Mechanistic links between underestimated CO ₂ fluxes and non-closure of the surface energy balance in a semi-arid sagebrush ecosystem

Abstract

The surface energy balance non-closure problem in eddy covariance (EC) studies has been largely attributed to the influence of large-scale turbulent eddies (hereafter large eddies) on latent and sensible heat fluxes. However, how such large eddies concurrently affect CO 2 fluxes remains less studied and mechanistic links between the energy balance non-closure and CO 2 fluxes are not well understood. Here, using turbulence data collected from an EC tower over a sagebrush ecosystem during two growing seasons, we decomposed the turbulence data into small and large eddies at a cutoff frequency and analyzed their contributions to the fluxes. We found that the magnitude of CO 2 fluxes decreased concurrently with decreased sensible and latent heat fluxes (and thus increased energy balance nonclosure), primarily caused by large turbulent eddies. The contributions of such large eddies to fluxes are dependent not only upon their magnitudes of vertical velocity (w) and scalars (i.e. temperature, water vapor density, and CO 2 concentration), but also upon the phase differences between the large eddies of w and scalars via their covariances. Enlarged phase differences between large eddies of w and these scalars simultaneously led to reductions in the magnitudes of both CO 2 and heat fluxes, linking the lower CO 2 fluxes to energy balance non-closure. Such increased phase differences of large eddies were caused by changes in the structures of large eddies from unstable to near neutral conditions. Given widespread observations in non-closure in the flux community, the processes identified here may bias CO 2 fluxes at many sites and cause upscaled regional and global budgets to be underestimated. More studies are needed to investigate how landscape heterogeneity influences CO 2 fluxes through the influence of associated large eddies on flux exchange.

Groundwater Regulates Interannual Variations in Evapotranspiration in a Riparian Semiarid Ecosystem

“…At the upland site, a 2‐month data gap occurred during the late spring to early summer of 2018 due to instrument failure, which was not gap‐filled. We evaluated the monthly surface energy balance closure at each site using two methods: (1) the energy balance ratio (Wilson et al., 2002) and (2) the slope of the linear regression between the half‐hourly measurements of the flux variables ( LE + H ) and available energy ( R n − G 0 ) (Gao et al., 2017, 2019). Both sites demonstrated reasonable energy balance closure, with the EBR being greater than 0.76 during the growing season (see SI for additional details).…”

Effects of Irrigation on Water, Carbon, and Nitrogen Budgets in a Semiarid Watershed in the Pacific Northwest: A Modeling Study

“…Observations from the AmeriFlux tower site US-Hn1 ( Figure 1) located in a semiarid upland sagebrush ecosystem without groundwater access are used to examine model performance over the naturally vegetated fraction (Gao et al, 2017(Gao et al, , 2019Missik et al, 2019). The eddy covariance measurements of water, energy, and carbon fluxes are collected from 2016 to 2018 at a half-hourly time step at the site.…”

Uncertainties in Turbulent Statistics and Fluxes of CO2 Associated With Density Effect Corrections

“…A mixture of shrubs and grasses dominates in the area (Figure S1 in the supporting information). During the spring with adequate soil water availability, the vegetation is active, whereas in summer months the vegetation activities are largely constrained due to the decreased water availability (Gao et al, 2017, 2019; Missik et al, 2019). One three‐dimensional sonic anemometer (model CSAT3, Campbell Scientific, Inc.) and one open‐path infrared gas analyzer (model LI‐7500A, LI‐COR, Inc.) were installed on the tower at 5 m above the ground with a separation distance of 0.2 m. The sonic anemometer measured longitudinal ( u ), lateral ( v ), and vertical ( w ) wind components and sonic temperature ( T s ), and the gas analyzer measured ρ v and ρ c .…”