30-37 %). Consequently, potential evaporation rates were extremely high, often exceeding 3000 mm/y.

To better understand the atmospheric processes governing precipitation formation and moisture sourcing in this environment, we analysed ten years of rainfall stable hydrogen and oxygen isotope compositions (δ¹⁸O and δ²H) from five sites. We also analysed 1,101 air parcel trajectories corresponding to rain events at five sites over the study period. Rainfall with low δ²H and δ¹⁸O values occurred predominantly during high-rainfall months, demonstrating a strong ‘amount effect’ at most locations. Both, stable hydrogen and oxygen isotope compositions were positively correlated with the stratiform fraction of total precipitation, indicating substantial sub‑cloud evaporation during stratiform events. On average, ~30 % of rainfall was lost to sub‑cloud evaporation, and back‑trajectory analysis showed that up to 47 % of wet-season moisture originated from recycled land evapotranspiration.

Differences between arithmetic and volume-weighted monthly isotope means highlight the seasonal importance of small-volume rainfall events. To address this bias, we introduce a new “cut‑off” method designed to reduce the disproportionate influence of low‑volume rainfall on monthly isotope compositions and on the construction of Local Meteoric Water Lines.