Turbulent diffusion within a wheat canopy: II. Results and interpretation

Abstract The coefficient of turbulent diffusion was measured in a 1·25m tall wheat canopy using a uniform release of nitrous oxide, the energy and momentum balance methods, and the humidity profile. The nitrous oxide profile gave the most accurate values, K N in the lower two thirds of the canopy, and a regression of the energy balance estimates, K E , against K N had a slope of 1·17 ± 0·10 with a non‐significant intercept of 17cm 2 s −1 . The mean day time profile of K N was exponential, K (z) = K ( h ) exp(−γ(1‐z/ h )) with γ = 3·9 when u( h ) 1·2m s −1 . But analysis of K N for the height interval 0·3 to 0·8m shows the importance of thermal stability, and the results are related to the non‐dimensional stability parameter g / T (Δ T /Δ z /( u ( h ) h 2 ) where Δ T /Δ z is the temperature gradient and u ( h ) is the windspeed at crop height, h . The large scatter in the results was attributed to the heterogeneity of the crop. Also the wind profile was frequently S‐shaped defying a simple one‐dimensional analysis and causing the momentum balance to give nonsensical results. Hence it seems unlikely that one‐dimensional analysis of transfer can give values of K allowing an accurate estimate of the source and sink distributions within a dense crop canopy.