Understanding the interplay between rainfall intermittency and streamflow events

Rainfall variability plays a key role in how we are impacted by flood events, but also how we manage our water storages for flood mitigation and replenish our water resources. The sequencing between wetter and drier periods typically results in a natural fluctuation of rainfall-streamflow response but also modulates streamflow extremes. This relationship is being be impacted by anthropogenic climate change, with unprecedented extreme events and changes to long-term catchment dynamics. Several factors contribute to how rainfall variability influences streamflow event variability, including the rainfall event total, rainfall frequency, rainfall intensity and antecedent catchment conditions. In this study, we investigate changes to these rainfall variability factors and their relationship to streamflow event variability.

Our investigation is performed at the catchment scale for 467 Hydrological Reference Stations (HRS) catchments across Australia, utilising catchment-aggregated daily rainfall and gauged daily streamflow from 1950 to 2022. We investigate long-term trends in the frequency, duration and intensity of wet and dry rainfall spells across annual and seasonal timescales. We also identify streamflow events for each catchment and calculate key hydroclimate conditions before and during the event, such as the length of the rainfall dry spell before the event. These conditions are then used to better understand the different drivers of streamflow event volumes across Australia.

We find that southern and eastern Australia experience a drying trend with more dry days, shorter wet spells and greater intermittency with increases in the number of wet and dry spells per year. Northern and northwestern Australia experiences a wetting trend with more wet days, longer wet spells and increases in annual rainfall totals and rain intensity. These results are seasonally dependent, with stronger trends during periods where the majority of rainfall falls. The most important factors in driving streamflow event volumes are rainfall and soil moisture. We also find that the relationship between dry spells and streamflow event volumes is weak across Australian catchments despite a strong correlation with annual streamflow volumes. This highlights that event scale dynamics differ from the annual scale and the need to expand this analysis of the drivers of streamflow events alongside drivers of annual streamflow.