EFFECTS OF WINDBREAKS ON AIR-FLOW, MICROCLIMATES AND CROP YIELDS

The mechanisms by which a porous windbreak modifies airflow, microclim ates and hence crop yields are addressed, based upon recent wind tunne l experiments, field observations and numerical modelling. This paper is thus an update to che excellent reviews in Brandle (1988). It shows how a turbulent mixing layer initiated at the top of the windbreak do minates the airflow behind a windbreak. This mixing layer spreads vert ically as it moves downwind, growing at a rate determined by the turbu lence in the approach flow and the windbreak's 'permeability'. The rou ghness of the terrain and land-cover upwind, windbreak height and poro sity are thus the main controls on the amount and extent of shelter pr ovided by a windbreak. The changes in temperature, humidity, heat and evaporation fluxes given these changes in turbulence are then describe d. Based on the turbulent mixing layer model, the highly sheltered 'qu iet zone' will be typically warmer and more humid while further downwi nd in the 'wake zone', cooler and drier conditions would be expected. The careful experimental studies needed to verify these theoretical pr edictions have not yet been published. Shade is also shown to modify t he heating in the quiet zone and, depending on the orientation of the windbreak, can offset the warming in the quiet zone. Lastly, the mecha nisms affecting plant productivity are described in light of these air flow and microclimate changes. A major effect of a windbreak is to red uce the incidence of low frequency, high magnitude damage events such as sandblasting or lodging. Microclimate effects, however, do not alwa ys improve productivity. For example, while shelter may improve water- use efficiency in irrigated crops by increasing yields and reducing wa ter-use, this may not be the case in dryland agriculture.