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Alluvial aquifers present a possibility for conjunctive use with surface reservoirs for the storage of water in ephemeral sand rivers such as the Mzingwane River in the Limpopo Basin, Zimbabwe. The Lower Mzingwane valley is a semi-arid region with high water stress, where livelihoods have revolved around the large rivers for thousands of years. However, current water allocation favours the commercial user: of the 2600 ha irrigated in the 5960 km(2) (596 000 ha) region, only 410 ha are for smallholder farmers. A water balance approach was used to model the surface water resources and groundwater resources to determine the potential for expanding irrigation and to explore water allocation options. Using a combination of field and laboratory investigations, remote sensing and existing data, the Lower Mzingwane valley was modelled successfully using the spreadsheet-based model WAFLEX, with a new module incorporated to compute the water balance of alluvial aquifer blocks.

Results showed that the Lower Mzingwane alluvial aquifers can store 38 x 10(6) m(3) of water, most of that storage being beyond the reach of evaporation. Current water usage can be more than tripled: the catchment could supply water for currently-planned irrigation schemes (an additional 1250 ha), and the further irrigation of two strips of land along each bank of the Mzingwane River (an extra 3630 ha)-without construction of any new reservoirs. The system of irrigating strips of land along each bank of the Mzingwane River would be decentralized, farmer or family owned and operated and the benefits would have the potential to reach a much larger proportion of the population than is currently served. However, there could be substantial downstream impact, with around nearly one-third of inflows not being released to the Limpopo River.

The approach developed in this paper can be applied to evaluate the potential of alluvial aquifers, which are widespread in many parts of semi-arid Africa, for providing distributed access to shallow groundwater in an efficient way. This can enhance local livelihoods and regional food security. Copyright (C) 2010 John Wiley & Sons, Ltd.