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 utilised 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.
In the Majjia Valley, the trees obtained large portions of their water from surface layers of the soil only after rain, when water there was abundant. During dry periods, roots of the trees extracted groundwater or deep reserves of soil water, while the millet crop extracted water from closer to the top of the soil profile. In contrast, at Sadoré, both the trees and crop fulfilled their water requirements from the top 2–3 m of the soil throughout the year. Thus, utilisation of water by windbreak trees and crops is more complementary where groundwater is accessible to tree roots. Competition for water is likely reduced at such locations as a consequence, but may affect the productivity of windbreak systems where groundwater is inaccessible. To maximise the benefits of establishing windbreaks, therefore, it is important that planners recommend strategies for reducing competition for water between trees and crops at sites where groundwater cannot be reached by tree roots.
Journal of Hydrology (1997) 198 (1-4) 140-153 [doi:10.1016/S0022-1694(96)03311-2]