Irrigation Engineering Chapter 1: introduction

1, INTRODUCTION
1.1 DEFINITION AND SCOPE OF IRRIGATION
Definition: Irrigation is the science of artificial application of water to the land, in accordance with the
crop requirements throughout the crop period for full nourishment of the crops. Irrigation Engineering is
the Engineering of controlling and harnessing the various natural sources of water, by construction of
dams & reservoirs, canals & head works and finally distributing the water to agricultural fields.
Water is normally supplied to the plants by nature through the rains. However, the total rainfall in a
particular area may be either insufficient, or ill-timed. In order to get the maximum yield it is essential to
supply the optimum quantity of water and to maintain correct timing of water. This is possible only
through systematic irrigation system by collecting water during the periods of excess rainfall and
releasing it to the crops as when it is needed. Generally the following are some of the factors that
necessitate irrigation.
o Inadequate rainfall
o Uneven distribution of Rainfall
o Increasing the yield of the crops
o Growing a number of crops
o Insuring against drought.
o Growing perennial crops.
Scope of Irrigation Engineering
Irrigation Engineering is not only confined to the application of water to the land for raising crops. It
includes all aspects and problems extending from the watershed to the agricultural fields. It deals with
hydrology, river engineering, design and construction of dams, weirs, canals and various other hydraulic
and irrigation structures. It also deals with surface and sub surface drainage system, soil reclamation,
water-soil-crop relationships. Other allied sciences such as flood control, hydropower, and inland
navigation are also studied in IRRIGATION ENGINEERING.
Aspects of Irrigation Engineering are:
1. Water resources and hydrology aspect – to locate various water sources and to study the
hydrology of the region. This includes study of meteorology, precipitation, stream flow, floods,
river engineering, reservoirs and flood control. The following information are required while
designing various irrigation structures.
 The quantity of water that will be available at a reservoir site for storage.
 Maximum discharge at a river site.
 Reservoir capacity that ensures adequate Quantity of water for various purposes.
 Quantity of ground water which can be economically exploited.
2. Engineering Aspect - involves the development of a source of water for irrigation and
construction of various irrigation structures.
 Dams and water power Engineering
 Diversion and Distribution structures
 Minor irrigation schemes (well, Tank / Pond, inundation Irrigation).
3. Agricultural aspect – Involves irrigation practice and the study of agricultural characteristics of
the land.
4. Management Aspect- deals with successful implementation and efficient management of
engineering aspects and agricultural works.
1.2BENEFITS AND ILL- EFFECTS OF IRRIGATION
There are various direct and indirect advantages of irrigation.

