On the consequences of the energy imbalance for calculating surface conductance to water vapour

Abstract

The Penman-Monteith combination equation, which is most frequently used to derive the surface conductance to water vapour (G s ), implicitly assumes the energy balance to be closed. Any energy imbalance (positive or negative) will thus affect the calculated G s . Using eddy covariance energy flux data from a temperate grassland and a desert shrub ecosystem we explored five possible approaches of closing the energy imbalance and show that calculated G s may differ considerably between these five approaches depending on the relative magnitudes of sensible and latent heat fluxes, and the magnitude and sign of the energy imbalance. Based on our limited understanding of the nature of the energy imbalance, we tend to favour an approach which preserves the Bowenratio and closes the energy balance on a larger time scale.

Improved understanding of drought controls on seasonal variation in Mediterranean forest canopy CO<sub>2</sub> and water fluxes through combined in situ measurements and ecosystem modelling

“…This assumption is frequently not fulfilled by the eddy covariance (EC) method (Wilson et al, 2002). Our understanding of the causes of energy imbalance is limited, but any imbalance could introduce uncertainty in the calculations applied in this study (Wohlfahrt et al, 2009). In particular, latent heat fluxes are often, though not always, underestimated by the order of 10-30% (e.g., Wilson et al, 2002).…”

Evapotranspiration and its main controlling mechanism over the desert riparian forests in the lower Tarim River Basin

“…The half-hourly turbulent fluxes and meteorological variables were again screened to remove spurious data points caused by sensor malfunction, rain events, and sensor maintenance. The energy balance was forced to close according to the Bowen ratio (Wohlfahrt et al, 2009) to obtain the sensible heat flux and latent heat flux data.…”

Components of near-surface energy balance derived from satellite soundings – Part 2: Noontime latent heat flux

“…Because eddy covariance is not a gradient method it is an attractive source of evaluation data. Sensible and latent heat flux measurements were used as reported in the FLUXNET data base; in other words no corrections for any lack in energy balance closure (Foken, 2008;Wohlfahrt et al, 2009) were applied. The spatial scale of the tower eddy covariance footprint is of the order of ∼ 1 km 2 and is hence on a scale approximately 4 orders of magnitude smaller than the 10 000 km 2 satellite data, which obviously has implications in heterogeneous environments (see above).…”

“…The more arid areas are also clearly delineated. Although somewhat variable, the Sahara gives a range of 1.5-3.5, which corresponds to the results of Kohler et al (2010) and Wohlfahrt et al (2009) for the Mojave Desert. The South American savanna gives a range between 0.5 and 1, which corresponds to values reported by Giambelluca et al (2009).…”