WINDBREAK-CROP INTERACTIONS IN THE SAHEL .1. DEPENDENCE OF SHELTER ONFIELD CONDITIONS

Growth of plants behind windbreaks is influenced by the seasonal chang e 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 variabili ty (V = sigma(u)/u) were normalized to conditions at a distance of 6h 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 o f u(0) at 1 m above the ground, but at 3 m above the ground the effect iveness of the windbreak increased at high incident wind speeds. The p attern of shelter changed over the cropping season because growth of t he millet crop occluded the gap below the tree canopy, decreasing the effective porosity of the windbreak and changing the wind speed profil e. This moved the zone of minimum wind speed (measured at crop height plus 0.1 m) from a distance of 6h towards 3h from the windbreak. When windbreak porosity (phi) was low the angle of incidence of the wind (T heta) changed u/u(0) by b(1 - sin Theta), where b is an empirical cons tant. This relationship did not hold as well at higher windbreak poros ities. An increase in wind speed was observed at a distance of 1h from the windbreak, resulting from preferential air-now under the tree can opy, but this effect decreased as the millet grew. High wind speed var iability 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 res ult of crop-influenced changes in windbreak porosity. It was concluded that the crop itself had an important influence on the effectiveness of the windbreak.