Abstract
Water and carbon exchanges between the land and atmosphere reflect key
ecohydrologic processes, from global climate change to local watershed
dynamics. Environmental stable isotope ratios of H2O and
CO2 fluxes have been used to study these processes, yet
measurement constraints have limited macroscale surface-atmosphere
isotope flux evaluations. Across North American biomes within the US
National Ecological Observation Network (NEON), we have worked as a team
to translate raw measurements of carbon and water stable isotopes into
calibrated daily surface-atmosphere flux isotope ratios for
precipitation, evapotranspiration, and net ecosystem carbon exchange.
Using information theory metrics, we demonstrate that these isotope
observations contain meaningful information about the bulk water and
carbon fluxes, with isotope measurements carrying about the same amount
of information as wind speed measurements. Decomposition of this
multivariate mutual information further shows that: (1) this information
is unique, i.e. not carried by other traditional ecosystem measurements;
and (2) the information added by isotopes is larger in more arid and
cool ecosystems. Combining these isotope fluxes with bulk hydrologic
fluxes drawn from a suite of land surface models in a first-order mass
balance framework also allows for evaluation of hydrologic model
structure and estimated uncertainties in partitioning of fluxes into
transpiration, evaporation, overland, and subsurface water fluxes. An
inter-model comparison suggests distinct patterns in isotope flux
composition associated with disparities in the relative contributions of
partitioned fluxes. Our results show that conservative isotope tracers
provide novel validation metrics for evaluation of land surface model
performance across ecosystems at a continental scale. Broadly, this
compilation of datasets - combined with both empirical and process-based
isotope modeling - suggests NEON stable isotope observations can improve
general understanding of land-surface processes influencing the water
and carbon cycles from regional to global scales.