Role of vegetation responses in hydrological shifts under multiyear droughts

Drought-induced vegetation responses are often hypothesized as one of the key drivers of hydrological changes under multiyear droughts. However, until now, this hypothesis has not been systematically tested on areas that experienced significant drought-induced reductions in streamflow generation. Our results do not support this hypothesis and suggest that vegetation changes are unlikely to be the main driver of observed hydrological changes.

We employed multiple remotely sensed vegetation indices (AVHRR NDVI & fPAR, MODIS NDVI & EVI, and Ku-VOD from multiple microwave satellite sensors) and rainfall-runoff shift indicators to investigate vegetation responses and their influences on streamflow generation during the Millennium Drought (from 1997 to 2009) in 156 catchments in Victoria, Australia. Many of these catchments experienced significant shifts in their rainfall-runoff relationship by severely reducing streamflow generation during the Millennium Drought. However, we show that vegetation indices are statistically similar or higher in many catchments during the Millennium Drought compared to pre-drought, consistent with published literature. Moreover, the spatial pattern of increase in vegetation indices does not match the spatial distribution of hydrological shifts, measured by significant streamflow reductions for a given rainfall. We argue that vegetation response is unlikely to be a primary driver of the observed hydrological shifts, although they are regarded as crucial in determining hydrological behaviour more generally. This finding has important implications for better understanding and modelling hydrological responses under future climate changes.