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XI IRCSA CONFERENCE – PROCEEDINGS RAINWATER HARVESTING BY UNDERGROUND TANKS FOR SUBSISTENCE FARMING Mesfin Shenkut 1. Introduction Ethiopia is located in the Horn of Africa. Its geophysical set up is dominated by highland complex of mountains, plateaus, and rugged terrain with a surface area of 1.1 million square kilometer. It has a scattered settlement pattern with population of 65 million. The annual rainfall varies from 600 mm in the eastern borders to 2000 in the southwest highlands. There are also a few areas in the northeastern Rift Valley that receive less than 200 mm. The estimated yearly average rainfall is 744 mm. Agriculture, which employs 85% of the population and make up 90% of the GDP is based on rain fed. The agricultural sector is facing sever failures due to inadequate rainfall management and recurrent drought. The yearly increase in agricultural productivity is 1% while the population growth is about 3%; the Country is therefore living on persistent international food aid. Currently Ethiopia is experiencing one of the worst food shortage in recent history with estimated 15 million people are affected by food shortage. In ordered to avert this harsh situation the government of Ethiopia is promoting a huge rainwater collection and management program together with integrated livestock and crop production. The development of over 100,000 rainwater harvesting underground water tanks of sizes 50 – 60 m 3 is the major components of the program. The paper presents a highlight of the most popular rainwater harvesting techniques for agriculture and discuses the technology, advantages and limitations of the underground tanks, which are now being promoted in mass in most parts of the countrty. 2 Popular Practices in Rainwater Harvesting Following the recurrent drought and associated food shortages, rainwater-harvesting technique is gaining a national attention to provide farmers with steady water source to XI IRCSA CONFERENCE – PROCEEDINGS improve agricultural productivity and the standard of living of the people in moisture stressed areas of Ethiopia. The following are the most popular methods of rainwater harvesting practises in the country: 2.1 Construction of stone/earth bunds This method deals with the laying of stones or earth along contours to slow down runoff and to increase infiltration into the soil. The main intention here is to conserve the soil and the water. 2.2 Runoff farming: Runoff farming involves the collection of direct runoff within the farm catchment system and gravitating it to adjacent lower elevation of the farmland. It involves a labor-intensive manual spreading of the run-off in the farm with precaution to avoid soil erosion. Micro basins semi circular bunds etc. are some of the examples of these technique. 2.3 Flood Diversion/Spate Irrigation During rains of high intensity on up-hills for short period of time, huge magnitude of flood is generated and flows along dry river basins, gullies and channels in the lowlands. The flood is diverted by making barriers across the runoff routes. The diverted flood is then spread into farmlands located along the banks using temporary structures, (see fig 1). XI IRCSA CONFERENCE – PROCEEDINGS Fig. 1 Flood Diversion 3 Underground Tanks Underground tanks adopted from China are now being promoted in the country. These tanks have proved to be useful in China where water is collected from concrete or stone masonry platforms, stored in two tanks, one for drinking and the other for agricultural purpose with a well managed water utilization system. In Ethiopian context it is usually one tank that is provided, and the collection is from the surrounding untreated ground surface hence the water is mostly turbid. Besides, the water abstraction mechanism, which is a rope and bucket system and the irrigation method which is direct surface application of the water on the crops, are highly labor intensive. In most cases these tanks are planned to be used in arid and semi-arid areas experiencing short rainfall and recurrent drought is occurring and for crops with short growing season, draught resistant variety and requiring small amount of water. 