Design of Jigessa Small Scale Irrigation in Dara Woreda, Southern Ethiopia ()
Subject Areas: Environmental Sciences, Hydrology
1. Introduction
Water is the greatest resource of humanity. It not only helps in survival but also helps in making life comfortable and luxurious, beside various other uses of water, the largest use of water in the world for irrigation lands [1] .
Based upon the various river basin master plans and land and water resources surveys, the aggregate irrigation potentials of Ethiopia have been estimated to be 2,523,000 million hectares net and about 3.7 million hectare gross [2] .
A large number of stream flows is available for utilization in irrigation, or any other use. With increasing demand for food production and maintaining agricultural sustainability, it is necessary to develop an integrated approach on land and water resource utilization. The availability of good agricultural land with a nearby flowing stream can enhance the utilization of the stream water by diverting the flow in to the canal system [3] .
Jigessa small scale irrigation project is found in SNNP Regional Government Sidama Zone Dara special woreda. The project site is located at 2 KM away from Tefrikella having a total distance of 10 km up to the head work site. The project is intended to irrigate about 100 ha land.
2. Statement of the Problem
On this project area there are some problems which are not tackled by the local farmers. Some visible problems observed in this woreda are:
Þ Food insecurity in the area.
Þ Backward tradition of agricultural practices, for the reason that they are dependent on yearly rainfall.
Þ They cannot afford modern irrigation system because of their economic problems.
Þ Rainfall in the study area is erratic, uneven and insufficient in its distribution and amount.
Why this problem exists their?
In adequate traditional and modern small scale irrigation schemes, this is due to
Þ Insufficient attention of irrigation institutions to the area.
Þ Lack of enough knowledge about importance and operation of modern irrigation.
Þ Economical problem of the farmer.
3. Literature Review
Irrigation is practically a science of planning and designing a water supply system for the agricultural land to protect the crops from bad effect of draught or low rainfall [4] .
A diversion head work (weir) is a structure constructed across a river for a purpose of raising water level in river so that it can be diverted in to off taking canals. Diversion head works are generally constructed on perennial rivers which have adequate flow throughout the year and therefore, there is no necessity of creating a storage reservoir [5] .
According to MoWR, the following criteria are used for classification of irrigation projects in Ethiopia: Small Scale under 200 ha, Medium Scale 200 to 3000 ha and Large Scale over 3000 ha [6] .
Diversion structure is a collection term for all (weir or diversion dam, head regulators, upstream and downstream river training work) at intakes of main or principal canal to divert and control river flow and to regulate water supplies in to the main canal or canal. A well-designed distribution system consisting of network of canal is required for caring water from the canal head works to the field [7] .
Across drainage work is a structure which is constructed at the crossing a canal and a natural drain, so as dispose drainage water without interrupting continues canal supplies. It includes aqueduct, siphon aqueduct, super passage, level crossing, etc. [8] .
4. Objectives
4.1. General Objective
To ensure food security of the society in Dara Woreda by supporting the rain fed agriculture with small-scale irrigation.
4.2. Specific Objective
ü To introduce modern irrigation system.
ü To analyze the agronomic and hydrology condition of the area.
ü To design the diversion head work structure and canal.
5. Description of the Study Area
Dara Woreda is located 120 Km from Hawassa, Ethiopia. Jigessa small scale irrigation project is found in SNNP regional government sidama zone Dara special Woreda, Particularly the command area is located in woinadega agro ecological zone with mean elevation 1860 m above sea level. Farmers undertake mixed farming system.
According to the 1:50,000 topo-map interpretations, the estimated area of watershed is 72 km2. The catchment condition characterized by mountainous ride steep gorge land scape and the river is meandering and tributary to Lake Abaya.
6. Material
Most of the task to be carried out in this project is the design part or the office work which is to be done using different software and by referring different irrigation engineering books.
7. Methodologies
The design and analysis of Jigessa small scale irrigation project shall be carried out using formula, principles and theories which have been commonly used in irrigation system design.
7.1. Irrigation Agronomy
The crop types those are irrigated by the project area are Maize, Sorghum, coffee, mango, Onion, Sweet potato and Tomato. The crop coefficient (Kc) value of the above crop type is given by standard table. From this table we are going to take the Kc value at middle stage which needs much amount of water.
