6th WaterNet/WARFSA/Global Water Partnership Annual Symposium - November 2005, Swaziland Theme 5: Water use in irrigated agriculture, challenges and opportunities in southern Africa OR Sub-theme 1: Effective Water Governance and IWRM Planning

Alluvial aquifers in the Mzingwane Catchment: their distribution, properties, current usage and potential expansion and risk

William Moyce1*, Pride Mangeya1, David Love1,2,3 and Richard Owen1,4

1 Department of Geology, University of , PO Box MP167, Mt. Pleasant, Harare, Zimbabwe 2 WaterNet, PO Box MP600, Mt. Pleasant, Harare, Zimbabwe 3 ICRISAT , Matopos Research Station, PO Box 776 Bulawayo, Zimbabwe 4 Minerals Resources Centre, University of Zimbabwe, PO Box Mp167, Mt. Pleasant, Harare, Zimbabwe

Alluvial deposits are common in southern Zimbabwe, occurring as sand filled ephemeral rivers. The Mzingwane River is one such sand filled channel, with extensive alluvial aquifers distributed along its banks on the lower catchment and is the major northern (left bank) tributary to the in Zimbabwe.

Selected alluvial deposits were investigated as potential groundwater resources for irrigation development. LandSat imagery was used to identify alluvial channel and plains deposits.

The alluvial aquifers in the Mzingwane River are extensive forming ribbon shapes covering over 20 km in length and areal extents ranging from 100 ha to 255 ha in the channels and 85 ha to 430 ha on the plains. They can sustain small-scale irrigation and infiltration galleries; and well point systems can be constructed to exploit the resource. Currently some of the aquifers are being used to provide water for domestic use (e.g. Maranda Village, J.Z. Moyo School), livestock watering and dip tanks (e.g. ), commercial irrigation (e.g. and Gem Farms) and market gardening (e.g. Bwaemura Village).

Field investigations focussed on two localities where small-scale irrigation has already been developed. The aquifer dimensions and properties were investigated using mechanical probing, grain size analysis, drilling and geophysical surveys. The alluvial sediments have thicknesses of up to 20 m with estimated hydraulic conductivity (K) values of between of 40 and 200 m/day. Yield ranges from 40 to 5200 m3/day, which can sustain perennial irrigation for thousands of hectares of land. The water quality in the aquifer was monitored by sampling alluvial channel wells. The water quality of the aquifers in general is fairly good due to regular recharge and flushing out of the aquifers by flood water. Water salinity was found to increase significantly in the months of October and November (the end of the dry season), and this effect was more pronounced in water abstracted from wells on the alluvial plains. Theoretical considerations suggest that such water is unsuitable for irrigation development.

Farmer-based programmes on water and crop management, along with careful management of abstraction rates can improve productivity of the available water and safeguard against depreciation in water quality.

Keywords: alluvial aquifer, irrigated agriculture, groundwater quality, remote sensing, Limpopo Basin

* Corresponding author: Tel.: +263-4-303211 ext1427 Fax.: +263-4-303557; email.: [email protected]