Irrigation Principles. Theory and Application

Research Paper (postgraduate), 2019

260 Pages, Grade: 1.0



1.1 Definition
1.2 Irrigation for sustainable food security
1.3 Historical perspective of irrigation
1.4 Total and supplemental irrigation
1.6 Spate irrigation
1.7 Importance of Irrigation
1.8 Advantages and disadvantages of Irrigation
1.9 Developmental aspects of irrigation
1.10 Development of irrigation in Kenya
1.11 Challenges of irrigation development
1.12 Strategies for improved irrigation development
1.13 Role of an Irrigation Engineer
1.14 Irrigation schemes in Kenya
Review Questions 1

2.1 Irrigation systems planning
2.2 Irrigation planning process
2.3 Field water balance-based plan
2.4 Fundamental factors in development of irrigation
2.5.1 Technical and physical factors
2.5.2 Economic factors
2.5.3 Social factors
2.5.4 Other factors
Review Questions 2

3.1 Introduction
3.2 Soil
3.2.3 Physical properties
3.2.4 Soil texture
3.2.5 Soil structure
3.2.6 Depth of soil
3.2.7 Chemical nature of soil
3.3 Soil-water relationship
3.3.1 Three phase diagram
3.3.2 Key parameters of soil in irrigation
3.4 Infiltration
3.5 Factors affecting infiltration
3.6 Infiltration models
3.7 Measurement of infiltration
3.7.1 Double-ring infiltrometers
3.7.2 Demerits of ring-infiltiltrometer
3.7.3 Infiltration indices
Examples 3
Review questions 3

4.2 Partitioning of evapo-transpiration
4.4.1 Effective rainfall
4.4.2 Factors affecting effective rainfall
4.4.3 Estimation of effective rainfall
4.6 Gross irrigation requirement
4.7 Estimation of evapotranspiration
4.7.1 Lysimeter experiment
4.7.2 Soil moisture depletion method
4.7.3 Field water balance
4.8 Estimation of ET using empirical models
4.8.1 Blaney-Criddle method
4.8.2 Crop Coefficient (Kc)
4.8.5 Thornwaite method
4.9 Irrigation scheduling
4.9.1 Determination of time to Irrigate
4.9.2 Plant Indicator Methods
4.9.4 Soil Indicator Methods
4.9 Delta (Δ)
4.9 Efficiency of irrigation systems
Examples 4
Review Questions 4

5.1 Introduction to methods of irrigation
5.2 Factors that influence the choice of irrigation method
5.3 Surface Irrigation methods
5.3.1 Surface irrigation process
5.4 Basin irrigation
5.4.1 Suitable crops
5.4.3 Basin Layout
5.4.4 Level basin size based on flow time
5.4.5 Shape and dimensions of bunds
5.4.6 Basin Construction
5.4.7 Water application into irrigating basins
5.4.8 Wetting patterns
5.4.9 Water storage in basins
5.4.10 Maintenance of basins
5.4.11 Advantages of basin irrigation
5.4.12 Disadvantages of basin irrigation
5.5 Border irrigation
5.5.1 Application of border irrigation
5.5.2 Border Layout
5.5.3 Maintenance of border irrigation system
5.5.4 Advantages of border irrigation
5.5.5 Disadvantages of border irrigation
5.6 Furrow irrigation
5.6.2 Evaluation of furrow irrigation system
5.6.3 Advantages of furrow irrigation
5.6.4 Disadvantages of furrow irrigation
5.7 Sprinkler irrigation system
5.7.1 Key Components of sprinkler irrigation system
5.8 Sprinkler layout
5.8.1 General rules for sprinkler system layout
5.8.2 Flow of water in sprinkler irrigation system
5.8.3 Performance of sprinkler irrigation system
5.9 Lateral System Design
5.10 Operation of sprinkler systems
5.11 Maintenance of sprinkler systems
5.12 Types of sprinkler irrigation systems
5.12.1 Based on the portability
5.12.2 Based on spraying pattern
5.12.3 Based on arrangement of spraying
5.13 Advantages of sprinkler irrigation
5.14 Drip irrigation
5.15 GSM based irrigation control system
5.15.1 Structure of the GSM module
5.15.2 Irrigation control system
5.15.3 Advantages of GSM based controlled irrigation
Examples 5
Review questions 5

6.1 Drainage engineering
6.2 Effect of excess water on agricultural land
6.3 Benefits of good drainage to agricultural land
6.4 Components of drainage system
6.5 Types of Agricultural land drainage systems
6.5.6 Surface drainage systems
6.5.7 Advantages of surface drainage system
6.5.8 Disadvantages of surface drainage
6.5.9 Design discharge and velocity for surface drainage system
6.5.10 Rational method
6.5.11 Curve number (CN) approach
6.6 Sub-surface drainage systems
6.6.1 Advantages of sub-surface drainage
6.6.2 Disadvantages of sub-surface drainage
6.6.3 Design of Sub-surface drainage system
6.6.4 Assumptions of Hooghoudt function
6.7 Tile drain
6.7.1 Diameter of tile drainage system
6.7.2 Ground water drainage
6.8 Mole drainage
Examples 6
Review questions 6

7.1 Salts in Irrigation Water
7.2 Origin of salts
7.3 Accumulation of salts in Soil
7.4 Quality of Irrigation Water
7.5 Method of Irrigation
7.6 Effect of salts on plants
7.7 Determinations of salinity and sodicity
7.7.1 Saline soils
7.7.2 Sodic soils
7.7.3 Saline-sodic soils
7.8 Managing salt-affected soils
7.9 Managing saline soils
7.9.1 Reclaiming saline soils
7.9.2 Controlling salinity with irrigation water
7.9.3 Salt-tolerant plants
Examples 7
Review questions 7

