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Water Management in Irrigated
Agriculture: A Training Manual for
Technical Staff
Improved Management of Agricultural Water in Eastern & Southern Africa
(IMAWESA)
IMAWESA TRAINING MANUAL No. 4
Compiled by:
Edward M. Muya, Moses O. Jura and Bancy M. Mati
September 2009
Citation
Muya, E. M., Jura, M. O. and Mati, B.M. 2009. Water management in irrigated agriculture: A training
manual for technical staff. IMAWESA Training Manual No.4. Improved Management of Agricultural
Water in Eastern and Southern Africa, Nairobi.
Contact us through:
The Programme Manager
Improved Management of Agricultural Water in Eastern & Southern Africa (IMAWESA)
IFAD Complex, United Nations Avenue, Gigiri,
P. O. Box 39063-00623, Nairobi, Kenya
Tel: +254 20 722 4110; Fax: +254 20 722 4001
Email: [email protected]; Website: www.imawesa.net
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About this Publication
IMAWESA (Improved Management of Agricultural Water in Eastern and Southern Africa) is
designed to improve and strengthen the sharing of knowledge, information and best practices
emanating from research, field experiences and the farmers themselves, in implementing
development programmes in agricultural water management (AWM). This is considered to be critical,
both for enhanced programme design and implementation, and for providing the substantive basis
upon which to engage in policy dialogue and influence investment support for AWM. The main
elements of IMAWESA include; enhancing policy for agricultural water management, studies on
key water management issues, capacity building, learning visits and workshops for programme
managers and staff, as well as building a community of practice in AWM through knowledge sharing
and networking. The project works directly in sample countries but its products cover 23 countries in
Eastern and Southern Africa (ESA) region, which include Angola, Botswana, Burundi, Comoros,
Democratic Republic of Congo (DRC), Eritrea, Ethiopia, Kenya, Lesotho, Madagascar, Malawi,
Mauritius, Mozambique, Namibia, Rwanda, Seychelles, South Africa, Sudan, Swaziland, United
Republic of Tanzania, Uganda, Zambia and Zimbabwe.
This manual was prepared as part of the training material used during the Training of staff of the
Post Conflict Transition & Reconstruction Programme (PCTRP) of Burundi, on agricultural
water management (AWM), conducted in Gitega, Burundi from 7th to 11th September 2009. The
training was for middle level technical staff comprising Agricultural Engineers, agriculturalists,
Hydrologists and Forestry specialists. The training was based on theory and practice with field
visits and exercises and was meant to build the technical capacities to plan and implement AWM
projects. This Training Manual has been compiled by resource persons who organized and
provided the technical training during the workshop. The information contained here is not
exhaustive and thus, readers are encouraged to seek further information from references cited as
footnotes in this publication and elsewhere.
This particular publication targets technical staff and middle level decision makers such as
extension workers, managers and implementers of programmes and projects, researchers,
development partners, public and private practitioners of irrigation. It is meant to inform,
educate and enhance knowledge and practice as regards irrigation in the region. This manual,
alongside other reports by IMAWESA are freely available on the internet as public goods and
can be downloaded from www.imawesa.net. For further information and comments on this
report, readers are welcome to contact [email protected].
Acknowledgement
The publication of this booklet was supported by the International Fund for Agricultural
Development (IFAD), while Kenya Agricultural Research Institute and Ministry of Water and
Irrigation provided technical experts to the manual. IMAWESA is a regional project supported
by IFAD and implemented by ASARECA in collaboration with ICRISAT and national
programmes on AWM. The authors wish to thank all the institutions and individuals who
supported the information and publication of this manual. The views expressed here are not
necessarily those of IFAD, as the content is solely the responsibility of the authors.
