Biosphere-Atmosphere Fluxes in Complex Terrain: Challenges and Opportunities

Abstract:

The occurrence of advection fluxes in complex terrain complicates the interpretation of eddy covariance data. As a result, flux monitoring networks are biased towards flat, homogeneous ecosystems. Here, we discuss both the challenges and opportunities presented by efforts to quantify ecosystem mass and energy fluxes in topographically complex sites. An approach for characterizing advection fluxes that relies on data collected from a single tower is described and applied as a case study in a flux monitoring site located in complex and heterogeneous terrain at the Coweeta Hydrologic Laboratory (North Carolina, USA). This site experiences significant nocturnal advection of mass and energy driven by downslope drainage flows, and significant daytime horizontal advection driven by the effects of anabatic and katabatic flow regimes on the flux footprint. While the advection regime at Coweeta may be especially challenging, the approach presented here is also relevant for flatter and more homogeneous sites where advection fluxes may be small but nonetheless non-negligible. Having addressed the methodological challenges presented by advection flows, we discuss novel opportunities associated with efforts to monitor ecosystem fluxes in complex sites. In particular, data from Coweeta are leveraged to highlight how topography buffers the site from mesoscale hydro-climatic variability via its influence on precipitation inputs, air and soil temperature, and vapor pressure deficit. We conclude by stressing that advances in flux measurement methodology now permit flux observation campaigns in complex and heterogeneous sites; furthermore, such data may be quite useful for exploring links between terrain, micro-meteorology, and physiological functioning.