Diffusion turbulente au-dessus et à l'intérieur d'un couvert végétal : application à l'étude des flux de chaleur et de masse

Chargé de Recherches, Station de Bioclimatologie, C.R.A.S.E (I.N.R.A.), domaine Saint-Paul, 84 -Monlfavet.

Résumé

Research is currently in progress at the Department of Bioclimatology of I.N.RA. with a view to the analysis of turbulent tranfer phenomena in and above growing crops in order to establish the photosynthesis of a full vegetation cover or a section of vegetation in situ and at any moment. A certain number of theoretical and experimental results which have been obtained are rapidly analysed. A theoretical internal velocity distribution for vegetation is established by exactly the same method as Prandtl used to find his expression for the logarithmic profile above a growth of vegetation. The two following new assumptions are made : (i) That the mixing length l is proportional to the reciprocal of leaf area per unit volume; (ii) A tangential stress varying with height and depending on leaf area density. An exponential velocity profile relationship (19) followed from these assumptions. Solutions are put forward for the transition to the lower and upper parts of the considered vegetation. The boundary conditions enable both the velocity profile and the momentum transfer coefficient to be expressed solely in terms of the crop's geometrical characteristics (26) (27) (28). Checks made against experimental data from other sources seem to confirm the various assumptions. A different research category is concerned with transfers taking place above vegetation. Windbreaks enable the transfer coefficients of the areas they protect to be modified. Possible applications of this research range from cropping techniques (crop associations, seeding densities, etc.) to the development of a whole area with a systematically laid out windbreak system.