8.1 Pumps
8.2 Types of pumps
8.3 Principles of operation of selected pumps
8.3 Single acting positive displacement pump
8.3.1 Discharge of a single acting reciprocating pump
8.4 Double-acting reciprocating pump
8.4.1 Discharge of a double-acting reciprocating pump
8.5 Principles of operation of hydraulic ram
8.6 Principle of operation of centrifugal pump
8.7 Pump characteristics
8.7.1 Affinity Laws
8.8 Power and energy requirements of a pump
8.10 Pump performance curves
8.11 Capacity of irrigation pump
8.12 Pump system configuration
8.12.1 Pumps in series
8.12.2 Pumps in parallel
Examples 8
Review questions 8

9.1 Importance of water measurement
9.2 Methods of water measurement
9.2.1 Tracer method
9.2.2 Water measurement by volume
9.2.3 Use of float
9.2.4 Velocity area methods
9.2.5 Manning"s formula
9.2.6 Weirs
9.2.7 Venturi meter
9.2.8 Flow nozzles
9.2.9 Orifice meters
9.2.10 Parshall flumes
Examples 9
Review Questions 9

10.1 Introduction
10.2 Importance of irrigation water management
10.3 Optimum use of irrigation water
10.4 Need for optimum use of irrigation water
10.5 Strategies to improve irrigation water management
10.6 Causes of poor irrigation water management
10.7 Conjunctive water use
10.8 Participatory irrigation management (PIM)
10.9 Principles of PIM
Review questions 10

11.1 Introduction
11.2 Feasibility of irrigation project
11.3 Increase in land value
11.5 Climate
11.6 Crop types
11.7 Water supply
11.8 Cost of irrigation works
11.9 Marketing of agricultural produce
11.10 Security of irrigation project
Review questions 11

12.1 Introduction to hydroponic system
12.2 Advantages of Hydroponics
12.3 Disadvantages of hydroponics
12.4 Irrigation within hydroponics
12.5 Water quantity
12.6 Water quality
12.7 Root zone environment
12.8 Layout and performance of hydroponic irrigation systems
12.9 Irrigation amount
12.10 Irrigation frequency in hydroponics
12.11 Hydroponics irrigation control
Review question 12



I express my sincere gratitude to the following people;

To my wife, Theresa Monthe, who has always made valuable assessment and suggestions for development of this book. My writing has been successful because she is part of my life.

To my students I have taught over the years specifically in the fields of Agricultural Engineering, Water and Environmental Engineering, General Agriculture, Agricultural Education and Extension. Through class participation, the students have helped me to revise numerous sections of the book to be more relevant to University setting and training fraternity.

Sincere gratitude goes to the book manuscript reviewers; namely Edwin Amisi and Jackson Mutinda for providing useful comments that were used to improve the quality and relevance of the book.

‘Irrigation principles (Theory and Application) ’ is part of my long-term personal interest that began numerous years ago when I was a small boy with acumen in writing and publishing. It has taken time to come up with a manuscript with worthwhile content. I hope the text book will be useful in learning and application scenarios.

Raphael M. Wambua


This book is dedicated to students who use it to gain theoretical principles of irrigation and in addition take a step in applying it in for increased food production; and

Instructors in higher education that find it useful in guiding and teaching at the institutions of higher learning; and

This book is also dedicated to relevant professionals and field officers in irrigation, agriculture and water resources particularly the ones who use this book for reference in matters or irrigation theory and application.


Irrigation Principles(Theory and Application) is a text book intended for students and instructors in University or higher education for Certificate, Diploma and Degree students in a number of courses such as Irrigation and Drainage, Agricultural Engineering, General Agriculture, Agricultural Education and Extension, Horticulture, Water Resources Engineering, applied irrigation engineering and other allied professions.

The content of the text book has been presented in a lucid style, arranged in coherent sequence that adheres to University and higher education curriculum. This makes the book suitable for relaxed reading. For the calculations, worked examples have been solved in a way of illustration and details are presented. Each chapter is concluded with the examples and review questions for the readers to expound on subject knowledge. For the purpose of improvement, any criticism from students, trainers and practitioners will be thankfully received by the author.

Raphael M. Wambua


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The objective of chapter 1to offer knowledge and skills on:

Introduction and important terms used in „irrigation principles, theory and practice"
Importance of irrigation in Kenya
Development of irrigation projects/schemes in Kenya
Role of irrigation engineer and agricultural officer in development of irrigation projects

1.1 Definition

Irrigation is a controlled artificial application, conservation and storage of water to plant root zone for the purpose of crop production. Its fundamental objective is to create an optimal soil moisture system for maximizing crop production and quality while at the same time minimizing any environmental degradation inherent in irrigation of agricultural lands.

In the context of this book, Irrigation Engineering is the application of scientific and mathematical principles to practical solutions in design, manufacture, and operation of efficient and economical irrigation structures, machines, systems and processes.

1.2 Irrigation for sustainable food security

Irrigated agriculture is always depended upon the natural rainfall for crop production and is thus difficult to manage especially in timing market demands for crops. However, irrigation technologies and crop production to meet a projected food shortage can simultaneously be controlled. Food security is a term used to refer to availability of sufficient quantity of food at the right time and at the right nutritional value to people. Sustainability is the capacity to be available at present and future generation without compromising the quantity, quality and nutrition. Innovation in irrigation to sustainably utilize available water resources for crop production is perceived as a solution to the food insecurity in any area. Through irrigation, limited water resources can be used in a sustainable manner for crop production and meet food demand throughout different seasons of the year.

