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

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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, KN in the lower two thirds of the canopy, and a regression of the energy balance estimates, KE, against KN had a slope of 1·17 ± 0·10 with a non-significant intercept of 17cm2s−1.

The mean day time profile of KN was exponential, K(z) = K(h) exp(−γ(1-z/h)) with γ = 3·9 when u(h) < 1·2m s−1 increasing to 5·3 when u(h) > 1·2m s−1. But analysis of KN 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/TTz/(u(h)h2) where ΔTz 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.

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