Modelled Sensitivity of Evapotranspiration to Vegetation Change: Reconciling Observations and Earth System Models

A significant portion of terrestrial surface water is returned to the atmosphere through vegetation transpiration, making vegetation dynamics—such as deforestation, reforestation, and Earth’s greening—a critical driver of land evapotranspiration and vegetation-climate feedbacks. However, Earth system models (ESMs) exhibit substantial discrepancies in simulating the direction and magnitude of vegetation effects on evaporation. These inconsistencies stem from the heterogeneity of land-cover changes and limitations in ground-based observations, which complicate the quantification of these responses. In this study, we identify key disparities among ESMs in simulating evaporation and transpiration responses to vegetation changes, which lead to divergent predictions of climate feedbacks. A central issue is the persistent underestimation of the transpiration-to-evaporation ratio (Et/E), despite observational evidence indicating that transpiration dominates terrestrial evaporation fluxes. This underestimation is further compounded by inadequate representations of groundwater processes and limited soil depth in models, which restrict the availability of water for vegetation transpiration. To address these shortcomings, we propose the integration of enhanced observation-based constraints on model sensitivity, improved transpiration parameterizations, and the explicit inclusion of groundwater processes in ESMs. These advancements are essential for reducing uncertainties in vegetation-climate feedback projections and improving the accuracy of Earth system modeling.