A data-driven analysis of energy balance closure across FLUXNET research sites: The role of landscape scale heterogeneity

Abstract

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

Carbon budgets for an irrigated intensively grazed dairy pasture and an unirrigated winter-grazed pasture

“…The DC values for both sites are closer to the ideal value of 1 than the average DCs found in all published FLUXNET intercomparison studies to date, see Stoy et al (2013) and references therein. Compared to other low-vegetation experiments, the DC values are close to the median (0.9) found by Laubach and Teichmann (1999) reviewing 11 studies over grass or crops, close to the grassland average (0.86) reported by Stoy et al (2013) and comparable to those for three Australian grasslands (0.87, 0.86 and 0.94) given by Leuning et al (2012). To conclude, the surface energy budgets at our sites give no indication of any quality issues beyond what is commonly accepted as inevitable methodical error of the EC technique.…”

Two years with extreme and little snowfall: effects on energy partitioning and surface energy exchange in a high-Arctic tundra ecosystem

“…Studies on the closure problem of eddy covariance measurements highlight the landscape heterogeneity as a driving factor for the lack of closure (Stoy et al, 2013) but also stress the importance of measurement scales (Foken, 2008). Due to the striking small-scale heterogeneity of soil moisture availability and vegetation cover in Arctic landscapes, measurements of point-scale G and radiative components may not represent the large source area of eddy covariance H and LE.…”

Energy balance closure on a winter wheat stand: comparing the eddy covariance technique with the soil water balance method

“…Low EBCs (60-80 %) were mainly observed at various agricultural sites and bare soil, whereas over forest they were typically higher (80-90 %) (Charuchittipan et al, 2014;Wilson et al, 2002;Foken, 2008a;Panin and Bernhofer, 2008;Stoy et al, 2013). The imbalance usually occurs during day time, particularly around noon, whereas during the night when fluxes are low EBC is often close to unity (Oncley et al, 2007).…”

“…This conclusion is supported by Heusinkveld et al (2004); they found a perfect EBC over a homogeneous surface: a desert in Israel. Tsvang et al (1991) and Stoy et al (2013) also concluded that the heterogeneities of the surrounding area are an important factor contributing to the lack of EBC. Several authors (Klaassen and Sogachev, 2006;Friedrich et al, 2000) reported an increase of the turbulent fluxes at forest edges.…”