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Water Harvesting

Managing the Process

A Story of Change

Typical Projects

Practical Technology

 

The average per capita income in Kitui is less than €2 per day
 
 
 
 

Rainfall harvesting is the term given to the conservation and storage of rainfall on outcrops and sheets of bare rock. The term can be extended to include sand dams formed in the beds of seasonal rivers. In many parts of the tropical pastoral regions there are rock outcrops which stand out prominently from the surrounding plains and receive additional precipitation from rainfall, dew and mist.  By encircling such rocks with a low masonry wall the runoff from the rock catchment can be guided into a natural storage area, blocked off with a concrete wall. Many rock catchments and storage tanks have been successfully constructed in Kenya as part of the surface water conservation programme of the Ministry of Agriculture. Even in the arid zone with an annual rainfall of less than 250 mm more water runs off the rock during a short storm of shower than can be effectively and economically trapped and stored.  

Simple Technologies for Water Harvesting and Conservation

Acknowledgements: This summary draws on material from several sources freely available on the Internet, particularly Research Report RR 6. The water resource in tropical Africa and its exploitation, by the International Livestock Centre for Africa, PO Box 5689, Addis Ababa, Ethiopia.

It is often said that the amount of rainfall in the arid and semi-arad lands (ARALs) is not the problem; the issue is in the distribution of that rain fall, in space and time. Kitui diocese has annual rainfall in the range 250mm-800mm, not hugely different from the eastern half of Ireland. However, unlike Ireland, the rainfall occurs in intense bursts of short duration, and at specific seasons. Thus for the Kitui district, the short rains are expected in November, and the long rains are expected during March and April. 

Some areas of Kenya have plentiful water supplies: however it is of no help to know that nearby humid areas have plentiful water if the costs of distributing that water far outweigh the economic returns. It is technically feasible to pump water from lakes and distribute it by pipelines to almost any pastoral area. Saline water can be desalinated and rivers impounded to provide supplementary water supplies. These methods, however, are extremely high-cost solutions and cannot usually be justified.
The simplest form of water exploitation is to store at the river bank or lake shore. All other forms of water exploitation require the use of technology, hand or machine powered, which must be appropriate to the local conditions. For the Kitui diocese, the only practical sources of water are from boreholes (expensive and far apart), shallow wells (require groundwater supplies, in turn recharged by rainfall), or from sand dams or rock catchments. Because of the long intervals between the rainy seasons, storage reservoirs with open surfaces are of limited benefit, due to the very high evaporation rates.  

Rock Catchments

Small dams

Perhaps the most common form of water conservation structure is a small earth dam across a water channel which may have permanent or seasonal flow, and which can be constructed without professional engineering advice. 

A narrow, steep-sided valley allows the design of a short dam wall of considerable depth and a small surface area of open water which will be less susceptible to evaporation losses. A wide flat valley calls for a long wall, which may be costly, and which will result in a large surface area of open water and high evaporation.  

Sub-Surface dams

How the Sub-Surface dam is constructed

Another method of evaporation control is to use sand rivers as storage reservoirs. This technique has been used successfully for some years in East Africa where the reservoirs are known as subsurface dams. The technique depends for its effectiveness on the availability of an extensive bed of coarse sand, and many such deposits can be found in the Kitui diocese. By constructing a low weir or wall at a convenient rock bar, coarse particles can be encouraged to settle upstream of the weir. If the height of the weir is raised after successive floods, fine particles tend to be carried over the weir and a deep bed of coarse sand can build up behind the wall. This is known as “phase lifting” of the dam and is usually done one or two years after the initial construction. 

