Energy partitioning between latent and sensible heat flux during the warm season at FLUXNET sites

The warm season (mid‐June through late August) partitioning between sensible ( H ) and latent ( LE ) heat flux, or the Bowen ratio (β = H / LE ), was investigated at 27 sites over 66 site years within the international network of eddy covariance sites (FLUXNET). Variability in β across ecosystems and climates was analyzed by quantifying general climatic and surface characteristics that control flux partitioning. The climatic control on β was quantified using the climatological resistance ( R i ), which is proportional to the ratio of vapor pressure deficit (difference between saturation vapor pressure and atmospheric vapor pressure) to net radiation (large values of R i decrease β). The control of flux partitioning by the vegetation and underlying surface was quantified by computing the surface resistance to water vapor transport ( R c , with large values tending to increase β). There was a considerable range in flux partitioning characteristics ( R c , R i and β) among sites, but it was possible to define some general differences between vegetation types and climates. Deciduous forest sites and the agricultural site had the lowest values of R c and β (0.25–0.50). Coniferous forests typically had a larger R c and higher β (typically between 0.50 and 1.00 but also much larger). However, there was notable variability in R c and R i between coniferous site years, most notably differences between oceanic and continental climates and sites with a distinct dry summer season (Mediterranean climate). Sites with Mediterranean climates generally had the highest net radiation, R c , R i , and β. There was considerable variability in β between grassland site years, primarily the result of interannual differences in soil water content and R c .