1 Department of Chemical and Biochemical Engineering, Technical University of Denmark2 Ecosystems Programme, Department of Chemical and Biochemical Engineering, Technical University of Denmark3 Department of Wind Energy, Technical University of Denmark4 Meteorology, Department of Wind Energy, Technical University of Denmark5 Montana State University6 Karlsruhe Institute of Technology7 University of Bayreuth8 Fondazione Edmund Mach9 Roskilde Universitet10 Department of Environmental Engineering, Technical University of Denmark11 McMaster University12 Lund University13 Finnish Meteorological Institute14 Dresden University of Technology15 European Commission - Joint Research Center16 Consiglio Nazionale delle Ricerche17 Commonwealth Scientific and Industrial Research Organisation18 University College Cork19 Georg-August University Göttingen20 Universite Laval21 Queen's University22 Swiss Federal Institute of Technology23 Free University of Bolzano24 University of Tuscia25 Max Planck Institute26 University College Dublin27 Estación Experimental de Zonas Áridas28 University of Sassari29 Russian Academy of Sciences30 Montana State University31 McMaster University32 Lund University33 Finnish Meteorological Institute34 Universite Laval35 Swiss Federal Institute of Technology36 University College Dublin37 University of Sassari
The role of landscape-scale heterogeneity
The energy balance at most surface-atmosphere flux research sites remains unclosed. The mechanisms underlying the discrepancy between measured energy inputs and outputs across the global FLUXNET tower network are still under debate. Recent reviews have identified exchange processes and turbulent motions at large spatial and temporal scales in heterogeneous landscapes as the primary cause of the lack of energy balance closure at some intensively-researched sites, while unmeasured storage terms cannot be ruled out as a dominant contributor to the lack of energy balance closure at many other sites. We analyzed energy balance closure across 173 ecosystems in the FLUXNET database and explored the relationship between energy balance closure and landscape heterogeneity using MODIS products and GLOBEstat elevation data. Energy balance closure per research site (CEB,s) averaged 0.84±0.20, with best average closures in evergreen broadleaf forests and savannas (0.91–0.94) and worst average closures in crops, deciduous broadleaf forests, mixed forests and wetlands (0.70–0.78). Half-hourly or hourly energy balance closure on a percent basis increased with friction velocity (u*) and was highest on average under near-neutral atmospheric conditions. CEB,s was significantly related to mean precipitation, gross primary productivity and landscape-level enhanced vegetation index (EVI) from MODIS, and the variability in elevation, MODIS plant functional type, and MODIS EVI. A linear model including landscape-level variability in both EVI and elevation, mean precipitation, and an interaction term between EVI variability and precipitation had the lowest Akaike's information criterion value. CEB,s in landscapes with uniform plant functional type approached 0.9 and CEB,s in landscapes with uniform EVI approached 1. These results suggest that landscape-level heterogeneity in vegetation and topography cannot be ignored as a contributor to incomplete energy balance closure at the flux network level, although net radiation measurements, biological energy assimilation, unmeasured storage terms, and the importance of good practice including site selection when making flux measurements should not be discounted. Our results suggest that future research should focus on the quantitative mechanistic relationships between energy balance closure and landscape-scale heterogeneity, and the consequences of mesoscale circulations for surface-atmosphere exchange measurements.
Agricultural and Forest Meteorology, 2013, Vol 171-172, p. 137-152
Eddy covariance; Energy balance closure; Enhanced vegetation index; FLUXNET; MODIS; Plant functional type