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To understand changes in land surface energy balance partitioning due to tropical deforestation, we use a physically based analytical formulation of the surface energy balance. Turbulent heat fluxes are constrained by the thermodynamic maximum power limit and a formulation for diurnal heat redistribution within the land-atmosphere system. The derived turbulent fluxes of sensible and latent heat compare very well to in situ observations for sites with intact rainforest and soybean land cover in southeastern Amazonia. The equilibrium partitioning into sensible and latent heat flux compares well with observations for both sites, except for the soybean site during the dry season where water limitation needs to be explicitly accounted for. Our results show that tropical deforestation primarily affects the absorption of solar radiation and the water limitation of evapotranspiration, but not the overall magnitude of turbulent heat fluxes that is set by the thermodynamic maximum power limit.

Plain Language Summary Tropical deforestation impacts the local energy and water exchange between land surface and atmosphere, typically resulting in regionally warmer and drier climates. General circulation models still disagree in reproducing these changes and little has been done to derive them from first principles. Here, we present an alternative approach to describe the effects of tropical land conversion from forest to soy agriculture, based on a physical theory of land-atmosphere interactions. We view land-atmosphere exchange as the result of a heat engine strongly shaped by turbulent heat exchange. This provides a framework to derive analytical expressions of the turbulent fluxes from the limit by how much work this engine can maximally perform. By comparing these with observations from a tropical rainforest and a soybean field in Amazonia, we find that the diurnal variations of turbulent fluxes are very well estimated. This means that turbulent land-atmosphere exchange is primarily constrained by the thermodynamic limit, irrespective of surface roughness and evapotranspiration, and suggests that one can estimate the primary impacts of tropical land use change from physical principles. Thus, using thermodynamic limits represents an alternative approach to investigate the highly complex nature of land-atmosphere interactions and global change from first principles.