Irrigation plays a key role in crop production and thus food security in many areas. It can be used to grow crops on those areas where there is no rainfall at all, or supplement water for crop production where and when the rainfall is not sufficient. Therefore, nothing can replace irrigation in meeting food security for all nations in the world. Rain-fed agriculture will not; nothing is more common than crop failure that depends on natural rainfall. New crop varieties alone will not; the world is full of new crop varieties that cannot grow without water. Good policies will not; unrewarded good food-security policies are very common. Applied irrigation with high water use efficiency is omnipotent in food security.

1.3 Historical perspective of irrigation

The earliest known irrigation systems started in 6000 BC (Before Christ) in Egypt and Mesopotamia. In Egypt, water was diverted from the Nile River into agricultural fields where farmers grew crops under controlled water application. Around 3100 BC large dams and canals of about 20 km length were constructed in Egypt to store and convey water for irrigation. It is believed that Egypt had the world's oldest dam of approximately 108 m long and 12 m high built for irrigation. In Mesopotamia, the Tigris and Euphrates rivers were used as water sources for irrigation. Canals were dug to divert and convey water to agricultural fields. . It has been recorded that Mesopotamia supported as many as 25 million people through irrigation programme.

Canal irrigation is an ancient method of irrigation in the world. Early irrigation canals were found in numerous places such as United States of America, China and India. In America, the Zana Valley exhibit an irrigation canal system dated to 4000 BC and may even have been used earlier than this. An artificial water reservoir that connects to a complex irrigation system was developed in Sri-Lanka around 300 BC. This system exists to date and present an incredible irrigation engineering design. Irrigation in Mexico has existed since 6000 BC when construction of storage dams was done. In such a system, construction blocks were joined together and canal systems created to regulate water flow. Romans have done irrigation in Britain since 2000 years ago. They began by constructing dams and reservoirs for irrigation and conveyed the water through aqueducts for irrigation purpose.

Surface irrigation systems are the oldest techniques of irrigated agriculture and are still being used today. Other advanced systems of irrigation have recently been developed including drip, sprinkler, and sub-irrigation. The irrigation techniques will continue to improve based on the basic process of artificially supplying water to soil or any other media for crop production. Use of remote sensing, GIS and Global Systems for Mobile Communications (GSM) technology for irrigation scheduling and prediction of crop yield has also been used in modern irrigation advanced technology will continue to allow human beings to increase food production to meet high demand for growing world population.

1.4 Total and supplemental irrigation

Based on the period of water application during the part or entire crop growing period which is also pegged on the rainfall distribution during the period, irrigation can be classified into total or supplemental irrigation.

(i) Total irrigation: it artificial application of irrigation water to the root zone as a principal source for meeting evapotranspiration requirements of the crop in areas with unreliable rainfall during all or large part of growing period of crop
(ii) Supplementary irrigation: it defined as artificial application of defined quantity of water crop root zone of essentially rain-fed crops during the period when rainfall fails to meet the evapotranspiration requirements of the crop. It a method for supplementing rainfall water for the crops.

1.5 Deficit irrigation

Deficit irrigation is an artificial application of water to crop root-zone at a quantity below the evapotranspiration requirements of the crop. It is a process of artificially applying water to cropland to partially meet the crop water requirement. It is an optimization strategy in which more irrigation water is applied during drought-sensitive growth stage. Deficit irrigation is mostly practiced in areas or periods when water scarcity is acute. In deficit irrigation, the crop root zone is not filled to the field capacity (FC). It is preferred in situations where or when the water application below full irrigation causes production cost to decrease without significant reduction in crop yield or revenue. This system allows some stress to the crop during part of the growing periods. However, sufficient water is applied during the critical growth stages of the crop. The critical growth stage of the crop is that period that if full water application is done, leads to the highest crop yield.

In deficit irrigation technique an irrigation scheduling strategy is used. Only a portion of net crop water requirement is attained through applying irrigation water. Correct application of deficit irrigation requires the understanding of crop yield as well as economic impact of reduced of produce. The principle is to get significantly high crop yield by applying a fraction of water needed to meet crop water requirements.

Deficit irrigation maximizes irrigation water productivity and increases farmer's profits. In deficit irrigation, the water management shifts from maximizing net income per land unit to maximizing net income per unit of water used. The technique of deficit irrigation has the following advantages, that it:

(i) Maximizes the productivity of water and good quality harvest
(ii) Allows for better economic planning and stable income due to stabilization of harvest as compared to the rain-fed agriculture
(iii) Reduces the risk of certain diseases linked to high humidity such as fungi which are common with total or full irrigation
(iv) Reduces nutrient loss by leaching of the root zone and thus lead to less fertilizer requirement
(v) Allows farmers to control and decide on sowing date which is not possible with rain-fed agriculture

1.6 Spate irrigation

Spate irrigation is a form of flood based farming system. It is usually practiced in large scale, by farmers in groups. Typically, spate irrigation utilizes flash floods generated from the upslope which is then conveyed to the fields using appropriate irrigation structures. The spate irrigation is mainly practiced in the arid lands and is influenced by the characteristics of the flash flood being harnessed for agricultural production. In Kenya, harnessing of flash flood is done by small scale farmers. Flood water has been used along the Tana River to grow rice and other crops. Spate irrigation has some advantages such as:

(i) It is a simple technology that can easily be maintained by the farmers;
(ii) It is a system which is less dependent on heavy machinery and imported materials and supplies thus low cost
(iii) Most of the construction works is simple and can be carried out by farmers themselves;
(iv) The structures for water conveyance and diversion can easily be repaired at low cost and can be executed faster since it uses local materials and/or skills
(v) The impact of failure is small and partial because the diversion structures have smaller command areas

