COS 193-2 - Coupled carbon and water fluxes at the ecosystem scale: Improved theory leads to simpler models

Background/Question/Methods

Evaporation (E) and transpiration (T) respond differently to ongoing global changes but are relatively difficult to measure compared to their sum, evapotranspiration (ET). New approaches to partition eddy covariance-measured ET into E and T at the ecosystem scale are promising and can be used to understand how canopy conductance responds to environmental variability to control coupled carbon and water fluxes. Here, we use a partitioning approach called flux variance similarity (FVS) to directly estimate E and T from 17 ecosystems in a 10 × 10 km area from the CHEESEHEAD19 experiment in northern Wisconsin, USA. The study sites included 10 deciduous forests, 4 evergreen forests, and 3 wetlands/lakes, and we use partitioned water and carbon (gross primary productivity and ecosystem respiration) fluxes from these diverse ecosystems to test simple ecosystem models based on new theoretical developments.

Results/Conclusions

T decreased more than E as the growing season progressed in the forest ecosystems but not the wetlands. The deciduous and coniferous forests showed similar E and T trajectories despite differences in vegetation phenology, with T/ET decreasing from 0.6 to 0.3, on average, from June to October. A model based on the principle of maximum entropy production (MEP) showed good agreement with E and T observations at each site if the thermodynamic environment of the evaporating surfaces was accurately simulated, suggesting that their variability across ecosystems can be modeled with minimal inputs and assumptions. We introduce a model that couples MEP, stomatal optimality theory, and the evaporative capacitor model for E and apply it to our observations to demonstrate how simple models based on theory can be combined for a parsimonious description of hydrology and carbon dynamics across diverse ecosystems.