Vegetation as a driver of shifts in rainfall-runoff relationship: Synthesising hydrological evidence with remote sensing

Drought-induced hydrological shifts and subsequent non-recovery have been reported globally, including in Australia. These phenomena involve changes in the rainfall-runoff relationship, so a year of given rainfall gives less streamflow than before. Some authors have indicated that vegetation dynamics played a key role in hydrological shifts during Australia's Millennium Drought (MD, 1997-2009), but such interactions are complex and are yet to be fully examined. This study investigates vegetation responses before, during, and after the MD for the same set of catchments in southeast Australia where hydrological shifts and non-recovery have been reported. The characterisation of vegetation behaviour relies on remotely sensed vegetation indices (VIs), namely Normalised Difference Vegetation Index (NDVI), Fraction of Photosynthetically Active Radiation (FPAR), Enhanced Vegetation Index (EVI), and Vegetation Optical Depth (VOD). Despite the severe multi-year drought, in most locations, the results indicate increased or maintained VIs over the entire period spanning pre-drought to post- drought. However, the link with hydrological shifts is nuanced and depends on how data are analysed. Contrary to expectations, an initial analysis (focussing on raw VI values) indicated that VI shifts were not correlated with hydrological shifts. It was only when the data were reanalysed to better account for the meteorological conditions that the expected correlations emerged. Overall, the results suggest that vegetation was able to maintain indices such as greenness and, by extension, actual evapotranspiration, leaving less rainfall for streamflow. More broadly, this approach provides greater insights into how vegetation affects hydrological behaviour through matched catchments during this and other multi-year droughts.