Flux-gradient relationships above tall plant canopies

Abstract

The classical flux-gradient relationships derived from the surface layer similarity theory have been known for a long time not to be valid just above plant canopies; within the framework of first-order closure, all the published studies have indeed shown that in the so-called ‘roughness sublayer’ the momentum and/or scalar diffusivities are enhanced above their inertial layer values. Alternate relationships that would be adequate in this region of the flow are, therefore, required for estimating heat and water vapour fluxes by the aerodynamic method or for calculating the aerodynamic resistances above tall canopies. A simple enhancement model may be a suitable basis for such relationships, provided it can be validated against existing or new experimental data. For this purpose, measurements of wind speed, air temperature and humidity profiles were made above a maize crop, along with other standard energy budget measurements. They confirm the existence of a roughness sublayer just above the canopy, in which the profiles depart significantly from the inertial layer similarity forms. This sublayer was found to extend up to about twice the canopy height. The measurements, performed throughout the entire sublayer and above it, give a precise picture of the evolution of the discrepancies with height, which can be interpreted in terms of enhanced diffusivities. Our results allow us to derive simple generalized flux-profile relationships that can be used to calculate the fluxes of heat and water vapour (and any other scalar) from two or more measurement levels, in the same way as with the classical aerodynamic method. These new relationships are shown to be compatible with most of the results already published, obtained over a wide range of canopies. The errors made when using the inertial layer relationships are quantified for typical experimental situations.