What Has Rainfall Got to Do With It? Landscape Antecedent Conditions Have a First Order Effect on Flood Inundation Extent in the Hawkesbury-Nepean Valley

The Hawkesbury-Nepean valley has a long history of hazardous flooding dating back to when records began, in the 18th century. It is one of the largest coastal basins in the eastern Australian region and supports a large population, agriculture, and industry, as well as being an important environmental asset. Multiple flooding events with differing socioeconomic impacts occurred in the valley between 2020 and 2022, brought about by a rare triple La Niña event—only the third since 1900. In this study, upon examination of the four significant floods in the area, the reasons for the variation in impacts are explored by investigating the climate drivers and antecedent conditions preceding these events, the streamflow during the events, and the hazard footprints themselves. It was found that antecedent landscape conditions exerted a first-order effect on the hazard footprint's size and thus impact. These results imply that with the current landscape drying trends and projected increase in landscape dryness for this region, the projected increase in extreme rainfall events will not affect the flood hazard footprint of medium-sized floods. Thus, impacts from flooding will not necessarily increase in severity in this region in the future.