Sources of water used by trees and millet in Sahelian windbreak systems

The extent to which water use by trees and crops is complementary in agroforestry systems may be affected by the proximity of groundwater to the soil surface. This may have important implications for the planning and management of agroforestry in semi-arid regions such as the Sahel of West Africa. A method of distinguishing uptake of water by plants from different sources was used, therefore, at locations with contrasting water table levels, to determine whether Azadirachta indica A. Juss (neem) trees in windbreaks utilized water from the same depths as adjacent crops of pearl millet (Pennisetum glaucum (L.) R. Br.). Comparisons of ratios of the stable isotopes of oxygen (18O/16O) in plant sap, groundwater and water in the unsaturated zone of the soil profile were made in the Majjia Valley, in south-central Niger, where groundwater was found at depths of 6-8 m, and at Sadoré in south-western Niger, where the water table was at a depth of 35 m.