Catchment scale changes to rainfall intermittency across Australia

Hydroclimatic variability at the catchment scale modulates spatiotemporal patterns of water availability, potentially inducing hydrological extremes such as flooding and drought. These events alter streamflow and pose significant challenges for water resources management, ultimately impacting local ecosystems and communities. To understand the changes in hydroclimatic variability we examine the patterns of rainfall intermittency using aggregated catchment average rainfall. 467 Hydrological Reference Stations (HRS) catchments are used with data spanning from 1950 to 2022 across the Australian continent. We investigate changes in intermittency characteristics such as spell duration, frequency and intensity at the annual and seasonal scale. There is a clear trend towards an increase in rainfall intermittency with an increase in the number of both wet and dry spells per year across Australia. Wet spells are becoming shorter across 80 % of catchments, with an increase in the number of dry days per year. Despite this increase in dry days, there are no robust trends for changes in dry spell length. Catchments with drying trends are typically in southern and eastern Australia, whilst the catchments in northern and northwestern Australia exhibit wetting trends. This wetting trend comes from fewer dry days and increases in both annual rainfall totals and rainfall intensity during wet spells. We find that the trends in the seasonal scale are regionally dependent and align with changes in the large-scale drivers of regional rainfall dynamics. In the south, winter rainfall and wet spells are the most impacted, whereas in the north, it is the summer monsoon that is most impacted by these trends. Our results show rainfall intermittency has increased in recent decades, suggesting that intermittency could potentially continue to change into the future. These results also highlight the need to investigate wet and dry spells concurrently to form a foundational understanding of how rainfall intermittency dynamics are changing. We conclude that changes in rainfall intermittency across Australian catchments have the potential to impact water resources management and need to be considered in future planning. • Rainfall intermittency is increasing across much of Australia. • The number of dry days per year is increasing across southern and eastern Australia. • Wet spells are becoming shorter, more frequent, and more intense. • Despite an increase in the number of dry days, dry spell lengths show no robust trends.