1.7 Importance of Irrigation

How can the future food production in conjunction with food security be projected? In the future, world food production will need to be increased to meet the demands of increasing human population. Most of the increased food production will have to be gotten from irrigated land. Without irrigation the world population will not be food secure; without irrigation farming is very limited especially with low water resources and changing climate change in many regions. Generally, Irrigation helps in socio-economic development of a country, area or region. Irrigation is important and necessary in the following situations

(i) On fertile land which receives insufficient rainfall for crop growth
(ii) During the dry season, it is used to extend the growing period
(iii) Where it is necessary to take advantage of high market prices during the dry period
(iv) Where high value crops such as horticultural crops are needed
(v) Where/when it is necessary to minimize fertilizer wastage fertigation is incorporated in the irrigation system
(vi) Providing insurance against short duration droughts
(vii) Reducing the temperature during hot spells (viii) Washing or diluting salts in the soil
(ix) Softening tillage pans and clods
(x) Delaying bud formation by evaporative cooling
(xi) Promoting the function of some micro organisms
(xii) Reducing the hazard of frost (increase the temperature of the plant)

Whether water is supplied through irrigation or via precipitation, generally plants need water for mainly the following benefits

(i) Cooling effect: this is caused by evaporation and transpiration in which use energy results in cooling plants and soil
(ii) Nutrient transport: after absorption of plant nutrients, subsequent movement within plant tissue is aided in water media
(iii) Dispersion of plant expelled waste: water is used as media through which plant wastes are expelled from the plant tissues

1.8 Advantages and disadvantages of Irrigation

Irrigation has a number of advantages when used for crop production. The following are some of the advantages of practicing irrigation:

(i) Irrigation helps to increase food production and can make a country to attain self-sufficiency
(ii) Through irrigation, cultivation of horticultural, cash crops and high value crops is possible
(iii) It increases land value and makes owners of land wealthy
(iv) It leads into prevention of floods though utilization of runoff water
(v) Irrigation systems can be used for generation of hydro-power at dam canals
(vi) Irrigation can be used for afforestation especially along canals and field boundaries
(vii) Generation of revenue alongside irrigation facilities such as boating, fishing and swimming is possible
(viii) Fish and other important wildlife may be preserved along the irrigation systems

However, if irrigation systems are not well managed, they may lead to some challenges. Some of the disadvantages of irrigation may include

(i) Irrigation may cause dump climate and lead to water borne diseases,
(ii) Over application lead to water logging of soils

1.9 Developmental aspects of irrigation

Irrigation is normally practiced to introduce and maintain differential parameters. The most important developmental parameters of irrigation include:

(i) Provision for proper and sustainable growth of crops
(ii) To increase soil moisture to the required level thus fill the soil moisture deficit
(iii) To make the harvesting of the crop more safe
(iv) Increase agricultural production by developing, using and adopting modern technology
(v) To shift crop growth from seasonal production
(vi) To promote intensive cultivation via multiple cropping patterns
(vii) To utilize cultivable waste land and thus improve production
(viii) To balance socio-economic development by making arid and semi-arid lands more productive

1.10 Development of irrigation in Kenya

Irrigation development in Kenya is currently quite low. At present, approximately a total area of 105, 800 ha is under irrigation compared to the potential of 539, 000 ha. Table 1.1 shows the summary of irrigation potential for different river basins in Kenya.

Table 1.1 Average irrigation potential verses the irrigated area in Kenya

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(Source, NIB 2008)

The total irrigated area is assessed based on all the categories of irrigation systems in Kenya. There are three main categories of irrigation systems in Kenya. These include

Public irrigation schemes: These are irrigation projects initiated, developed and centrally managed by the government agencies such as National Irrigation Board (NIB).

Private irrigation schemes: they are irrigation projects developed, owned and managed by individual farmers or companies.

Smallholder irrigation schemes: This refers to irrigation projects which are community-based. They may be owned, operated and managed by the farmers through their own irrigation water users" associations (IWUAs).

1.11 Challenges of irrigation development

Although irrigation development has far reaching benefits, it can be faced by a number of challenges either at its conceptualization, development and or implementation stages. Some of the challenges of irrigation development in Kenya and other countries may include

(i) Inadequate development of irrigation infrastructure and water storage structures or strategies
(ii) Insufficient technical capacity at the farm level and technical officers in some aspects
(iii) Limited resources support such as loans/credit and extension services in irrigation
(iv) Inadequate national irrigation policy, legal and institutional frameworks in irrigation
(v) Some farmers or leaders have a wrong perception about irrigation potential and importance in the country
(vi) Limited farmers" organizational structures and participation in key activities
(vii) Scanty public-private partnership in terms irrigation investment
(viii) Unsuitable topography in some areas such as steep topography irrigation practices may be a challenge. Installation of the irrigation systems may be difficult and even basic infrastructure such as roads may be limited to aid in marketing of agricultural produce.
(ix) Desertification this is a situation where water becomes scarce on land. This leads to lack of water needed for irrigation
(x) Depletion or degradation of natural resources including soil and water multiply degradation of plant nutrients
(xi) Salinization and sodification which is caused by presence of salts and mineral salts in the soil around the root zone of crops makes the soil to be low productive

1.12 Strategies for improved irrigation development

To overcome the above challenges of irrigation and thus fast track irrigation-induced development, the following strategies can be adopted:

(i) Creation of enabling environment for key stakeholder participation in irrigation
(ii) Re-structuring of government and re-engineering of ministerial functions for optimum irrigation development
(iii) Innovation, development and application of appropriate irrigation technologies which are affordable and increase income for the farmers
(iv) Improved integrated development approaches for provision of critical irrigation services and consultancy services
(v) Increased capital investment in development of irrigation infrastructure
(vi) Develop technical capacity of the agricultural officers and the farmers
(vii) Improved community participation in irrigation activities including perception, development and management
(viii) Enhanced public-private partnerships in developing and managing irrigation activities
(ix) Promotion of water harvesting and storage facilities especially at farm or scheme level

