Inferring the contribution of advection to total ecosystem scalar fluxes over a tall forest in complex terrain

• Data from a single tower are used to characterize advection regimes in a new site in complex terrain.
• Positive vertical advection was associated with cold-air drainage.
• Horizontal advection driven by landscape heterogeneity was inferred in mid- to late-morning.
• Roughly 50% of observations, collected when advection is significant, will need to be filtered.
• Similar physiological functioning in morning and afternoon periods simplifies gapfilling.

Multiple data streams from a new flux tower located in complex and heterogeneous terrain at the Coweeta Hydrologic Laboratory (North Carolina, USA) were integrated to identify periods of advective flow regimes. Drainage flows were expected a priori, due to the location of the measurement site at the base of a long, gently-sloping valley. Drainage flow was confirmed by examining vertical profile measurements of wind direction and by estimating vertical advection fluxes. The vertical advection flux of CO2 was most significant in early morning (000–0600 h) during the growing season, when it averaged ∼5 μmol m−2 s−1. Horizontal advection flux of CO2 was not directly measured in this study; however, an expected exponential relationship between nocturnal ecosystem respiration (RE) and air temperature was recovered when horizontal advection of CO2 was assumed to be negatively correlated to vertical advection, or when data were limited to periods when measured vertical advection fluxes were small. Taken together, these data imply the presence of a negative horizontal advection CO2 flux during nocturnal periods characterized by positive vertical advection of CO2. Daytime periods were characterized by consistent anabatic (up-valley) flows in mid- to late-morning (0500–1200 h) and consistent katabatic (down-valley) flows in the afternoon. A combination of above-canopy flux profile measurements, energy balance closure estimates, and flux footprint estimates suggest that during periods of up-valley wind flow, the flux footprint frequently exceeds the ecosystem dimensions, and horizontal advection fluxes related to landscape heterogeneity were a significant component of the total ecosystem flux of CO2. We used sap flux from individual trees beneath the tower to explore diurnal patterns in stomatal conductance in order to evaluate gapfilling approaches for the unreliable morning data. The relationship between stomatal conductance and vapor pressure deficit was similar in morning and afternoon periods, and we conclude that gapfilling morning data with models driven by afternoon data is a reasonable approach at this site. In general, results were consistent with other studies showing that the advection and wind flow regimes in complex terrain are highly site specific; nonetheless, the site characterization strategy developed here, when used together with independent estimates of components of the ecosystem carbon flux, could be generally applied in other sites to better understand the contribution of advection to the total ecosystem flux.