Windbreak-crop interactions in the Sahel. 1. Dependence of shelter on field conditions

Abstract

Growth of plants behind windbreaks is influenced by the seasonal change of air-flow caused by a combination of seasonal changes in incident wind conditions and windbreak characteristics. This paper examines the factors that influence the pattern of wind speed and turbulence for a field of millet over a cropping season behind a two-row windbreak in Niger, West Africa. Horizontal wind speed ( u) and wind speed variability ( V= σ u / u) were normalized to conditions at a distance of 6 h windward of the windbreak ( u 0, V 0), where h is height of windbreak. It was found that while no crop was present, u u 0 was independent of u 0 at 1 m above the ground, but at 3 m above the ground the effectiveness of the windbreak increased at high incident wind speeds. The pattern of shelter changed over the cropping season because growth of the millet crop occluded the gap below the tree canopy, decreasing the effective porosity of the windbreak and changing the wind speed profile. This moved the zone of minimum wind speed (measured at crop height plus 0.1 m) from a distance of 6 h towards 3 h from the windbreak. When windbreak porosity (φ) was low the angle of incidence of the wind (φ) changed u u 0 by b(1 − sinφ), where b is an empirical constant. This relationship did not hold as well at higher windbreak porosities. An increase in wind speed was observed at a distance of 1 h from the windbreak, resulting from preferential air-flow under the tree canopy, but this effect decreased as the millet grew. High wind speed variability was found at the top of the windbreak and at a distance of 10 h in the lee of the windbreak. Wind speed variability changed as a result of crop-influenced changes in windbreak porosity. It was concluded that the crop itself had an important influence on the effectiveness of the windbreak.