1.13 Role of an Irrigation Engineer

For a successful irrigation project, the role of an Irrigation Engineer, Agricultural engineer, or agricultural officer involves and not limited to

(i) Identification and development of water resources for the irrigation
(ii) Means and ways of supplying water at farm turnout through design of water supply systems
(iii) Design and installation of water storage structures for supply during critical drought period
(iv) design and development of Water conveyance systems with appropriate conveyance efficiency
(v) Supplying water when needed and by the quantity needed by way of proper irrigation scheduling
(vi) Plan, design and develop drainage in cases where the surface and sub-surface soil mass may have water logging
(vii) Community mobilization in adoption of irrigation methods and management on agricultural land.

1.14 Irrigation schemes in Kenya

An irrigation scheme is a combination of systematic planning and infrastructural development aimed increasing food security through application of irrigation water as well as allocation of land to individual farmers and or farmer groups. In Kenya seven irrigation schemes have been developed in different parts of the country for growing certain crops as shown in the Table1.2.

Table 1.2 Public irrigation schemes in Kenya

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Source: National Irrigation Board (NIB, 2018)

Review Questions 1

1.1 Explicitly describe the role of an irrigation/agricultural engineer in development of irrigation projects in context of meeting food security in any country.
1.2 A large-scale Galana-Kulalu irrigation project is to being established in Tana River County, Kenya. Clearly outline the importance and any disadvantages of such a project.
1.3 A commercial farmer has consulted you on water productivity issues. Describe what is deficit, total and supplemental irrigation and how they can be applied to the commercial farm.
1.4 Give a breakdown of the uses of irrigation water by the plants on farm fields.



The objective of chapter 2 is to offer knowledge and skills on:

Introduction to irrigation systems planning concepts
Irrigation planning process and stages
Feasibility study and factors affecting irrigation projects
Key differences between Irrigation and drainage systems
Review questions

2.1 Irrigation systems planning

Irrigation systems planning is a process that involves evaluation of potential alternatives of interaction between soil, water, plants, air, animals, and human beings and its associated on-site and off-site environmental impacts. To accomplish the irrigation planning process a nine step procedure may be applied as illustrated in Figure 2.1.

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Figure 2.1 Schematic representation of nine step irrigation planning process

Identification of possible opportunities/challenges: In this step the possible water sources, water quality, quantity, energy and labour sources, soil erosion for the perceived irrigation project are evaluated.

Identification of the objectives: The community resources of concern including water needs and its sustainability are assessed.

Inventory of resources: this involve examination of soil types and its variables, the plant and animal resources, availability of irrigation systems, labour, drainage and salinity within the land where irrigation is planned.

Analysis of the resource data: This is where the effect of exploiting each resource on the other resources is determined for the purpose of developing mitigation measures.

Formulation of alternatives: In this step, analysis of appropriate irrigation methods, components and systems, scheduling methods is conducted for the purpose of optimizing the mixture of the water application methods.

Decision on water use: Evaluation of potential impacts of water extraction and drained water from irrigation is done with respect to the environment, skilled and non-skilled labour availability.

Implementation of the decision: in this level, the type of irrigation method to use, components and management approaches selected

Monitoring and evaluation: This is a continuous process where the on-going activities are checked against the results of the plan implementation, on-site and off-site; frequently revise the planning as needed for continual improvement.

An irrigation system plan can in addition be designed by adopted a few of the above parameters and appropriately combining and networking them to achieve an overall objective. The effect and selection of the best option of a system plan is considered as an overall farm production and conservation plan that includes and aims at:

i) Sustainable soil condition improvement
ii) Improvement of quality and quantity of both surface and ground water
iii) Provision of favourable conditions for plant growth without degradation of other resources
iv) Minimizing impact of soil erosion and deposition
v) Provision of the needs of favourable conditions of flora and fauna

2.2 Irrigation planning process

The planning process for proper irrigation management system involve the following steps

(i) irrigation systems design, installation and component
(ii) soil, crops and tillage management
(iii) irrigation systems operation and maintenance
(iv) water management

2.3 Field water balance-based plan

A data function that shows all the components of water resources utilization, distribution and losses for a particular farm is called water balance. This is a critical irrigation planning tool as it used to determine

i) the time to irrigate and quantity of water used by the crops
ii) time and quantity of water available and or supplied from different sources
iii) amount of water going to losses from root zone via deep percolation, runoff among others

The water balance data may be available on yearly, monthly or daily bases. For easy design, operation and management of irrigation systems daily data is preferred. The following function is used to represent field daily water balance on crop land

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Where, Fg =Gross water requirement during a defined period

ETC=crop evapotranspiration

SDL=spray and drift losses from irrigation water in air and evaporation from plant canopies and other surfaces

RO=surface runoff that leaves the field during irrigation period

P =total precipitation during the period

GW=ground water contribution to the crop root zone during the period

ΔSW=change in soil water in the crop root zone (it may be plus or minus)

Dp =deep water loss

The above equation for gross water determination provides for all water losses. However, if net application is used instead of gross value, then losses would be estimated by use of average irrigation efficiency. (Irrigation planning aids to be put on appendices). Sometimes the equation might be modified by addition of auxiliary (Aw) water requirements to the right side. This refers to other crops and soil water needs such as leaching requirements for salinity and sodicity management, seed germination, plant disease control, wind erosion and dust control, crops and soil cooling.

