An alternative approach for WPL: contribution from water vapor

Energy and CO2 fluxes are commonly measured above plant canopies using an eddy covariance system that consists of a three-dimensional sonic anemometer and an H2O/CO2 infrared gas analyzer. By assuming that the dry air is conserved and inducing mean vertical velocity, E. K. Webb et al. (Quart. J. Roy. Meteorol. Soc., 106, 85-100, 1980) obtained two equations to account for density effects due to heat and water vapor transfer on H2O/CO2 fluxes. In this study, directly starting with physical consideration of air-parcel expansion/compression, we derive two alternative equations to correct for these effects, which do not require the assumption (i.e., the dry-air is conserved) and the use of the mean vertical velocity. We then applied these equations to eddy flux observations from a black spruce forest in interior Alaska during the summer of 2002. In this ecosystem, the equations developed here led to increased estimates of CO2 uptake by the vegetation during the day (up to about 20%), and decreased estimates of CO2 respiration by the ecosystem during the night (approximately 4%) as compared with estimates obtained using the Webb et al. approach.