Changes in wet and dry spell characteristics in Australian catchments

Hydroclimate variability results in sequencing between wetter and drier periods at both day-to-day and longer timeframes. Variability at the day-to-day scale can result in sudden water surpluses or deficiencies resulting in extreme events such as floods and flash droughts. Longer-term variability, however, can significantly influence water security through the impact of droughts and reduced streamflow. Variability at both timescales poses significant challenges to water resources management, with follow-on impacts on local ecosystems and communities. In this study we investigate changes in day-to-day hydroclimate variability, focussing on the intermittency patterns of rainfall (wet and dry spells).

Our investigation, at the catchment scale, uses rainfall for 467 Hydrological Reference Stations (HRS) catchments from 1950-2022 across the Australian continent. We look at long-term trends in rainfall frequency, duration, and intensity characteristics at annual and seasonal timescales and break down our analysis by similar climatic regions. We find a clear trend towards more dry days per year across most catchments in Australia. Interestingly, there are no consistent trends in annual rainfall totals or annual mean dry spell length, despite the increase in the dry days per year. There are however consistent declining trends in annual mean and maximum wet spell lengths with shorter spells over ~80% and ~50% of catchments respectively, with the majority being in southern and eastern Australia. Northern Australia sees the opposite of this drying trend with fewer dry days per year and more intense rainfall during wet spells. Depending on the season, some regions are experiencing an increase in the number of wet spells, potentially suggesting there are changes to the dominant weather systems delivering rainfall to the region.

The presence of trends towards shorter wet spells and an increase in their frequency aligns with the change towards more episodic rainfall and highlights the need to further investigate both wet and dry spells concurrently. We conclude that wet and dry spell characteristics are changing and will continue to do so under the influence of climate change and need to be considered to manage water security across Australia.