Trends in the frequency and duration of high flow events across climates

Flood events play a critical role in infrastructure design, floodplain management, water supply and natural ecology. While large floods are a risk to life and infrastructure, smaller flood events, here termed high flow events, are essential for maintaining biodiversity of wetlands and filling our water supplies. With climate change altering extreme rainfall magnitude, frequency, and timing there is a need to understand the impact of these changes on catchment flow response. Globally, studies have generally focused on changes in flood magnitude, and the drivers of these changes, and there remains a gap in understanding the historical changes in the frequency and duration of high flow events – those critical for sustaining ecosystem health and water supply. Here, using a set of 476 long record streamflow gauging stations across the continent of Australia, which span a wide range of climate zones, we investigate the trend in the frequency of high flow events and their duration. We find that, across most arid and temperate regions, the frequency of high flow events has decreased and their duration shortened, suggesting a reduction in water availability. In the tropics, there have been increases in the frequency of high flow events. Testing the robustness of the historical trends to the assumptions made in the analysis, we find that the rarer the event, the more likely it is to be increasing in frequency. These results provide some evidence that climate change is impacting both streamflow magnitude and frequency, which has significant implications for water supply, as well as the health of our ecological and environmental systems.