Evaluating the ACASA model as a tool for flux interpretation at 4 distinct AmeriFlux Sites

Many eddy-covariance flux sites are equipped with only one measurement height above the canopy. Due to power requirements, cost of instruments, and the maintenance involved it is often unrealistic to expand to a full profile of measurements throughout and above a canopy. Here we apply a higher-order closure turbulence and flux model to 4 distinct flux sites in order to produce profiles of turbulence and fluxes as well as storage in and above canopies from single point measurements. We used the UC-Davis Advanced Canopy-Atmosphere-Soil Algorithm (ACASA) to study a range of ecosystems including a tall old-growth forest with a complex canopy (Wind River Ameriflux), a two-layered oak savanna (Tonzi Ameriflux), a grassland (Diablo Ameriflux), and a wheat field (Southern Great Plains Ameriflux). We assess the conditions under which applying a complex model like ACASA is beneficial in interpreting within-canopy dynamics and flux signal quality. We describe ACASA performance at the 4 sites as well as the changes to the model and the tuning necessary to optimize performance for each of the unique sites. Novel improvements to the model include the incorporation of seasonal dynamics in leaf area index and photosynthetic capacity, adjustment of plant water use during seasonal drought, and adjustments for a clumped canopy.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.