1
Lecture note
Irrigation Engineering Chapter 1: introduction

- Increase in food production: Irrigation helps in increasing crop yields through controlled and
timely supply of water to the crop.
- Optimum benefits: optimal utilization of water is made possible by irrigation. Optimum
utilization implies obtaining maximum crop yield with any amount of water. In other words, yield
will be smaller for any quantity lesser than or in excess of optimum quantity.
- Elimination of mixed cropping in areas where irrigation is not ensured, generally mixed
cropping is adapted. Mixed cropping is growing two or more crops simultaneously in the same
field. If the weather condition is not suitable to one of the crops it may be suitable for the other;
and thus at least some yield is obtained. Mixed cropping can be adopted when irrigation facilities
are not available, but if irrigation is assured it can be eliminated. Mixed cropping is generally not
acceptable, because different crops require different types of field preparations and different types
of manures, amount of water etc.
- General prosperity: Revenue returns are sometimes quite high and help in all round
development of the country.
- Generation of hydroelectric power: cheaper power generation can be obtained on objects
primarily designed for irrigation alone. Also falls on irrigation channels can be utilized to
generate electricity which may help in industrializing the rural area and so in solving the problem
of fuel shortage.
- Domestic water supply:- irrigation helps in augmenting the town water supply where water is
available with great difficulty. It also provides water for swimming bathing, cattle drinking etc.
- Facilities of communication: Irrigation channels are generally provided with embankments and
inspection roads. These inspection paths provide a good road way to the villagers for walking,
cycling or even motoring.
- In land navigation
Ill-effects of irrigation
Ill-effects of irrigation occur only when the scheme is not properly designed and implemented. Most of
these are due to excess irrigation water application. Some of the common ill-effects are
1. Water logging: when cultivators apply more water than actually required by the crops, excess
water percolates in to the ground and raises the water table. Water logging occurs when the water
table reaches near the root zones of the crops. The soil pores become fully saturated and the
normal circulation of air in the root zones of the crop is stopped and the growth of the crops is
decreased. Thus crop yield considerably reduces. When the water table reaches the ground
surface, the land becomes saline.
2. Long term application of pesticides under large scale irrigation system might have a negative
influence on soil microbial activities, on the quality of surface and sub-surface water resources
and the survival of the surrounding vegetation. Irrigation may contribute in various ways to the
problem of pollution. One of these is the seepage in to the ground of the nitrates that has been
applied to the soil as fertilizer. Sometimes up to 50% of the nitrates applied to the soil sink in to
the underground reservoir. The underground water thus get polluted.
3. Irrigation may result in colder and damper climate causing outbreak of disease like malaria.
4. Irrigation is complex and expensive in itself. Sometimes cheaper water is to be provided at the
cost of the government and revenue returns are low.

1.3 IRRIGATION DEVELOPMENT IN ETHIOPIA

Traditional small scale irrigation was practiced in Ethiopia since ancient times producing subsistence food
crops. These traditional irrigation schemes are, in general, simple river diversions, rudimentary and
subjected to frequent damage by floods. However, modern irrigation systems were started in the 1960s
with the objective of producing industrial crops such as cotton, sugarcane, and horticultural crops in
Awash Valley. The Awash Valley saw the biggest expansion in view of the water regulation afforded by
the construction of the Koka dam and reservoir that regulated flows with benefits of flood control,
2
Lecture note
Irrigation Engineering Chapter 1: introduction

hydropower and assured irrigation water supply. In addition, the construction of the Addis Ababa-Assab
road opened the Awash Valley to ready markets in the hinterland as well as for export.
Ethiopia has an estimated irrigation potential of 5.3 million hectares of which the 3.7 million hectare can
be developed using surface water resources, and 1.6 million hectare can also be irrigated by managing
groundwater and rainwater. The estimated total irrigated area during 2005/2006 in Ethiopia was about
625,819 ha, which, in total, constitutes about 18% of the potential that can be developed through the use
of surface water. Irrigation schemes in Ethiopia are classified according to their size, technology use, and
management system. Based on the size of the command area irrigation schemes are classified as large
scale irrigation if the size of command area is greater than 3,000 hectares, medium scale if it falls in the
range of 200 to 3,000 hectares and small scale if it is covering less than 200 hectares. The second
classification is by the differences in the technology used to control and divert water, which have
implications on water availability, water loss and establishment, and for operation & maintenance (O&M)
costs. With respect to the management system, schemes are classified as namely traditional, modern,
public and private. The distribution of irrigation schemes in Ethiopia is quite skewed. Almost 74 percent
of the irrigated area served by large and medium irrigation schemes is located in Awash basin.
Irrigation development has been identified as an important tool to stimulate economic growth and rural
development, and is considered as a cornerstone of food security and poverty reduction in Ethiopia.
Ethiopia has a significant irrigation potential identified from both available land and water resources.
Many irrigation schemes had been developed in many parts of the country at different scales. Irrigation is
one means by which agricultural production can be increased to meet the growing food demands in
Ethiopia.
The Water Sector Development programme of MoWR (2002) organizes irrigation schemes in Ethiopia
under four different ways with sizes ranging from 50 to 85,000 ha
 Traditional small scale schemes: These includes up to 100 ha in area, built and operated by farmers
in local communities. Traditionally, farmers have built small scale schemes on their own initiative with
government technical and material support. They manage them in their own users’ associations or
committees and irrigate areas from 50 to 100 ha with the average ranging from 70 to 90 ha. A total of
1,309 such schemes existed in 1992 covering an estimated area of 60,000ha.
Water users’ associations have long existed to operate and manage traditional schemes. They comprise
about 200 users who share a main or branch canal and further grouped in to several teams of 20 to 30
farmers each.
 Modern communal schemes: schemes up to 200 ha, built by government agencies with farmer
participation. Modern communal schemes were developed after the catastrophic drought of the 1973 as a
means to improve food security and peasant livelihoods by providing cash incomes through production
and marketing of crops. Such schemes are capable of irrigating about 30,000ha of land.
These schemes are generally based on run-of - diversion of streams and rivers and may also involve micro
dams for storage. On-farm support from the respective agricultural departments and maintenance of
headworks by water, mines and energy sections as well as technical support from the authorized irrigation
development Bureaus in different regions is giving supports and trying to strengthen the system.
 Modern private schemes: up to 2000 ha, owned and operated by private investors individually, in
partnership, or as corporations. Medium to large scale irrigation schemes in Ethiopia are private
enterprises. The private estates are the pioneers in the development of medium and large scale irrigation
development projects in the upper Awash during the 1950s and 1960s. During the 1990s some private
schemes, mostly in the form of limited companies re-emerged with the adoption of market based
economic policy but have expanded relatively slowly.
 Public Schemes of over 3,000 ha, owned and operated by public enterprises as estate farms. They are
recently developed irrigation schemes during the late 1970s. Gode West, Omo Ratti and Alwero- Abobo
3
Lecture note
Irrigation Engineering Chapter 1: introduction

