|Sourcebook of Alternative Technologies for Freshwater Augmentation in Africa (International Environmental Technology Centre - United Nations Environment Programme, 1998, 182 p.)|
|Part B - Technology profiles|
|1. Agricultural technologies|
|1.1 Fresh water augmentation|
Water is abstracted from sand-filled river beds during periods in which there is no surface water flowing in the river. In such situations, the sand is fairly saturated. The method involves the use of slotted cast iron or PVC pipes drilled into the sand-filled river bed, connected to a mainline or manifold, and a pumping system. The water pumped out and distributed for use in agricultural or domestic applications. In small-scale, domestic systems wells or casing sunk into the sandy river bed are fitted with simple hand pumps, or a bucket and windlass assembly.
Extent of Use
Freshwater abstraction from sandy river beds is widely used in Zimbabwe and Botswana for farming and domestic supply purposes. Commercial enterprises, such as ARDA and the Hippo Valley Estates in Zimbabwe, have successfully used sand abstraction for irrigation purposes. At Chisumbanje, ARDA has installed some sand abstraction units for winter irrigation of wheat and early cotton crops. Hippo Valley Estates successfully used sand abstraction during the drought of 1991-1992 for sugarcane irrigation in one of their Estate sections. Sand abstraction has great potential for freshwater augmentation in agriculture in areas where runoff is stored in surfacial aquifers such as “dry” river beds.
Operation and Maintenance
Adequately designed pumping units are essential for the successful operation of this technology. Care must be taken to balance the yield of the sand aquifer and the pumping rate, and also spacing of abstraction points, to minimize excessive drawdowns.
Maintenance involves ensuring that well points do not collapse. This is achieved by casing the well. Further, the screening at the well-point intake has to be good to minimise the intake of sand grains which can damage pump impellers and shorten the life of pumping plant.
Level of Involvement
For large-scale operations, the technology has been limited to applications in the governmental and private sectors. For small-scale operations, local communities may be involved.
For large-scale schemes, there is a relatively high capital cost in the form of pumps and pumping accessories, and relatively moderate operating costs, primarily related to the need for fuel for the pumping equipment.
Effectiveness of the Technology
The technology is quite effective as has been proven by its application in Zimbabwe. As an example, the sand abstraction units at Chisumbanje (ARDA) are capable of delivering over 340 l/s, which is adequate for irrigation of up to 300 ha of summer cotton and winter wheat. The volume of water available is a function of the depth of sand in the river bed. A general guideline, based on the ARDA experience, is that 20% of the volume of sand is water.
This technology is suitable for sandy river beds that are usually seasonally dry.
This technology may permit maintenance of ground cover vegetation during drought periods. However, over abstraction may reduce downstream flows.
Use of this technology has the advantages of:
• Lower financial requirements compared to surface water developments (e.g., dams)
• Providing a reliable source of water even during the dry season
• Requiring management and operational skills not much different from those required by borehole systems
• Being applicable over a range of operations from small scale- to large-scales.
The technology has the disadvantages of having:
• Relatively high capital costs related to the purchase of hardware
• The potential for considerably shortened equipment lifespans, primarily arising from the effect of sand on the pumps.
Traditionally, sand abstraction has always been practised in the whole region, albeit on a small scale, for human supply and livestock watering. Therefore, it has a high degree of acceptability.
Further Development of the Technology
A study for relating to the various productivity parameters (e.g., drawdown and well spacing, yield, casing and sand depths, sustainable pumping rates, etc.) is required before this technology is widely used on a large-scale. In Zimbabwe, research to answer some of these questions is already underway.
The hardware requirements necessary to implement this technology are not a problem in the region.
Agricultural and Rural Development Authority, Box CY 1420, Causeway, Harare, Zimbabwe.
Department of Geology, University of Zimbabwe, Box MP 167, Mt Pleasant, Zimbabwe.
Departments of Water Resources Development (Botswana, South Africa, Namibia and Swaziland).