3.1 Types of Tanks XI IRCSA CONFERENCE – PROCEEDINGS The main types of tanks considered are bottle shapes, brick cap type, dome cap and semicircular type. .1.1 Bottle shape tank As the name indicates these tanks have a bottle shape. They are suitable in thick soil deposition with high stability and bulk density. They are not suitable for poorly structured and sandy loam soil with high moisture content. 3.1.2 Brick cap tank These tanks have the top parts of the structure built from brick. These types of tanks are: ? Suitable for thick soils deposition with relatively poor stability and sandy soil; ? Excavation is relatively easy and ? Report indicate that they have long span of life and 3.1.3 Dome Cap Tank These tanks have the top part of the structure dome shaped (see fig 2). They are ? Suitable for most types of soils including loose soil with relatively poor stability and sandy soil; ? Excavation is easy; ? Reports indicate that they have also long life span. XI IRCSA CONFERENCE – PROCEEDINGS Fig. 2 Design of Dome Cap Tank XI IRCSA CONFERENCE – PROCEEDINGS 3.2 Catchment The rainfall catchment area is the land around the tanks without any modification or treatment. The key issues that are considered in selecting suitable catchment areas are the following: ? Sufficiency of water to be collected to meet the storage requirement; ? Water collected can be easily diverted into storage tanks; ? Catchment having good gradient to gravitate the water in to the tank without causing erosion; ? Grassy and bushy areas to filter out dirt, ? Catchment selected for rainwater harvesting not affecting other production activities such as farmland and ? Areas located sufficiently away from pollution sources such as latrines. 3.3 Construction Site Selection The following aspects are considered when selecting tank sites: ? Water flow from the catchment; ? Land to be irrigated; ? Soil (depth and texture); ? Effect on other facilities such as road, buildings and ? Effect of other natural formations such as joints, landslide and gullies. 3.4 Determination of tank type Tank specification is determined according to soil, water availability, land to be irrigated and the existence of construction materials and skills to construct a particular type of tank. Though theoretically storage capacities are determined on the above criteria, a 60 m3 is mostly adopted as an optimum size. This is considering sufficiency to satisfy the need at household level for drinking, water for small ruminant livestock and some vegetable gardening. XI IRCSA CONFERENCE – PROCEEDINGS 3.5 Construction Techniques The steps followed during construction of the tanks are the following: ? Excavate the ground circularly with the diameter of the base and the depth of the dome as shown in fig 3; ? Trim the earth to the shape of the dome as shown on fig 4; ? Lay reinforcement bar as shown on fig 5; ? Concrete the dome as shown on fig 5; ? Dig out the soil entering through the manhole of the dome in line with the shape of the tank as shown on fig 6 and ? Plaster the inside of the well with selected lining material, Fig 3 Excavation – Dome Fig 4 Shaping Dome XI IRCSA CONFERENCE – PROCEEDINGS Fig 5 3.6 Casting Concrete on Dome Fig 6 Underground Excavation Lining The following are the adopted types of lining: ? Red clay, suitable for stable and homogeneous soil with high clay content; ? Cement mortar, suitable for unstable soils; ? Concrete suitable for very unstable soils and ? Stones or bricks, suitable for places impossible to excavate a tank for a given shape. 3.7 Sediment Pond In order to trap suspended and settleable matter, a small sediment pond is provided. The issues considered in constructing these units are: ? It is normally located at least 3 m away from the storage tank and ? Its size is determined according to sediment characteristics and flow discharge The sediment tanks are not mostly compatible with the turbidity load of the water. 3.8 Operation and Maintenance: The following are the main operation and maintenance activities: XI IRCSA CONFERENCE – PROCEEDINGS ? Frequently observe water level in storage tank. If water stored in the tank comes up to the designed storage level, block inlet and divert runoff to some other areas sufficiently away from the tank; ? Frequently observe every component of the system to identify possible problems; ? Sediment deposited in storage tank should be removed at least once a year ? During wet seasons, de-silt the sediment trap after each rain; ? Inspect channels, sediment pond and pipes so that they are not blocked; ? Leave some water in a depth of about 20 c.m. in the storage tank to prevent cracking of the lined layer; ? Catchment area should be cleaned from visible debris and ? Remove deep-rooted trees from storage tank area. 3.9 Cost The cost of these tanks is about 10 USD to store 1 m 3 of water. This is some 10 times cheaper compared to masonry or brick reservoirs of the same size. The cost breakdown of a 60 m3 tank constructed on strict control and excluding overhead costs is shown on Table 1. 