began late in the 1980s and early in the 1990s but have not yet been completed. Public involvement
towards large scale schemes was withdrawn due to government changes and most of such schemes with
the exception of Fincha sugare estate have been suspended. Large scale schemes being operated by public
enterprise extend over an area estimated at 61,000 ha. Oromiya and Affar account nearly 87% of all
irrigation schemes and about 73% of this is located in Awash valley. The SNNPR and Somali regions
contain 9.9 and 3.3 percent respectively.

1.4. FEASIBILITY STUDIES OF IRRIGATION PROJECTS

A feasibility study for irrigation development would assess the physical aspects of land, water
and climate, and evaluate crop production potential and cropping programmes within the context
of the physical aspects. The same study reviews and assesses alternative engineering options in
terms of benefits and costs, operation and maintenance, compatibility with the available land and
water resources, their impact on the environment, the health of the users and the social life and
welfare of the irrigators. Finally, market potentials and access to markets are critically reviewed
through such studies and the financial and economic aspects of the development are evaluated. In
summary, the feasibility study is expected to provide options for the client with
recommendations for the best option combining technical feasibility, financial and economic
viability and social desirability and environmental sustainability.
For irrigation projects, the feasibility study is expected to cover the following areas:
1. Climate and natural resources
2. Agriculture
3. Credit and marketing
4. Engineering aspects of the project
5. Social aspects of the project
6. Organization and management aspects of the project
7. Health and environmental aspects of the project
8. Economic and financial analysis
1, Climate and natural resources
As a rule, climate and the assessment of the potential and availability of natural resources (land
and water) are among the first areas to be addressed in the preparation of a feasibility study.
 Climate
The most important climatic data are rainfall, maximum and minimum temperatures, maximum
and minimum relative humidity, wind and sunshine hours. Climate is an important factor in for
irrigation projects. Different crops have different requirements in terms of temperature, humidity
and light. Also, occurrence of frost at certain times may exclude a number of crops from the
cropping programme. All in all, the analysis of climatic data with respect to crop production is
needed before a cropping programme can be prepared.
Accurate estimates of crop water requirements also rely heavily on the availability of accurate
meteorological data. Errors of only 20% in crop water requirement estimates can significantly
affect the economics of the project, especially in Africa where the water development cost is
high. Hence the need for long-term accurate meteorological data, especially long-term rainfall
data.

