UNDERGROUND RAINWATER STORAGE FACILITIES
Introduction
Rainwater storage tanks are more important in semi arid areas than any other areas because they are useful in storing limited water supplies for agricultural purposes. In semi-arid regions, where precipitation is low or infrequent during the dry season, it is necessary to store the maximum amount of rainwater during the wet season for use at a later time, especially for agricultural supply. One of the methods frequently used is the underground storage t anks. The in situ technology consists of making storage available in areas where the water is going to be utilised. All rainfall-harvesting systems have three components: a collection area, a conveyance system, and a storage area. In this application, collection and storage is provided within the landscape.
Tank design criteria
Consider five main factors when designing your underground tank. 1. Seepage
It is very important to store rainwater and not to lose it! The tank should have a durable,
watertight, opaque exterior and a clean, smooth interior. A common method used in rural areas is puddling as shown in picture 2. Below ground tanks must also be plastered well and correctly installed, otherwise they can collapse. 2. Evaporation All storage tanks should have a roof made from locally available materials as indicated in the material list. A tight fitting top cover prevents evaporation, mosquito breeding and keeps insects, rodents, birds and children out of the tank. A suitable overflow outlet(s) and access for cleaning are also important. 3. Length of the dry spell This is important in determining the size and number of tanks to be constructed. The longer the dry spells the more water you would need. Sometimes sophisticated calculations are involved, but these tend not to take into account human behaviour and the willingness to use water if it is available and not to conserve it for future use, in the hope that the dry spell will soon be over. 4. Daily usage This is related to the above factor but ensure that t he crops are hardened to withstand some stress during the dry spell. Determine how much area you would want to save in case of a very long dry spell lasting more than three weeks.
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Underground rainwater harvesting
Practical Action
5. Construction costs To save costs use as many available local materials as possible as shown below. Work in groups to share labour, as this is a labour intensive system. It takes approximately fours for ten people to construct a 10 000litre tank.
Materials required
1 rammer locally made from dry wood, Ant-hill soil (termitaria), water, 5 x 1,5-2m long dry poles, Pick, shovel (depending on labour available), 10 farm/earth bricks, Tape measure or ruler, string 2m, knife/hacksaw/small axe, 50 wooden pegs, old Hessian or plastic bags. 25kg cement.
Constructing the underground tan k
1. Along any dug dead level contour (which is your collection area) choose where you can construct your storage tank. All soils that allow rapid percolation the closer should be the storage tanks. This because your soil quickly loses water and that’s where your plants are likely to show signs of moisture stress during the dry spell. 2. Measure along the channel the length of the tank you want and mark using pegs. Dig the marked area up to a further depth of 500cm or more. The deeper it is the more water will be harvested.
Picture 1: Foundation of the underground tank ©Practical
Action/Douglas Gumbo
3. Mark with a shovel the root zone depth within your tank. 4. Place termitaria-cement mixture at the base of the tank and sprinkle water to facilitating easy ramming. 5. Using homemade rammer, start ramming up to a maximum depth of 2 -5cm. Sprinkle whenever required. 6. All the tanks must have a non-permeable layer therefore a thicker layer may be required to avoid seepage and deep percolation. Picture 2 below the process of ramming. 7. Mix termitaria with water to make mortar and plaster up to the marked root zone level of your deep-rooted crops as shown in Picture 3. The table below indicates the recommended options for your tank lining when plastering the walls.
Material use Cement-anthill/termitaria Cement-clay soil Cement-sand soil
Cement –soil ratio 1:8 1:8 1:3
Thickness (mm) 50 60 50
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Covering the top of the underground tank 1. Using earth bricks mark the inlet and outlets of your tank from both ends of your ta nk. 2. Lay your wooden poles across your tank spaced at 2m.Ensure that the poles are not totally fixed for ease removal during maintenance usually during the dry season.
Root zone depth
Picture 2: Preparing to ram the bottom of the underground tank ©Practical Action/Douglas Gumbo 3. Decide where you can place two openings at the top for collecting a nd checking water inside your tank. 4. Lay old plastic or hessian bags onto the poles. 5. Use same mortar as referred above to plaster the roof of your tank.
Picture 3: Tank lining/ Plastering
©Practical Action/Douglas Gumbo
Water conveyance devices
Fittings are usually built into the tank during construction. These include: one or more outlets for water collection; a drainage tap (or wash-out) at the bottom of the tank, to be used when cleaning; an inlet pipe; and an overflow pipe. This must be screened to prevent insect entry.
Some advantages of using underground rainwater storage tanks
Rainwater is generally much lower in salinity than other water. This is good for plants in the garden and fields.
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Underground rainwater harvesting
Practical Action
Once the tank is installed, rainwater is cheaper to collect than water conveyed from expensive storage facilities such as dams, which are usually located far away from the fields. Using rainwater tanks regularly may reduce storm water run -off from your field. This may reduce flooding, erosion and environmental damage to streams. Rainwater tanks provide a secondary backup water supply to your crops should there be a failure or erratic rains or long dry spells.
