Download rainwater harvesting by underground tanks for subsistence farming

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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
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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).
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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
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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.
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Fig. 2
Design of Dome Cap Tank
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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.
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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
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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:
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? 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: -
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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.
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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,
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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.