Shallow groundwater in the northeast arid zone of Nigeria

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Abstract
The natural resources of the northeast arid zone of Nigeria have been the subject of a recent five-year, European Union-funded research programme, carried out in support of rural development initiatives in the area. The shallow groundwater resources of the extensive river floodplain alluvial aquifers, and of the interfluves of uplands, have been an important focus of the research. The main issue in relation to sustainable development of these resources is the estimation of their recharge. Recharge to the alluvial aquifer of the Yobe river downstream of Gashua is showmn to be limited by an extensive clay cove overlying the aquifer. As a consequence of this the main mechanism of recharge is channel seepage, vertical infiltration being severely restricted. if, as is though by most consultants, this is the only source of recharge to the area as a whole, then it is shown that recharge to the area amounts to less than 1 mm/a. This is probably less than the present abstraction from the shallow aquifers. The paper challenges the current presumption that rainfed recharge to the shallow aquifers beneath the upland is nil. Observations from ther parts of the Sahel are used to show the typical magnitudes of water table decline which have coincided with the Sahel drought (early 1960s to date). These rates of decline cannot be explained by increases in abstraction. They can however be explained by significant reductions in recharge (of several mm/a) over the period in which rainfall has reduced, and this in turn implies recharge values significantly greater than zero in the very recent past, or to date. Detailed studies by the first author in the Manga Grasslands of north east Nigeria, in which recharge of 30–60 mm/a was estimated, are used to exemplify the likely situation in at least some parts of the upland. Ignorance or uncertainty of both the natural replenishment processes, and of the impact of climatic and anthropogenic changes, is no basis for sound water resource management policy. Because of the importance of establishing a more sound understanding of the recharge processes in the northeast arid zone, an outline programme of further research is presented.