4
Lecture note
Irrigation Engineering Chapter 1: introduction

 Land
The topography of the land when combined with the soil characteristics will provide the means
of assessing the irrigability of the land and select the most suitable areas for irrigation. In this
respect, soil and topographic surveys, provide the means for this assessment.
 Water
Long-term data of river flow and water quality are needed to assess the potential of the water
resources. In the absence of hydrological data, rainfall records or flows of nearby streams are
used for estimates. In the case of groundwater resources, hydrogeological studies are carried out
and records from existing wells and test wells are used to establish long-term and short-term
yields of the aquifer
Nevertheless, irrespectively of water availability, the right to using the water should be
investigated. This is becoming very important with the establishment of water boards, water
strategies and policies as well as water legislation in many countries in Africa. Hence, a water
right should be obtained from the relevant authorities that permit the use of the water. Since the
use of transboundary water resources is bound by agreements between the states sharing the
same river basin as well as international law, the feasibility study should deal with such matters
as and when they arise.
Wherever a new scheme is planned, existing established demands for water upstream and
downstream should be investigated and taken into consideration. A formal system of water rights
might be in operation, or local people may have an agreement by traditional custom over the way
in which water for irrigation is allocated. Proposed changes in water demand must be fully
discussed with the national authority responsible for regulating abstraction.
Water quality and flow rates are very important for the selection of crops to be grown and the
irrigation method to be adopted. As such they should be included in the water resources surveys
to be undertaken. Of particular importance is the potential siltation of water reservoirs and the
need to protect the catchment areas, in order to avoid the rapid decline in the yield of dams.
2, Agriculture
As irrigation development aims at agricultural production the engineering works should be
designed for this purpose. The objective is not the conveyance of water but the irrigation of
crops. Thus the engineering approaches used should be considered as part of a broader system
(irrigated crop production) for which the designed scheme will be constructed to serve.
 Existing farm practices
The existing agricultural practices are assessed to analyze the without-project situation. Data is
gathered from the baseline socio-economic survey. The data is aggregated to reflect the average
production cost and gross margins and incorporated in the financial and economic analysis. The
same surveys will provide information on the availability of family labour for use under rainfed
farming and irrigation in the future, and assess the need for hired labour.
 Land tenure
The land tenure for smallholders varies from country to country in Sub-Saharan Africa. In some
countries smallholders have the right to use the land, while in others smallholders have title

5
Lecture note
Irrigation Engineering Chapter 1: introduction

deeds of their land. How one or the other type of land tenure affects the various aspects of the
project should be elaborated in the feasibility study.
 Proposed agricultural system
Based on the climate and the natural resources potential, crops are selected for consideration and
alternative cropping programmes and rotations are developed for discussion with the
smallholders. The cultural requirements of each crop and expected yields should be elaborated
and the crop water requirements estimated for alternative cropping programmes. Crop budgets
for these crops will be prepared and presented later on in the feasibility study, under financial
and economic analysis. The marketing potentials of these crops will also be discussed under the
relevant chapter of the study.
3, Credit and marketing
As a rule, irrigated crop production is a high-input high output system. Smallholders therefore
need to procure seeds, fertilizers and chemicals in order to optimize their production system.
However, the poor cash flow from conventional rainfed farming is too low for such an
investment. Consequently, the need for credit is great indeed. It is therefore necessary that the
study reviews potential options and makes recommendations under the prevailing land tenure in
the scheme.
The choice of crops to be grown and the cropping patterns influence the field layout and
irrigation method. However, the choice of crops as well as the cropping programmes are
influenced by their marketing potentials. Therefore, an assessment of the existing markets and
transport system and road infrastructure, as well as their potential for development, should be
made. Market prices, transport costs and farm prices must be predicted, as related to the expected
increased volume of production. Processing and/or storage facilities should be considered as part
of a marketing strategy.
4, Engineering aspects
This part of the feasibility study covers the rehabilitation and/or extension of existing irrigation
schemes, as well as the development of new schemes. It deals with the water development, the
distribution system, the water storage and control structures and measuring devices, the on-farm
irrigation works and the drainage. For these and other engineering works preliminary designs are
made and cost estimates prepared.
 Social aspects
The project’s objectives and expectations cannot be realized unless farmers’ considerations on
benefits and costs, feasibility and desirability and their priorities in life match that which the
project requires of them. At times, smallholders’ priorities differ from the project’s priorities.
Hence the need to assess the acceptability and desirability of the farmers to participate in the
development of the irrigation scheme. The nature of the population must be understood in order
to match the rate of development with the absorptive capacity. Elements such as the level of
literacy, farming knowledge and skills, past experience with irrigation, gender issues and
attitudes to change are among the several parameters to be considered when analyzing the social
aspects of the project.

