Turbulent diffusion within a wheat canopy: I. Measurement using nitrous oxide

A new method is described for measuring the vertical component of the coefficient of turbulent diffusion, K, within a wheat canopy. A uniform controlled flux of nitrous oxide was released from a network of nylon tubes at ground level, and K was calculated from the concentration gradient measured at the centre of the network. The method was first used above the canopy when the crop was short, and the results agreed with those obtained from the wind profile and energy balance. Experiments with different sizes of distribution area showed that in most conditions a radius of only 12m was adequate to give a vertical flux that was constant within 15% to a height of 1m in a canopy 1·25m tall. The possible error in K was estimated to be 10 to 15% in the lower half of the canopy increasing to 30 to 40% above 1·0m. This is a useful complement to the energy and momentum balance methods that are very inaccurate in the lower half of the canopy.