THE ISOTOPIC COMPOSITION OF RAINFALL FROM AN ENERGY BALANCE PERSPECTIVE

The spatial distribution of isotopes in meteoric water is primarily controlled by the transport of atmospheric vapor and the balance between precipitation (P) and evapotranspiration (ET). In continental settings that receive moisture from a single source, isotope gradients are largely determined by the net distillation from an airmass (P minus ET). Here, we use a one-dimensional vapor transport model to demonstrate how the energetic limit on evapotranspiration influences net distillation. Our model solves the water balance while tracking the isotopic composition of rainfall over space. It can be applied to both mountainous and lowland environments. Across mountain ranges the energetic limit on ET forces Rayleigh-style distillation on the windward slope, consistent with other stable isotope paleoaltimetry models. In lowland settings, the energy limit can modify net distillation when rainfall exceeds available energy. To illustrate these effects we apply our model to the eastern Himalayan Range (mountain) and the Amazon basin (lowland). For the Himalaya case, we demonstrate how changes in the windward water balance can influence leeward isotope records. In the Amazon case the isotope gradient across the basin can be used to infer the modern water balance, but only when the energy limit is constrained. Our analysis suggests that the negative correlation between δ¹⁸O and rainfall amount—the ‘amount effect’—on climatological timescales in the Amazon basin is best quantified by the balance between rainfall and energy. Interpreting paleo-records in this context will improve our understanding of regional hydroclimate evolution and help disentangle topographic and climatic signals through Earth history.