Turbulent transfer in a barley canopy

Abstract As a first step towards estimating the vertical flux of CO2 in the canopy of a barley crop, the eddy diffusivity for turbulent transfer was estimated by two independent techniques: the energy balance method which needs measurement of net radiation, temperature and humidity throughout the canopy; and a flux method which needs measurements of individual leaf photosynthesis together with in-canopy CO2 gradients and the soil CO2 flux. At the top of the canopy the two sets of diffusivity measurements agreed with each other and with an aerodynamic estimate of the diffusivity. In the lower part of the canopy, however, large systematic differences occurred between the two estimates. Hence fluxes of CO2 and flux divergences calculated from diffusivity profiles may be seriously in error. The basic assumptions and approximations embodied in the two sets of calculations are reviewed. Horizontal inhomogeneity in the crop is the most likely source of error. Lack of uniformity may be responsible for horizontal fluxes of heat, water vapour and CO2 large enough to invalidate the one-dimensional approximation inherent in the micrometeorological methods used. In the current state of the art, inserting leaves in transparent chambers in a gas flow system or dosing them with 14CO2 appear to be the most reliable methods of estimating photosynthetic rates layer by layer in a canopy. These techniques will be compared in a future paper in the series.