3.10 Benefits ? Water availability primarily for drinking, livestock watering and supplementary moisture for vegetable gardening; ? Reduced cost compared to any other known water storage tanks, such as masonry, brick or concrete tanks; ? High support from the government hence encouraging users and professionals to make effort to mitigate the water shortage problem. 3.11 Problems Encountered: - XI IRCSA CONFERENCE – PROCEEDINGS 3.11.1 General Problems ? The experience of such underground storage tanks was not available in Ethiopia; hence introduction of the technology is taking time, ? Limitation on meteorological, soil, hydrology, runoff factor not available and designs are made on theoretical estimates and professional guesses; ? Low community involvement and commitment in cost sharing and availing credit for self implementation not incorporated and ? Direct implementation of the technology without pilot trials, hence the long-term impact is not known. 3.11.2 Technical ? Water extraction system is mainly bucket and rope type. Laborious and exposed to pollution; ? Water loss thorough seepage; ? Inadequacy of the quantity of water for irrigation; ? High silt deposits in the tanks sharing the space for water storage and creating difficulty for cleaning; ? The limitation of the structures to meet all types of soil formation, ? High expectation by the people and the government from this technology and concentrating most effort on it and not giving the same attention to other options. 3.12 Conclusion The recognition of the government to take up the importance of rainwater harvesting techniques is commendable. The cost of the tanks is attractive. The potential of these tanks to avail water for drinking purpose is very promising provided the catchment is from rooftop or from treated ground by concrete or similar materials. The inadequacy in size of the tanks for agricultural purpose, full fledged implementation without pilot trials, minimal participation of the sector partners such as the Ministry of Water Resources, the unbalanced expectation of the technology to cope up with the huge water conservation techniques to fight hunger are some of the critical weaknesses that need due consideration. It is therefore imperative to look into solving the shortcomings of the program and building on the achievements. XI IRCSA CONFERENCE – PROCEEDINGS Table 1 3 Description Cost of the concrete dome tank (60m3) Unit Quantity Unit Total Price Price (Birr) 8.6 Birr = 1USD 1. Materials Cement Kg 1950 0.58 1131.00 Sand m3 5 36.00 180.00 Bricks No 623 0.65 404.95 3 Gravel m 2.5 64.00 160.00 Concrete pipe No 9 16.00 144.00 Steel bar (6mm) kg 45 3.85 173.25 Steel bar (12mm) kg 11.04 3.80 42.00 Wire (4mm) kg 25 11.00 275.00 Block wire kg 2 10.00 20.00 Metal sheet (cover) m2 0.72 140.00 100.80 Wire mesh (sieve) m2 0.33 12.00 4.00 Water It 5000 0.01 50.00 Total 2685.00 2. Labor Excavation pd 70 10.00 700.00 Masons pd 10 35.00 350.00 Assistant masons. pd 12 10.00 120.00 Total 1170.00 Grand Total 3385.00 If farmers contribute (Birr) External Support (Birr) 180.00 404.95 160.00 100.80 50.00 895.75 1131.00 `73.25 42.00 275.00 20.00 4.00 1789.25 700.00 120 820.00 1715.75 350.00 350.00 1669.25 Notes: ? 1 US Dollar = 8.6 Birr ? The above cost does not include overhead cost, a safe estimate would be an increase by 50%. . References: 1 ALEM Getachew, 1999; Rainwater Harvesting in Ethiopia, an overview, Proceeding of 25th WEDC Conference, Addis Ababa August 1999. 2 Shenkut Mesfin, Desta Lakew and Waktola Tsedale, 2000, Rainwater Harvesting in Ethiopia - Status, Potential and Problem, Nairobi Kenya, XI IRCSA CONFERENCE – PROCEEDINGS 3 Mesele Solomon, Desta Lakew and Fanta Birhanu, (unpublished) Collected data and photos on Underground Tanks Development Project of the Ministry of Agriculture of Ethiopia.