Energy balance closure at FLUXNET sites

Abstract

A comprehensive evaluation of energy balance closure is performed across 22 sites and 50 site-years in FLUXNET, a network of eddy covariance sites measuring long-term carbon and energy fluxes in contrasting ecosystems and climates. Energy balance closure was evaluated by statistical regression of turbulent energy fluxes (sensible and latent heat (LE)) against available energy (net radiation, less the energy stored) and by solving for the energy balance ratio, the ratio of turbulent energy fluxes to available energy. These methods indicate a general lack of closure at most sites, with a mean imbalance in the order of 20%. The imbalance was prevalent in all measured vegetation types and in climates ranging from Mediterranean to temperate and arctic. There were no clear differences between sites using open and closed path infrared gas analyzers. At a majority of sites closure improved with turbulent intensity (friction velocity), but lack of total closure was still prevalent under most conditions. The imbalance was greatest during nocturnal periods. The results suggest that estimates of the scalar turbulent fluxes of sensible and LE are underestimated and/or that available energy is overestimated. The implications on interpreting long-term CO 2 fluxes at FLUXNET sites depends on whether the imbalance results primarily from general errors associated * Corresponding author. Tel.: +1-510-642-2874; fax: +1-510-643-5098. E-mail address: baldocchi@nature.berkeley.edu (D. Baldocchi).0168-1923/02/$ -see front matter. Published by Elsevier Science B.V. PII: S 0 1 6 8 -1 9 2 3 ( 0 2 ) 0 0 1 0 9 -0 224 K. Wilson et al. / Agricultural and Forest Meteorology 113 (2002) with the eddy covariance technique or from errors in calculating the available energy terms. Although it was not entirely possible to critically evaluate all the possible sources of the imbalance, circumstantial evidence suggested a link between the imbalance and CO 2 fluxes. For a given value of photosynthetically active radiation, the magnitude of CO 2 uptake was less when the energy imbalance was greater. Similarly, respiration (estimated by nocturnal CO 2 release to the atmosphere) was significantly less when the energy imbalance was greater. Published by Elsevier Science B.V.

Analysing surface energy balance closure and partitioning over a semi-arid savanna FLUXNET site in Skukuza, Kruger National Park, South Africa

“…The EBR results for the Skukuza eddy covariance system, which vary between 0.76 and 1.09 with an annual mean of 0.93 (only the years with high-quality data), are generally within the reported accuracies as shown in most studies that report the energy balance closure error at 10-30 %, across different ecosystems. For instance, Wilson et al (2002) also recorded an annual mean EBR of 0.84, ranging between 0.34 and 1.69 in an extensive study investigating 22 FLUXNET sites across the globe; EBR in ChinaFLUX sites ranged between 0.58 and 1.00, with a mean of 0.83 (Yuling et al, 2005); according to Were et al (2007), EBR values of about 0.90 were found over shrub and herbaceous patches in a dry valley in southeast Spain, whereas Chen et al (2009) showed a mean of 0.98 EBR for their study in the semi-arid region of Mongolia, and an EBR value of 0.80 was found by Xin and Liu (2010) in a maize crop in semi-arid conditions, in China. Using data from the Tibetan Observation and Research Platform (TORP), Liu et al (2011) observed an EBR value of 0.85 in an alfalfa field in semi-arid China.…”

“…For instance, Wilson et al (2002) comprehensively investigated the energy closure of the summer and winter seasons for 22 FLUXNET sites for 50 site-years. They also reported higher energy balance correlation during the wet compared to the dry season, with the mean R 2 of 0.89 and 0.68, respectively.…”

“…climate (Wilson et al, 2002). How the turbulent fluxes (H and LE) are partitioned in an ecosystem plays a critical role in determining the hydrological cycle, boundary layer development, weather, and climate (Falge et al, 2005).…”

“…net radiation minus soil heat flux, is equal to the sum of the turbulent fluxes (Rn − G = LE + H); however, in most instances, the measured available energy is larger than the sum of the measured turbulent fluxes of sensible heat and latent heat. Extensive research on the issue of surface energy imbalance in EC observations has been done (Barr et al, 2012;Chen et al, 2009;Foken et al, 2010;Franssen et al, 2010;Mauder et al, 2007), and closure error (or imbalance) has been documented to be around 10-30 % (Wilson et al, 2002;von Randow et al, 2004;Sánchez et al, 2010).…”

“…This imbalance has implications on how energy flux measurements should be interpreted and how these estimates should be compared with model simulations. The surface energy balance closure is an accepted performance criterion of EC flux data (Twine et al, 2000;Wilson et al, 2002), and different methods have been used to assess the energy closure and partitioning, including ordinary least squares regression (OLS) method, i.e. a plot of turbulence fluxes (H + LE) against available energy (Rn − G), the residual method, i.e.…”