Field and Numerical Investigation of Models for Flux Partitioning of Water Vapor and Carbon Dioxide Surface Fluxes

Yearly budgets of water vapor and CO₂ exchange between ecosystems and the atmosphere can now be measured through a growing worldwide network of eddy-covariance towers continuously measuring data across seasons. On the other hand, the quantification of water vapor and CO₂ exchanges from specific sources and sinks, namely from the soil and from the plant canopy, remains an elusive yet critical research objective. For instance, understanding the connection between environmental variables and soil evaporation and respiration or plant transpiration and photosynthesis is essential to characterize the influence of climate change and variability on ecosystem functioning. To this end, methods capable of reliably partitioning the total water vapor and CO₂ fluxes into their respective soil and plant components are highly valuable. In this work, we present existing models and propose new ones for flux partitioning that are based on analysis of conventional high frequency eddy-covariance (EC) data. The evaluation of the models uses actual observational times series, as well as data from virtual eddy covariance systems in large eddy simulations. The physical validity of the assumptions of all methods, as well as their partitioning skill, are examined. Our findings elucidate how the methods’ performance depends on the flux magnitude of the different components, and how it is modulated by the turbulent mixing directly caused by different canopy sparseness and arrangements. Overall, this research enables the development of best practices for partitioning ecosystems fluxes for each method, as well as the joint application of these methods to obtain reliable partitioning under a wide range of conditions.