Heat and mass exchange within the soil - plant canopy-atmosphere system : a theroretical approach and its validation

Heat, mass and momentum transfer between the canopy air layer and the layer of air above has a very intermittent nature. This intermittent nature is due to the passage at the canopy top of coherent structures which have a length scale at least as large as the canopy height. The periodic passage of these coherent structure at the canopy top leads to the ejection of the air inside the canopy and the replacement of this air by fresh air from above. It is through this process of ejection and sweep that the coherent structures become responsible for most of the large time average flux.This study considers the effect of these coherent structures on the modelling and the dynamics of interaction between the plant canopy and the soil with the layer of air above and the effect of these coherent structures on the soil temperature profile. so, three parts are considered: Modelling , mathematical analysis and validation.In the Modelling part: a discussion of the limitations of the available approaches and a suggestion of an intermittency approach are given.First, there is a qualitative analysis of the effect of these coherent structures and their role in the momentum, heat and mass transfer on the validity of the Eulerian approaches used to describe canopy flow. We outline the limitations of these approaches and later suggest an intermittency approach to describe heat and mass transfer between the canopy layer and the layer of air above. We describe the used averaging procedure, the resulting correlations, the closure parameterization used and their justification.Then we give a discussion of the effect of these coherent structure on the Lagrangian model approach qualitatively and then quantitatively and a method to correct for this is suggested.From this, a mathematical analysis of the effect of coherent structure on the soil temperature profile is done by first analysing the effect of coherent structures on the mean temperature and vapour pressure deficit of the air. It is shown from the equations governing the system's behaviour that there is a non linearity in the canopy system. The effect of this non linearity depends on the ratio between the period between consequent gust intrusions into plant canopy with respect the air time constants. The effect of this non linearity on the soil temperature profile is shown through its effect on the coefficients of an Eigenfunction expansion of the soil temperature profile. Different scenarios for the effect of different parameters such as the stomatal resistance, the turbulent transport coefficient and the period between gust intrusion are studied and explained.In the validation part, a comparison of a simulation for 7 days against a data set shows that the model gives very good agreement between the radiative environment and the temperature and vapour pressure of the air. Anyhow there is a interplay between three degrees of freedom. These are represented by the turbulent transport coefficient , the stomatal resistance and the gust intrusion into plant canopy