The Coupling of Soil Water and Leaf Phenology Observed by Satellite During Australian Bushfires in 2019–2020

Water and energy fluxes at the interface between the surface of the earth and the atmosphere depend strongly on soil moisture. Surface soil moisture (SSM) participates in natural phenomena such as evaporation, infiltration and runoff. From 2019 to 2020, bushfires in Australia caused changes in the runoff of surface water, the transpiration of vegetation, and evapotranspiration of the soil, subsequently affecting the soil water cycle. However, the dynamics of surface soil water content during large-scale bushfires are still not completely understood. Here, we report on patterns of seasonal variation in SSM and vegetation leaf phenology in Australia, identified using L-band radiometric measurements of soil moisture taken by the soil moisture and ocean salinity mission from 2011 to 2020. This study reveals a decrease in the leaf area index in affected regions of New South Wales, Victoria, Queensland, Western Australia, Australian Capital Territory, Northern Territory, and Tasmania. The observed seasonal variations in SSM are highly synchronous with those of leaf phenology in northern and northeastern Australia. However, the development of the leaf area lags behind that of SSM for up to 90 days in the southeastern subtropical woodlands. The time lag between leaf area development and SSM in the southwest is about 60 days. Bushfires can lead to the decrease of evapotranspiration hence shortening the time lag between SSM and leaf phenology. Our results offer insights into the variations in SSM during bushfires, the relationship between SSM and leaf phenology in Australia, and lay a foundation for improving models of ecological hydrology during bushfires.