Some disadvantages of using underground rainwater tanks
It is difficult to detect the leaks and take corrective measures in case of under ground tanks. Water from under ground tanks cannot be drawn by gravity. Some kind of manual or power lifting devices need to be used for drawing the water. Further, in coastal areas, under ground tanks are prone to water contamination due to fluctuation in groundwater table and leakage of stored water. A rainwater tank may be costly to purchase and install and will require some maintenance. If rainwater from the tank is supplied using a pump then this will need to be maintained and occasionally repaired or replaced. If a small tank, less than 10 000 litres is used to store rainwater, it will probably empty frequently, particularly during summer. (However, it is possible to arrange your system so that you have rainwater available for drinking all year round.)
Rainwater tank maintenance
Ongoing planned maintenance will maintain your rainwater quality and extend the life of your system. Maintenance should include the following: Regularly check the tank to ensure there are no unscreened or damaged openings that allow insects, rodents or animals to get into the tank. Check gutters and strainers devices at least twice before and after the rainy season. Keep them clean and free of leaves and debris. Ensure the tank lid is tight. Check the tank every two years for sludge and have the tank cleaned if there is a thick layer of sludge on the bottom of the tank.
Further information
Lang, J., Allen, M and van der Wel, B. (2002). Good Alternative Rainwater Tanks: Fact or Fiction? 2002 Water Symposium Proceedings. Office of Sustainability, Department for Environment and Heritage. Nissen-Petersen, E., (1992) How to Build an Underground Tank with Dome, ASAC Consultants Ltd., Kitui, Kenya, 1992. Runoff Rainwater Harvesting Practical Action Technical Brief The Sri Lankan ‘Pumpkin’ Tank ~ Case Study Practical Action Technical Brief The Underground Brick Dome Water Tank ~ Case Study Practical Action Technical Brief Cement Mortar Jar ~ Case Study Practical Action Technical Brief Rainwater Catchment Systems for Domestic Supply, by John Gould and Erik NissenPetersen, Practical Action Publishing 1999. Ferrocement Water tanks and their Construction, S. B. Watt. Practical Action Publishing 1978 Rainwater Harvesting: The Collection of Rainfall and Runoff in Rural Areas , Arnold Pacey and Adrian Cullis Practical Action Publishing 1986 Water Harvesting – A Guide for Planners and Project Managers, Lee, Michael D. and Visscher, Jan Teun, IRC International Water and Sanitation Centre, 1992 Water Harvesting in five African Countries, Lee, Michael D. and Visscher, Jan Teun, IRC / UNICEF, 1990. As snapshot of the status of RWH in five African countries. Waterlines Journal Vol. 18, No 3, January 2000 and Vol. 14, No.2, October 1995 Both issues are dedicated to rainwater harvesting, available through Practical Action Publishing
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Practical Action
Photo-manuals by Eric Nissen-Petersen. A range of manuals on how to build a number of tank types including: cylindrical water tanks with dome, an underground tank, smaller water tanks and jars, installation gutters and splash-guards, available from the author at: P.O. Box 38, Kibwezi, Kenya. Rainwater Catchment Systems – Reflections and Prospects, John Gould, Waterlines Vol.18 No. 3, January 2000. Domestic Water Supply Using Rainwater Harvesting, by T.H.Thomas, Director of the Development Technology Unit (DTU), University of Warwick. The article is available on DTU’s Website Waterlines back issues containing rainwater harvesting articles: Vols 17(3), 16(4), 15(3), 14(2), 11(4), 8(3), 7(4), 5(4), 5(3), 4(4), 4(3), 3(3), 3(2), 3(1), 2(4), 2(1), 1(1).
Video
Mvua ni Maji – Rain is Water, Rainwater Harvesting by Women’s Groups in Kenya, FAKT,
1996. 27 min VHS/PAL. A Kenyan film team documented this success story on the occasion of the visit of a delegation of Ugandan women who came to learn the skills of rainwater harvesting from their Kenyan sisters. Available through FAKT A Gift from the Sky – An Overview of Roofwater Harvesting in Sri Lanka. Available from the Lanka Rainwater Harvesting Forum Construction of Water Tanks for Rainwater Harvesting – a video manual prepared by Eric Nissen-Petersen (see above). Rock Catchments. Several designs of rock catchment system looked at in detail by Erik Niseen-Petersen. Author: Douglas Gumbo Practical Action Southern Africa 4 Ludlow Raod (off Enterprise Road) Newlands Harare Zimbabwe T +263 4 776 107 / 776 631 F +263 4 788 157 E practicalaction-zimbabwe@practicalaction.org.zw
Practical Action The Schumacher Centre Bourton-on-Dunsmore Rugby, Warwickshire, CV23 9QZ United Kingdom Tel: +44 (0)1926 634400 Fax: +44 (0)1926 634401 E-mail: inforserv@practicalaction.org.uk W ebsite: http://practicalaction.org/practicalanswers/
Practical Action is a development charity with a difference. We know the simplest ideas can have the most profound, life-changing effect on poor people across the world. For over 40 years, we have been working closely with some of the world’s poorest people - using simple technology to fight poverty and transform their lives for the better. We currently work in 15 countries in Africa, South Asia and Latin America.
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