Irrigation may be seen as a special case of intensive agriculture in which technology intervenes to provide control for soil moisture regime in the crop root zone. The aim of irrigation is to achieve a high standard of year-round agriculture. Irrespective of rainfall availability, Technology intervenes by:

(i) Adding water when it is needed by the crops
(ii) Removing excess water from the soil profile
(iii) Removing excess water from the surface of the cropped land.
(iv) Protecting the cropped land from flooding (from higher lands and from adjoining rivers)

When planning for any irrigation, it is important to establish whether it will be done in all years or some of the years and all year round or just part of the year. in this context total or supplemental irrigation may be applied.

2.4 Fundamental factors in development of irrigation

For the purpose of developing irrigation schemes and or projects, technical, economic, social and political factors should be considered. Based on technical aspects, irrigation development can be said to be feasible or unfeasible. The former means that a proposed irrigation project is acceptable while the latter means it is rejected since it cannot technically be successful.

2.5 Feasibility Study of irrigation projects

This is expert advice based on a concise presentation of all relevant information which must be sought before commitment to investment in a proposed development. The type of project under consideration governs the intensity of feasibility study. The scale of an irrigation development project is determined by the relative disposition of the land and water resources and can vary from millions of hectares to a couple of hectares. The importance of scale of irrigated agriculture varies for different regions of the world and is a very relative concept. It can be depicted by size as in the case of Africa; south of the Sahara or by cost in other regions of the world. The Food and Agriculture Organization (FAO) of the United Nations (UN) categorizes irrigation projects for Africa South of the Sahara as shown in Table 2.1.

Table 2.1 Categories of irrigation projects

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The process by which irrigation development in a given locality is rendered feasible or unfeasible given a set of conditions and environmental factors is termed “irrigation planning” The first part of the following discussion will introduce the technical factors and criteria underlying irrigation planning for Kenya and any other part of the world. General economic human social and political considerations underlying the irrigation planning will in addition be discussed. .

2.5.1 Technical and physical factors

Certain conditions of technical and physical nature should be met and are determinant for the technical feasibility of irrigation and drainage projects such conditions are often standard and can be considered as pre-requisites to every irrigation development in whatever part of the world. The major technical aspect of irrigation planning is briefly considered below.

i) Availability of skilled labour: the source of technical labour such as setting up, operation and maintenance of automated systems, fertigation, irrigation interval and gross application of water at maximum efficiency.
ii) Crops: information of the present potential agricultural production is needed. The types and varieties of crops grown, crop yields suitability to climate soil, farming practices and their value to the community. Also to be included are areal distributions and area of each crop to be grown. Planting dates and how the stakeholders will receive investigations on proposals for a new pattern of irrigated agriculture.
iii) Climate: The amount of rainfall and its annual distribution may or may not justify the need for irrigation (or supplementary irrigation) Climatological data are further required to estimate the crop water requirements. The variability and probability distribution of rainfall should also be taken into consideration.
iv) Soils: Soils suitability is often one of the major constraints to irrigation. Soil texture structure, fertility, infiltration, and water holding capacity salinity, alkalinity and susceptibility to erosion should all be considered and play a decisive role in the selection of irrigation sites.
v) Quantity of available water: A survey of both land and water resources are essential. Water is one of the most obvious pre-requisites to irrigator. Sixty to ninety five percent of physiologically active plant tissue is water. Water is required for such plant processes as:

(i) cell and tissue metabolism
(ii) photosynthesis
(iii) transport of minerals and photosynthesis
(iv) structural support
(v) Growth and transpiration
(vi) Cooling of the plant

The important considerations in water availability are the amount of water available, the minimum flow and the seasonal distribution. Equally important is whether or not extraction from the source can be done by gravity and the technical implications of conveyance and distribution system. If storage facilities are required there is need to know the availability of feasible dam sites. If ground water is to be the main source of water the safe yields should be known. Due to lack of adequate hydrological information in Kenya water availability is often one of the most difficult and uncertain aspects of irrigation development planning.

vi) Water quality: The water source(s) to be used for irrigation should not contain any matter which is harmful to the crops to be grown (it should be free from pollution). The salt concentrations of irrigation water should be within the limits of crop tolerance and should not affect the soil structure due to sodium or alkalinity hazard. Moreover, the quantity and content of sediment may cause excessive siltation in structures, canals, reservoirs and even may completely block the head works. It should be determined early enough whether sand traps settling tanks and other desiltation devices are required for an irrigation system.
vii) Topography : The suitability of land topography for irrigation should be evaluated well in advance as the irrigation site. The type of irrigation, which best suites the prevailing topography, should be identified. The risk of soil erosion and the need for soil conservation measures necessary should be considered. Land leveling if necessary should be taken into account and the impact this will have on soil depth and structure.
viii) Drainage and flood protection: It should be considered whether excess water could be adequately removed from the irrigation area. Also important is whether the groundwater table can b kept at a level which is not harmful to the crop at least during the cropping season. Also to be considered as determinant for the technical feasibility for irrigation development is whether it is technically possible to protect the area against seasonal flooding.
ix) Infrastructure: The availability of transport and communications within the project area and linkages with external markets is of major importance in irrigation planning. The irrigation area should be accessible (for bringing in construction material and for marketing the produce) and should be provided with some basic infrastructural facilities (stores shops offices). The environmental conditions should be suitable for housing and free of diseases (in particular water and vector-borne diseases). Also important to be considered is the availability of credit facilities and whether agricultural inputs can be adequately supplied to the farmer.

