EOMF-28. A new inverse modeling tool for understanding plant drought stress using atmospheric 13C of CO2 measurements

Abstract
Feedbacks related to exchanges of water and carbon between the atmosphere and the terrestrial biosphere are key uncertainties in our understanding of Earth’s climate. Of particular importance for climate projections is the response of diverse biomes’ to changes in water stress, due to increasing vapor pressure deficit and changing precipitation regimes, as well as responses to increasing atmospheric CO2. δ13C of CO2, hereafter δ13C, is a useful proxy for detecting plant water stress, since against 13C during photosynthesis, and this discrimination is reduced under periods of moisture stress. Indeed, recent studies have shown that atmospheric measurements of δ13C of CO2 are sensitive to changes in plant response to water stress, over regional and global scales. However, fewer studies have tried to directly assimilate these observations to constrain regional-scale plant water stress. Here, we first present a simple mathematical formulation that links δ13Catm to changes in photosynthetic fractionation (Δph). This framework allows for an examination of the fundamental limits of δ13Catm to constrain Δph, specially given the analytical uncertainty of the atmospheric measurements. We then present a novel and rigorous regional data assimilation system using synthetic measurements from a net work of highly-calibrated CO2 and δ13Catm measurements. The model simultaneously solves for net ecosystem exchange of CO2, δ13C discrimination and disequilibrium fluxes that are optimally consistent with pseudo-measurements. We find that the model is able to resolve signals that are considerably smaller than the limits of the theoretical mathematical framework. However this improvement is contingent on both the analytical uncertainty of measurements and the ability of a gridded model to represent point measurements. Therefore, a dense network of highly calibrated measurements of δ13Catm can be useful towards constraining regional-scale carbon and water fluxes between terrestrial ecosystems and the atmosphere.