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, R₁, 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.