6
Lecture note
Irrigation Engineering Chapter 1: introduction

As a rule, irrigation development brings cultural shock to a smallholder community. With

monomodal rainfall conditions, smallholders work for a few months in a year under rainfed
conditions. In a sense they are underemployed and have ample time to attend to their social
aspects of the society. In contrast to this, irrigated crop production requires almost daily attention
throughout the year if it is to be profitable. How able the community is to adjust to these and
other changes becomes critically important and should be thoroughly discussed with the farmers.
5, Organization and management aspects of the project
An analysis of the structures and competence of the agencies or bodies responsible for the
organization and management of the project is necessary. A number of problems or difficulties
should be expected to arise during the planning, construction and operation of a fairly large
project. Hence the need for the presence or establishment of competent agencies is to manage the
planning and implementation of the project.
 The organization of planning and construction
The planning and construction of a smallholder irrigation scheme involves several stakeholders.
Rural authorities, traditional leaders, farmers, relevant Department or Ministry at central level,
consultants and contractors are the major stakeholders. At times, sub-contractors are also
involved with the construction of some parts of the project.
Hence the need for a competent agency to coordinate and supervise the work of all involved in
the planning and implementation of the project. The same agency, through established
procedures, would be responsible for the selection of the contractor and sub-contractors. As a
rule, selection of inexperienced contractors on the basis of a cheaper offer does not always cost
less. Delays from one contractor can have snowball effect on other contractors, and the on
project as a whole.
 The organization of operation, maintenance and management
Irrigation development, especially in sub-Saharan Africa, is very costly. It is therefore necessary
for this investment to be utilized productively as soon as possible. Thus, provision should be
made from the feasibility study stage onwards for the needed trained engineers, agronomists and
technicians to be available on time. Equally important is the assessment of the farmers’ training
needs, which will enable them to make well-informed decisions and to undertake the operation,
maintenance and management of the infield part of the system.
6, Health and environmental impact assessment
Very often the health and environmental aspects of irrigation development are not given
deserved attention in the feasibility studies. Water-related diseases affect the health of the
irrigators and thus the overall performance of the scheme. Measures to reduce such problems
through engineering and other solutions should be incorporated in the feasibility study. The
impact of irrigation development on the environment is equally important, as it affects the
quality of the water resources and thus downstream water users as well as the ecosystem at large.
7, Economic and financial analysis
Economic and financial analyses are carried out in order to appraise a project. The economic
analysis provides the justification for an irrigation development. The financial analysis evaluates
the project’s capability to repay the investment and the operation costs of the project. In other
7
Lecture note
Irrigation Engineering Chapter 1: introduction

words, the economic analysis assesses the economic viability of different alternatives and assists
with the selection of one. The financial analysis evaluates different financial alternatives with
respect to interest rates, repayment schedules and length of the loan period.

8
Lecture note