Determination of ETo: ETo can be determined using different empirical method in accordance with the available data.
Reference Crop Evapotranspiration (ETo): We use Blaney-criddle formula to determine ETo. Because it is suitable for calculating our data we got from the meteorology station.
(1)
where, ETo = reference crop evapotranspiration (mm/day).
P = mean daily percentage of annual day time hours.
To determine P, it is essential to know latitude of the area and read from the table. Teferikela is situated on latitude of 6.30 N. But in the table there is no exact value for 6.30 N. Therefore, to estimate the value of P for 6.30 N, we interpolate b/n 5 and 10.
Dependable Rainfall: We calculated the probability of the rainfall using the following formula.
(2)
where, P = Probability (%), R = Rank and N = Number of years.
The rainfall for our irrigation design should have a probability of 80%. But since there is no exact value of 80%, we get the value by interpolation.
Effective Rainfall: Effective rainfall can be computed using the following formula.
(3)
Crop Coefficient (Kc): Different crops have different Kc values at different stage. Among these crops we took the largest Kc value. I.e. Kc = 1.2. Thus not to make other crops stressed.
Actual Crop Evapotranspiration (ETc): ETc can be calculated by the formula:
(4)
NET Irrigation Requirement (NIR)
(5)
Duty: We take the maximum value of GIR to compute the duty.
(6)
Design Discharge
Now, we are going to irrigate a command area of 100ha of the Jigessa kebele by considering the downstream users of the river.
The demand discharge can be computed by the following formula.
(7)
The base flow of the river is 250 lit/sec, which means it can satisfy the project’s water requirement at any time of the season.
7.2. Hydrology Analyses
Since “Jigessa” river is not gagged, it is not possible to determine the peak flood from the gauged varieties of hydrological phenomena.
7.2.1. Estimation of Design Rain Fall
The maximum flood is determined in order to design the diversion head work capable of withstanding any disaster caused by the peak flood which may come in 50 years return period.
(29)
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Figure 4. Cross-section of unlined canal.
For economic cross section, R = D/2
, 
, 
and 
By using manning’s formula
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Adding freeboard 0.2 m the full supply depth of the canal will be,
D = 0.7 m, A = 0.44 m2, P = 1.80 m and R = 0.243 m.
By using manning’s formula for not silting and scouring the canal velocity should be between (0.3 and 0.6) m/sec. ![]()
8. Conclusions
The design analysis of Jigessa small scale irrigation project is generally carried out by integrating different disciplines like: irrigation agronomy, soil and water conservation engineering, surface water hydrology, survey, etc.
The structure we design in this project will resist the 50 year return period peak flood. As a result of this, the structure will be stable, safe and effective in terms of delivering adequate amount of water to the beneficiaries through a conveyance system (canal) without clogging by silting. So there will not be water scarcity and moisture stresses.
Therefore, this project has been proposed mainly to overcome food insecurity in the Woreda and to produce throughout the year using irrigation.
Generally, the project will be effective if and only if the command area and the water are managed and used appropriately.
9. Recommendation
In general, based on the problems we encounter and for sustainability of this project, we recommend the following:
Ø As we know that most irrigation projects need data from meteorology station for effective and reliable design. But we could not get such type of data because in our project area there was no meteorology station and/or the nearby station could not give us available information. In order to reduce the problem, the government should spread wide metrology stations on local areas to give efficient and valuable data for the implementation of an irrigation scheme and other purposes. .
Ø Different conservation practice should also be applied after construction for the sustainability the structures.
Ø Development without participation beneficiaries is like a house without foundation. The community should participate during the construction phase.
Ø Even though farmers of the project area have an experience in crop production through irrigation, encouraging them to have adequate experience on operation, maintenance and managements of modern irrigation system is obligatory for the success of the project.
Acknowledgements
We would like to thank heartily and deeply our advisor Mr. Mihret. D. (Msc.) for his valuable advice, constructive comment and giving much of his time for us. Our deepest thanks also go to our coordinator Mr. Desalegn. J. (Msc.) for his encouragement. And special thanks for all our friends for their idea sharing and help.
Appendix
Graph from the discharge curve, the design flood equivalent to the flood mark (3.24) was 104.88 m3/sec. Here, the flood mark is equivalent to the Tail Water Depth (TWD).
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