2.5.2 Economic factors

When planning irrigation projects, the economics of every irrigation activity should be considered. This is due to the fact that economics will influence the final decisions. The following economic aspects should be taken into account

i) Costs and benefits: Economic and financial analysis for an irrigation project should be carried out to determine total irrigation project costs and projected benefits. The benefits should be higher than total discounted costs which include purchase, operation and maintenance of the project. The ratio of total benefits to the total cost also called internal rate of return is an index for preliminary economic viability of the irrigation project. The index is mainly used to make a choice between two or more irrigation projects when capital resources are limited. The higher the return index, the more the profit from the project. Based on the national economy, costbenefit analysis should be done to give detailed economic implications of an irrigation project.
ii) Financial analysis: The financial aspects to be provided to the farmers should be evaluated. These may include; total monetary requirement, sources of project funds, marketing chains, availability of loans and their interests and national pricing policy. Irrigation projects that run at a loss may not be allowed to develop. However, under special circumstances, it might still be more advantageous and cheaper for the Government to subsidize it rather than to carry on with famine relief. The two options may be economically analyzed to select a cheaper option between food from the irrigation project and the one to purchase by government as relief food during famine.
iii) Project or family income: The actual increase of cash income in at family or project level is one of the most important incentives form starting an irrigation project. Family cash income is farm income less production cost and substance requirements including storage losses. Water charges and repayment of loans should be in reasonable proportion to the estimated cash income.
iv) Availability and cost of labour: The costs of the available labour greatly affect the profit of any irrigation project. It is a key factor in production and should be investigated when irrigation project is being conceptualized.

2.5.3 Social factors

Social and human factor are the most difficult to deal with when evaluating the feasibility of an irrigation project especially since they are not easy to value in monetary or economic terms. They are nevertheless extremely important and often prevail over technical and economic consideration. Some of the social and human factors which may adversely affect the implementation and operation of irrigation projects include:

(i) Land tenure or Ownership: Land tenure or ownership in irrigation project has too often been the cause of complete irrigation failures. Land tenure problems must be solved prior to the implementation of an irrigation project. The proposed solutions should be acceptable to all parties and leave no doubts over the rights and responsibilities of the landowners. In case of communal land ownership it may be desirable to allocate plots to scheme members or family clans. Traditional land use rights should be respected as much as possible in particular in the case of trust land. Proposed irrigation projects which are expected to be profitable are sometimes not considered feasible due to disputes over land ownership. Land consolidation may have to be considered in certain situation but this has always proved to be a delicate issue and should be avoided as much as possible.
(ii) Organization: Although not considered a typical problem organization is one of the crucial factors of an irrigation scheme. Scheme organization and in particular maintenance has often caused the failure of initially well-designed productive schemes. For each type of irrigation scheme the scheme members should accept organization and management proposals, whether owners or tenants and their rights and duties clearly define. It will often be desirable to lay down these management rules and regulations in a special By-law or Act. Depending on the form of organization of the scheme. Responsibilities of irrigation committee manager and his staff should also be clearly defined. If no agreement about a suitable form of organization can be reached between future scheme members the overall viability of the scheme becomes doubtful and alternative forms of organization must be proposed. If not the project may have to be abandoned altogether. Farmers in an irrigation scheme normally have to share the scarce production resource water. In particular when farmers do not pay for water or in areas where irrigation is being introduced to them water distribution rules should be strictly adhered to. The management should be vested with all powers to act against contravention of the rules.
(iii) Motivation: Although the farmers motivation is often difficult to assess there must be certain indications that local people are interested in a proposed scheme and are prepared to cooperate with the Government Farmers who have already practiced a kind of traditional irrigation are generally more motivated than people who are predominantly pastoralists. It should be noted that an improved standard of living (higher cash income) might not always be a sufficient proposals and extension to local people at an early stage of the project may contribute much towards a better understanding and increased motivation of the farmers.

2.5.4 Political factors

Political and government policies significantly affect irrigation projectsat different level and in a number of ways including

(i) Institutional infrastructure: institutional framework and infrastructure is key to evaluating policy issues with regard to design, construction, operation, maintenance, cost recovery and administration of irrigation projects.
(ii) Product market: there should be a reliable marketing system (marketing institutions) for agricultural products that come from an irrigation project. Farmers should be encouraged to grow high value products that give them maximum gain after marketing the products.
(iii) National policy and priority: Government intervention promote irrigation development as a way of ensuring food security in a country.
(iv) Tribal issues and management accepted by farmers.
(v) Introduction of new crops and varieties not known by the local people
(vi) Not sufficient possibilities to keep livestock in the vicinity of the scheme
(vii) Loss of confidence in the Government due to delays in implementation
(viii) Too high water rates or too low (gazette) producer prices and not sufficient freedom in marketing of their products.
(ix) Insufficient protection against wildlife and cattle rustlers.
(x) Better paid employment available outside the irrigation scheme.

2.6 Irrigation systems verses drainage system

Irrigation engineering involves the design of two complementary supply and drainage system. The systems have the task of conveying water from a source and timely distributing it equitably over an agricultural area. The drainage system removes unwanted drainable excess water which has passed from field supply ditch onto the field, through the plant-soil system and into the field collector drains. On a broad scale irrigation is more than just supplying the plants with the water they need. Major irrigation projects have had a profound effect on the farming community. They also have attenuated effects spreading through other regions. The economic and social patterns of life may be radically altered. Nomadic people may be settled ancient values and customs set aside and the appearance of money consciousness where none existed before. All implications of a proposed irrigation development should be borne in mind before deciding to proceed.

