Impact of shelter on crop microclimates: a synthesis of results from wind tunnel and field experiments

The purpose of this paper is to synthesise data from the literature, and acquired during an extensive set of wind tunnel and field experiments, to quantify the effect of porous windbreaks on airflow, microclimates and evaporation fluxes. The paper considers flow oriented both normal (i.e. at right angles) and oblique to the windbreak, in addition to the confounding effects of topography. The wind tunnel results confirm the validity of the turbulent mixing layer as a model for characterising the airflow around a windbreak and for predicting the locations of the quiet and wake zones. This mixing layer is initiated at the top of the windbreak and grows with distance downwind until it intersects the vegetation or surface, marking the downwind extent of the quiet zone where the maximum shelter occurs. The 3 factors that determine the growth of this mixing layer are the windbreak porosity, windbreak height and the nature of the terrain upwind. For wind that is flowing normal to a porous windbreak in the field, the latter 2 have the primary influence on the size of the sheltered zone, while windbreak porosity is the main factor determining the amount of shelter. Analyses of the effect of porosity revealed that the amount of wind shelter increases as windbreak porosity is reduced, but the downwind extent of the sheltered zone does not vary with windbreak porosity. Thus, the suggestion from older studies that low-porosity (i.e. dense) windbreaks lead to a reduced sheltered area is not supported by the wind tunnel measurements. In the absence of shading effects, temperature and/or humidity are increased in the quiet zone, mirroring the pattern and magnitude of wind shelter. Thus, the increase in temperature and humidity is greatest where the minimum wind speed occurs, and the magnitude of the increase is smaller for more porous windbreaks. The humidity and air (but not surface) temperatures are decreased very slightly in the wake zone, although these small changes were not significant in a field situation. Microclimate changes, therefore, occur over a much smaller distance downwind than wind shelter, and are negligible for the very porous windbreak. For example, at 20 windbreak heights downwind, the wind speed may still be 80% of its upwind value, while the air and surface temperature and humidity have returned to their upwind values after 12-15 windbreak heights. Furthermore, these changes in temperature and humidity vary with the type of land cover, surface moisture status and the temperature and humidity of the 'regional' air. Over the course of a growing season, these changes can be masked by soil and climate variability. The turbulent scalar fluxes, i.e. evaporation and heat fluxes, also differ from the pattern of near-surface wind speeds. While significantly reduced in the quiet zone, they show a very large peak at the start of the wake zone the location where the mixing layer intersects the surface. Thus, caution is required when extrapolating from the spatial pattern of shelter to microclimates and turbulent fluxes. Wind flowing at angles other than normal to the windbreak has 2 effects on the pattern of wind shelter. First, for the medium and low porosity windbreaks used in the wind tunnel, the amount of wind shelter is increased slightly in the bleed flow region near the windbreak, i.e. there is an apparent reduction in windbreak porosity as the wind direction becomes more oblique to the windbreak. Second, the profile of near surface wind speeds is similar to that for flow oriented normal to the windbreak, providing that the changes in distance from the windbreak are accounted for using simple geometry. The field data agree with these results, but show an even greater influence of the windbreak structure on the pattern of wind shelter in the bleed flow region, extending from the windbreak to at least 3 windbreak heights downwind, precluding any generalisations about the flow in this region.