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Table of contents
1. INTRODUCTION ........................................................................................................... 1
1.1. Main focus........................................................................................................................................ 1
1.2 Major factors considered in planning an irrigation system ........................................................ 1
1.3 Hydrologic cycle ............................................................................................................................... 5
1.4 Water availability for irrigation....................................................................................................... 6
1.5 Rivers as sources of irrigation water ............................................................................................. 7
1.6 Analysis of hydrological data for irrigation .................................................................................. 8
1.7 Sedimentation ................................................................................................................................... 8
1.8 Use of analogue and digital maps ................................................................................................ 11
1.9 Conjunctive use of surface water and groundwater .................................................................. 11
2 CLIMATE, HYDROLOGY AND GEOMORPHOLOGY ......................................... 12
2.1 Climatic, hydrologic and geomorphologic characteristics........................................................ 12
2.2 Hydrological processes and geomorphic characteristics .......................................................... 12
2.3 Rainfall, Runoff and Water balance............................................................................................. 13
2.4 Influence of climatic variables on crop water requirements .................................................... 14
2.5 Methods of determining reference crop evapotranspiration (ETo) ........................................ 15
2.5.1 Pan Evaporation Method ...................................................................................................... 15
2.5.2 Blaney-Criddle Method ......................................................................................................... 15
2.5.3 Radiation Method ................................................................................................................... 17
3 DESIGN OF IRRIGATION SYSTEMS...................................................................... 19
3.1 Data requirements and preparations ........................................................................................... 19
3.1 Procedures for calculating irrigation water requirements ........................................................ 19
3.2 Effect of growth stage on crop water needs .............................................................................. 21
3.3 Irrigation efficiencies ..................................................................................................................... 22
3.4 Assessing soil parameters .............................................................................................................. 24
3.5 Flow measurement in canals ........................................................................................................ 24
3.6 Design of canals ............................................................................................................................. 25
4 IRRIGATION METHODS AND THEIR APPLICABILITY ................................... 31
4.1 Factors considered in selection of irrigation system ................................................................. 31
4.2 Choice of irrigation method ......................................................................................................... 31
4.3 Surface irrigation systems ............................................................................................................. 32
4.4 Borderstrip irrigation ..................................................................................................................... 33
4.5 Basin irrigation................................................................................................................................ 35
4.6 Furrow irrigation ............................................................................................................................ 38
4.6 Sprinkler system ............................................................................................................................. 40
4.7 Localized Irrigation systems ......................................................................................................... 41
4.8 Drip Irrigation ................................................................................................................................ 41
4.9 Bucket irrigation ............................................................................................................................. 42
4.10 Pitcher Irrigation .......................................................................................................................... 43
5. DRAINAGE TECHNIQUES ...................................................................................... 44
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5.1 The need for drainage ................................................................................................................... 44
5.2 Factors that affect drainage .......................................................................................................... 44
5.3 Design of agricultural drainage systems...................................................................................... 45
5.4 Surface drainage ............................................................................................................................. 47
5.5 Subsurface drainage ....................................................................................................................... 50
5.5.1 Horizontal subsurface drainage ............................................................................................ 50
5.5.2 Vertical subsurface drainage ................................................................................................. 53
5.5.3 Salt problems in irrigated agriculture ................................................................................... 53
6 OPERATION AND MAINTENANCE OF IRRIGATION SCHEMES ................. 54
6.1 Participatory irrigation development ........................................................................................... 54
6.2 Participatory and strategic change process ................................................................................. 54
6.3 Identification of stakeholders ....................................................................................................... 55
6.4 Participation of farmers in planning and design........................................................................ 55
6.5 Farmer participation in irrigation development......................................................................... 56
6.6 Farmer participation in the implementation of an irrigation scheme..................................... 57
6.7 Operation and maintenance responsibilities .............................................................................. 57
6.8 Monitoring and evaluation of smallholder irrigation schemes ................................................ 57
6.9 Indicators of sustainability ............................................................................................................ 58
7 SUMMARY CONCLUSIONS AND RECOMMENDATIONS ................................ 59
8. REFERENCES ............................................................................................................. 60
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List of Figures
Figure 1: The Hydrological cycle....................................................................................................................... 6 Figure 2: Examples of streamflow hydrographs ................................................................................................. 7 Figure 3: Sedimentation in a reservoir created by a dam .................................................................................... 9 Figure 4: Illustration of the partitioning of rainwater in relation to irrigation ................................................... 13 Figure 5: Water balance and irrigation water (Source: Barron et al. 1999) ..................................................... 14 Figure 6: Class A Evaporation method (circular pan) .................................................................................... 15 Figure 7: Illustration of water distribution system through irrigated fields......................................................... 20 Figure 8 Cross sectional view of a canal .......................................................................................................... 25 Figure 9: Canal parameter for different cross-sectional shapes .......................................................................... 25 Figure 10: Sample calculations of hydraulic parameters for different canal shapes ............................................. 28 Figure 11: Variety in surface and ground water abstraction ............................................................................ 31 Figure 12: Border strip irrigation layout (Source: Kay, 1986) ......................................................................... 34 Figure 13: Cross-sectional view of border strip irrigation ................................................................................. 34 Figure 14: Layout of basin irrigation system (Source: FAO, 1985) ............................................................... 36 Figure 15: Orientation of feeder canals for basin irrigation .............................................................................. 37 Figure 16: Sketch of furrow irrigation and cross-section................................................................................... 39 Figure 17: Typical soil moisture distribution in a sandy soil (left) and a clay soil (right) (Source: Kay, 1986) .. 39 Figure 18: Sprinkler irrigation method........................................................................................................... 40 Figure 19: Schematic representation of drip irrigation in the field..................................................................... 42 Figure 20: Illustration of pitcher pot irrigation ................................................................................................ 43 Figure 21: Flow chart indicating drainage design ............................................................................................ 46 Figure 22: Rainfall- Duration curve .............................................................................................................. 48 Figure 23: Subsurface drainage systems at field level ....................................................................................... 50 Figure 24: Subsurface drainage parameters ..................................................................................................... 51 Figure 25: Nomograph to determine equivalent sub-stratum depth (Source: FAO, 1985) ............................... 52 Figure 26: Salt accumulation in the root zone showing effect of capillary rise .................................................... 53 v
Term
Definition/Brief description
Coefficient of
variation (CV)
A mathematical measure of the variability of runoff from year to year.
