Some of the main mechanisms by which windbreaks are believed to modify plant growth - the creation of a more favourable microclimate, reductions in evaporation and improved water-use efficiency - depend on the way that a windbreak modifies scalar transport. However investigations of windbreak effects on scalar transport, especially the turbulent heat and water vapour fluxes, and the resulting microclimate, are much less common than studies of the mean and turbulent airflow.
This paper presents results from a series of wind tunnel experiments and field observations whose specific aim was to study scalar fluxes, and microclimates, around windbreaks. The focus is upon the turbulent scalar fluxes; the diurnal variation of the mean scalar concentrations; and the effects of obliquity. Windbreaks are shown to have quite different effects on the turbulent scalar fluxes and mean scalar concentrations, compared to the mean and turbulent velocity fields. A conceptual model of windbreak effects on evaporation and the temperature and humidity regime downwind of a windbreak is developed as a result of these measurements. The implications of these results for windbreak effects on crop productivity are examined, and the conceptual model tested, using 5 years of crop growth measurements obtained at a number of windbreak experimental sites across Australia, gathered as part of the National Windbreaks Program.