Accounting for Vegetation Feedbacks in Hydrological Models Using a New Vegetation-aware Evapotranspiration Formulation

Streamflow in several catchments in eastern Australia has decreased considerably (up to 40%) over the past 20 to 30 years, despite stable rainfall levels. This decoupling of streamflow and rainfall undermines the predictive accuracy of rainfall-runoff models used by catchment managers, which typically rely on the assumption of a stable relationship between these variables. Similar non-stationarity in streamflow has been observed in other catchments in the world, and evidence suggests that vegetation processes may be driving this non-stationarity due to increases in temperature and CO₂. Current rainfall-runoff models fail to capture the impact of these vegetation changes on evapotranspiration (ET). While these models account for ET's dependence on soil moisture, they do not consider changes in vegetation biomass and health, which can significantly alter ET and, consequently, runoff.

This contribution presents a methodology for estimating a vegetation-aware ET based on the Penman-Monteith equation and emulators that can account for changes in vegetation biomass and health. The emulators are developed using data from the Australian and New Zealand Flux Research and Monitoring network (TERN OzFlux). This network provides extensive measurements of energy, carbon, and water exchanges across various ecosystems, from which vegetation effects can be estimated under different environmental conditions, and across different vegetation types. Through this research we aim to contribute to understanding evapotranspiration dynamics and offer a reliable and simple tool for estimating vegetation effects, ultimately adding it to more realistic rainfall-runoff simulations.