Review Questions 2

2.1 Explain the main steps undertaken in irrigation systems planning
2.2 Describe five main aspects of irrigation planning listed under (i)Technical (ii)
Economic and (iii) Social and political factors
2.3 Categorize and discuss seven main variables used in determination of gross water requirement in an irrigated field
2.4 A small-holder irrigation scheme is to be developed in Kirinyaga County, Kenya. Illustrate the main technical and physical factors that should be considered for its feasibility study



The objective of chapter 3 is to offer knowledge and skills on:

Introduction to factors that influence crop water storage and extraction
Crop water extraction based on 4-3-2-1 rule
Key soil parameters that influence irrigation
Infiltration processes and infiltration models
Examples, solutions and review questions

3.1 Introduction

Soil-plant-water relationship is the study of the interaction between soil as a media of water storage for the crops to extract to meet their evapotranspiration or consumptive use needs.

3.2 Soil

Soil is a media of storage for plant nutrients, habitant for plant roots, soil organisms and a reservoir for water required to meet plant evapo-transpiration demands across different growth periods. The quantity of water that can be held by any soil is a function of its physical and chemical properties. This quantity of water determines the longest period of time that a plant can be sustained before an irrigation event or rainfall without significantly affecting the crop yield. Thus the type of soil influences the frequency of irrigation, quantity of water to be applied for optimal crop production.

3.2.1 Plant root zone

The depth of crop root zone is important in planning irrigation scheduling program. The crop root zone is affected by the soil texture, presence of hardpan, crop type and level of bed rock. For most annual crops, root development is assumed to depict a linear increase in rooting depth from germination to maturity. The root zone influences the water uptake by crops based on a 4-3-2-1 rule described below.

3.2.2 Crop water extraction based on the 4-3-2-1 rule

The 4-3-2-1 water extraction rule by crops provides a general trend of water uptake levels from different portions of the root zone. The rule applies when most of the root zone is irrigated to field capacity and depends upon the distribution of the root mass. The 4-3-2-1 rule requires that the crop root zone be partitioned into quarters as shown in the following figure.

Abbildung in dieser Leseprobe nicht enthalten

Figure 3.1 portions of the crop root zone

Beginning from the top quarter downwards:

(i) The first (upper) quarter provides 40 percent of water extraction by crops
(ii) The second quarter provides 30 percent of the water uptake by the crop
(iii) The third quarter provides 20 percent of the water extraction
(iv) The last (lowest) quarter provides 10 percent of the water uptake by a crop

The rule provides critical information regarding the key root zones where water application should take place. For instance, applying water in top and second quarter provides approximately 70 percent of the water extraction by the crops. However, applying water to the lowest quarter of root zone allow for only 10 percent water extraction, thus most water in this portion may be a loss.

3.2.3 Physical properties

Soil comprise of mixture of inorganic particles, decaying organic matter, air, water and living organisms. The parent material of mineral soils consists of weathered rocks or unconsolidated sediments. When the parent rock is weathered over long period of time, the resulting fragments combine with the other mentioned materials to form soil layers. The characteristic of these layers influence the root growth, retention and transmission of water in the soil. There are two main physical properties of soil; texture and structure. These soil properties influence the permeability of soil with respect to water, air and root systems. The permeability of soil is greatly dependent on porosity and how the pores are distributed within the soil mass.

3.2.4 Soil texture

Soil texture refers to the size and range of the soil particles and their distribution. Most soils consist of mixture of soil particles with different sizes such as sand (0.05-1.0 mm in average diameter), silt (0.002 to 0.05 mm) and clay (less than 0.002 mm) Table 3.1 shows the textural classes of different soil particles. The texture of the soil influences the water flow, aeration, the rate of chemical processes and water holding capacity.

Texture is a term used to describe the predominant particles in any soil type that may be stated as large, small or intermediate sizes. In addition the term is used classify soil into coarse and gritty or fine and smooth depending on feel by hand. The usual practice is to measure distribution of soil particle sizes or the proportion of the various size ranges of the particles which occur in a given soil to define its texture. Distinct broad textural fractions are

i) Sand
ii) Silt and
iii) Clay.

The procedure of separating these fractions and measuring their proportions is called mechanical analysis. The results of mechanical analysis are the mechanical composition; a term used interchangeably with soil texture. Various groups and institutions have devised various criteria of classifying soil particle size. These include:

(i) United states Department of Agriculture (USDA)
(ii) American Society for Testing Materials (ASTM)
(iii) Massachusetts Institute of Technology (MIT)
(iv) German Standards (DIN)
(v) British Standards Institute (BSI)

A commonly used textural classification method is the USDA classification. USDA classification scheme requires information on predominant sand, silt, clay and their particle sizes as showed in Table 3.1.

Table 3.1 Particle size of different soil types

Abbildung in dieser Leseprobe nicht enthalten

A plot of percent total sample weight against the sieve size results in grain size distribution curve provided by the USDA. A soil sample is said to be poorly graded if the particles fall predominantly in one size category or well graded if the sample tends to have a significant percentage of various particle sizes. Once the percent by weight of sand silt and clay have been determined the USDA textural triangle (Figure 3.2) is used to classify the soil into the relevant textural class.

Abbildung in dieser Leseprobe nicht enthalten

Figure 3.2 Textural triangle (Source USDA-NRCS, 1999)

3.2.5 Soil structure

Soil structure is described by the manner in which individual particles of sand, silt and clay are assembled. When single particles are assembled together, they may appear as larger particles than original particles, and are called aggregates. The soil aggregates form different patterns that form different soil structures. The soil structure influence soil water movement, aeration and soil workability. The soil structure and its granules may be broken down due to excessive irrigation, ploughing or compacting the soil when it is too wet. The structure of an irrigated soil can be maintained and improved by proper farming practices which include;


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Irrigation Principles. Theory and Application
Egerton University
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irrigation, principles, theory, application
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Raphael Muli Wambua (Author), 2019, Irrigation Principles. Theory and Application, Munich, GRIN Verlag,


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