This document provides an overview of irrigation engineering. It defines irrigation as the artificial application of water to land to supplement natural precipitation for optimal crop production. Irrigation is needed in arid, semi-arid, and humid regions where rainfall is unreliable or insufficient. The objectives and scope of irrigation engineering are outlined, along with the factors, benefits, and potential ill-effects of irrigation. Standards for irrigation water quality and procedures for feasibility studies of irrigation projects are also discussed.

1. CHAPTER ONE
INTRODUCTION

3. Definition and Scope of Irrigation
•Irrigation is a science of artificial application of
water to the land, in accordance with the ‘crop
requirements’ throughout the ‘crop period’.
•Irrigation is the application of water to the soil to
supplement natural precipitation and provide an
environment that is optimum for crop production.

4. Necessity of Irrigation
Irrigation is needed for a continuous and
reliable water supply to the different crops
in accordance with their different needs.
When water supply is not sufficient and
timely, crop yield gets reduced and
consequently famines occur.
Irrigation can be supplementary irrigation or
total irrigation based on rainfall availability.

5. Irrigation is needed in:
•Arid and semi-arid regions - natural rainfall
does not provide all the water needed, and
therefore, irrigation can make up this
deficiency.
•Humid regions –ideal rainfall patterns seldom
occur in these regions.
•Thus, the need of irrigation in humid regions is
mainly to supplement the supply of water
received from precipitation.

6. Objectives of Irrigation:
To supply water partially or totally according to crop
needs
To cool the plant or protect them from frost
To leach or dilute excess salts present in the soil
To facilitate continuous cropping
To enhance fertilizer application- Fertigation

7. Scope of Irrigation Engineering
Irrigation engineering involves:
Conception,
Planning,
Design,
Construction,
Operation and
Management of an irrigation system.

8. •The irrigation engineer needs to know:
Basic Soil Science/Physics
Plants
Water
Plant/Soil/Water Relations
Hydraulics
Hydrology
General Engineering Principles

9. Benefits and Ill-Effects of Irrigation
Benefits of irrigation
Increase in food production- food security
Optimum utilization of water
Protect crops from high temperatures and frost
Elimination of mixed cropping
General prosperity- high revenue
Enhance fertilizer application through fertigation
Domestic water supply- by the installed facilities
Inland navigation- large irrigation canals

10. Ill- effects of irrigation
Problem of ground water pollution by
fertilizer chemicals due to seepage
Irrigation may result in marshy land causing
breeding of mosquitoes
Over-irrigation may lead to water-logging
and may reduce crop yield
High initial investment

11. Factors affecting irrigation practices:
Land Capability- Irrigation should be confined
to soils that are capable of producing sustained
high yields of adapted crops.
Adequate Water Supply- to meet crop needs.
Adequate Labor
Climate
Adequate initial capital-to install, operate and
maintain irrigation projects

12. Irrigation development in Ethiopia
Facts
Agricultural sector - contributes about 40% of the GDP and
employs 80% the labor force
• Virtually all food crops come from rain fed agriculture
• Generally, 75% of the rainfall is concentrated in a period of 3 to 4
months.
• Drought and famine are common phenomena in Ethiopia. These
drought and famine are mainly due to failure of rainfall both in
amount and variation in time.
• Nowadays, highly variable rainfall in spatial and temporal scales
make farming a risky business

13. Government policy
•Transformation and intensification of agricultural
sector.
•Optimal development of water resources of the
country
Water resource of the country:
•Surface water is about 112 billion m3
•Ground water potential is about 2.6 billion m3
•Potential irrigable land of the country = 3.7 Mha (only
0.19Mha is currently irrigated)
•Irrigation projects:
i. small scale (<200ha)
ii. medium scale (200ha < A < 3000ha)
iii. large scale (>3000ha)

14. Cognizant of these facts, irrigation is now a
national issue and is given a priority to meet the
following objectives.
To increase production and supply of food crops
both quantitatively and qualitatively so that the
population will be fed adequately.
To improve the quality of life of rural population
through the generation of higher incomes and
reduction of poverty.
To increase and diversify the production of raw
materials for industry and to promote linkage with
the industrial sector
To increase and diversify production for export and
maximize the country’s foreign exchange earnings
from agriculture

15. Problems of Irrigation Development in Ethiopia:
Lack of mapping data:- Mapping information not
available at a scale suitable for detailed studies of
irrigation areas, dam sites, etc. Similarly, data related to
soils are not found in sufficient detail to all potential
areas.
Lack of Hydrologic Data:- The collection of hydro
meteorological information are not adequate. River
gauges are mostly available for major rivers and bigger
tributaries.
Lack of resources:- The level of skilled manpower
available in the country is not adequate to carry out the
number of projects being demanded by the regions.
Inadequacy in construction:- due to inadequate
machinery, equipment and spare parts.

16. Lack of Finance:- Despite ambitious plan of the
government, sources of finance for implementing
the planned irrigation projects is expected from
foreign aid and loans.
Inadequate research undertakings in the area:-
So far, little emphasis for irrigation in agricultural
research.
Operation and Maintenance:- Institutional water
operation and management is in a very poor
condition with regard to the administration of
supply, maintenance and repair of the networks
due to lack of trained manpower, sufficient
maintenance tools and equipment.

17. Standards of Irrigation Water
•Every water is not suitable for plants.
•Water containing impurities, which are injurious to
plant growth, is not satisfactory for irrigation.
•The various types of impurities are discussed below
Sediment
•Eroded soils may decrease soil fertility and
permeability.
•Sediments create siltation in canals and causes
high maintenance cost.

18. Total concentration of soluble salts:
•Excessive salt contents reduce plant osmotic activities
and prevent aeration
Proportion of sodium ions to other cations
•Increase in the proportion leads to disaggregation of
soil grains
•High sodium soils become plastic and sticky when wet
and cloddy when dry

19. Concentration of potentially toxic elements
•Boron, Selenium etc. in high concentration are very
toxic to plants
Bicarbonate concentration
•High conc. of bicarbonate ions leads to precipitation
of Ca and Mg bicarbonates from soil solution,
increasing the relative proportion of Na+ and causing
Sodium hazards
Bacterial contamination
•This will be harmful to human if crops irrigated with
highly contaminated water are directly eaten, without
being cooked.

20. Procedures for Feasibility Studies of
Irrigation Projects
Considerations in Planning Irrigation Systems include:
Location: - involves investigating availability of
resources in the area such as:
•Adequate water in quality and quantity
•Land with good agricultural potential and good
topography
•Availability of labor (skilled and unskilled)
•Land tenure
•Market
•Transport facilities, etc.

21. Crops to be grown:- should be determined by available
resources as well as marketability of the crops especially
in terms of what people like to eat/buy.
Water Supply: Consider
•Sources of water
•Quantity and quality of water
•Engineering works necessary to obtain water e.g. if
underground, pumping is needed
•Conveyance System: can be by gravity e.g. open
channels or canals or by closed conduits e.g. pipes.

22. •Method of water application:
•Surface (90% worldwide),
•Sprinkler(5%),
•Trickle(5%).
•Water Demand: The water requirement for the given
crop has to be determined. This is by calculating the
evapotranspiration
•Project Management: Consider how to manage the
irrigation system