How a sand-dam is constructed

 

A shallow well can be sunk into the sand bed about 100 metres or so from the wall of the dam, and with suitable sand filtering can provide potable grade water for human consumption.  With careful construction, such subsurface dams can function without trouble for many years. The storage capacity of the reservoir is determined by the voids or spaces between the coarse sand particles, and typically amounts to  30 and 50% of the gross reservoir volume. Well chosen sites can drain quite considerable stretches of river, albeit slowly, and this method can sustain small livestock populations throughout the dry season. No particular skill is required in constructing the low weir, and pipes can be laid to cattle troughs, incorporating valves or taps to control wastage.  
The cost of constructing a sand dam, or sub-surface dam, is approximately €4,000- as measured by the external funding contribution required by the community. Such a dam can provide water for a community of 200 to 300 people, including their livestock. The system which has evolved for constructing these dams is a co-operation between a community group and a donor organization.    The community group makes an application to the for support, citing the number of participants in the project, and committing to providing local labour and materials. The community contribution is typically 60% of the total project cost, as measured by the value of the labour and materials they provide. The subsequent “phase lifting” of the dam, described above, will cost a similar amount of €4,000, and increases capacity further.  

A SASOL sand dam in Kitui

Shallow Wells

In addition to sub-surface dams, shallow wells are another suitable source of water for much of the Kitui diocese. If the water level is less than 10 m from the surface, it will be relatively cheap and quick to dig a well by hand. Such wells are known to have been dug to great depths, up to 120 m. Because of the cost and scarcity of mechanical equipment for digging wells, hand-dug wells will continue to be constructed to considerable depths, even though they may not represent the most economical option. 

Hand-dug wells are one of the oldest means of water supply. Begun as simple water holes in sand rivers, the concept of finding water by digging in riparian areas has spread away from the river course itself and, particularly in West Africa, deep hand-dug wells, reaching up to 100 m in depth, are used to tap deeper shallow aquifers and areas of basal seepage around rock outcrops and escarpments. One advantage of a dug well over a costly borehole is that community participation is assured from the start.
 Self-help labour is usually used to dig the well, and women and children can all help with the fetching and carrying of sand and gravel. A rural community thus identifies itself with the construction of the dug well, and this sense of communal ownership is vital if the water point is to continue to function. Open wells afford very little protection against pollution, even when low parapets are constructed to prevent the ingress of surface water.  Any water-lifting techniques involving the introduction into the well of ropes and buckets which are handled or have been exposed to contamination will create a possible source of faecal pollution. More sophisticated lifting devices, such as hand pumps, will require maintenance of one sort or another. If no maintenance facilities can be provided, open wells will be a better option. 

If, on the other hand, the hand pump is suitable for village or community level maintenance, and the frequency of use of the water point is high, it is preferable to aim for a fully protected well. For this reason, well digging should take place at the end of the dry season when static water levels are at their lowest.

In nomadic societies, water is still being raised from wells by leather buckets and ropes, or by a team of several men standing one above the other and passing the water upwards in small containers. 

 

To anyone who has observed this operation used to water a large herd of cattle the hourly output is impressive, and is witness to the energy which men are prepared to expend to safeguard their herds. Such traditionally developed methods, however primitive, are adapted to the situation and are an appropriate technology, if not always the most efficient. In contrast the reluctance of these same people, who are willing to expend great manual effort, to contribute money for the operation and maintenance of modern mechanical equipment is manifest in many instances, even if the money is there and the service is generally appreciated.  

Appropriate Technology for the Area

The appropriateness of any mechanical device is determined not only by its reliability and simplicity in operation, but also by its acceptance by the users. For example, it has been the East African experience in pastoral areas that attempts to replace the bucket and rope, or chain of men, with a hand pump, which allows the top of the well to be sealed and the water kept free from pollution, have not been successful because it was claimed by the users that the hand pump was more energy consuming and had a lower output. Hand pumps fitted on wells were neglected, frequently vandalised or abused to the point of breaking the surface equipment. They were soon discredited and abandoned.   It has certainly been the East African experience that equipment bought from the developed world by the State or by private individuals, or presented by well-meaning donors, without insisting that the manufacturer at the same time establishes a spares holding and servicing agency, has soon failed. Borehole and other installations have been known to stand idle for months or even years, awaiting the arrival of a small but essential spare part. Many have, in the end, been replaced by other plants supplied by locally established agents, in whose interest it is to ensure that their reputation, and hence their business turnover, is maintained, and that their products and their back-up organisation give good service.