It is the ratio of standard deviation of annual inflow to the mean
annual inflow.
Drainage
The process of managing excess surface water and controlling water
logging from shallow water tables.
Evaporation (E)
the annual net water loss from a free water surface (mm)
Evapotranspiration
(ET)
The sum of water lost from an area through the combined effects of
evaporation from the ground surface and transpiration from the
vegetation.
Gravity-fed Irrigation The type of irrigation in which water is available or made available at a
higher level so as to enable supply to the land by gravity flow.
Irrigation
Any process, other than by natural precipitation, which supplies
water to crops or any other cultivated plants
Irrigation efficiency
It is the ratio of irrigation water consumed by the irrigated plants to
the water delivered from the supply source.
Overhead irrigation
A method of irrigation water application in which the water is ejected
into the air to fall as spray on to the crops or on the ground surface.
Supplementary
irrigation
Providing additional water to stabilise or increase yields where a
rainfall is insufficient for crop growth
Surface irrigation
The supply of irrigation water to the ground surface for crop use.
Examples are: basin, border, furrow, corrugation, wild flooding, and
spate
Water Application
efficiency
The ratio of water applied as net increase in soil moisture in the crop
root zone to the total amount of water applied at the field level.
Water Conveyance
Efficiency
The ratio of water delivered in the fields at the outlet head to that
diverted into the canal or pipe system from the source.
Water logging
State of land in which the water table is located at or near the surface
resulting in poorly drained soils, adversely affecting crops production.
Drainage can be used to solve the problem
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1. INTRODUCTION
This Training Manual summarizes the major components of irrigation planning, design,
development and management and the requisite factors considered. It is meant to improve the
skills of engineers, technicians, managers and practitioners of irrigated agriculture, especially
those working in scheme-based and smallholder irrigation in Africa. The manual is meant to
enhance human capacities in; (i) estimation of water flows within a system, (ii) procedures for
calculating irrigation water requirement and application in design, (iii) irrigation and drainage
methods, (iv) factors affecting choice of irrigation systems, (v) irrigation design, planning and
development, and (vi) improving stakeholder participation in irrigation planning, development
and management and facilitating sustainability.
1.1. Main focus
This training manual puts together technical notes, some sample calculations, figures, tables and
examples so as to capture pertinent factors influencing the design and management of irrigation
infrastructure, its applications and selection of best practice for each system. The training manual
targets theory and practice, covering the following key areas:
o Analysis of climatic data
o Analysis of hydrological data and water requirements
o Irrigation and drainage network design
o Implementation of irrigation and drainage
o Management and maintenance of irrigation schemes.
To make more meaningful use of these notes, users are encouraged to:
a) Seek and understand the long-term, medium and short term plans of the country and
organization for which they are working on irrigation and drainage,
b) Prepare a personal medium, short term and immediate (daily) plan of work,
c) Involve farmers and other key stakeholders in the planning of activities in their respective
areas of work and to subsequently ascertain farmers’ participation in the entire project
cycle.
d) Exploit possibilities of organizing periodic consultative fora with field staff, expose them
to the strategic plan of the institution and to review and revise with them the procedures
followed in project preparation, its duration and team composition; including approval of
designs.
In making use of this manual, some practicals, illustrations, worked calculations and case study
examples are necessary as well as field visits and discussions so as to expose users to the real
challenges and actual solutions. At the end of the training, the trainees should acquire skills that
enable them manage agricultural water in irrigation projects. More specifically, the manual equips
the reader with knowledge on how to:
o Measure the discharge/ flow of water,
o Improve management of irrigation schemes,
o Apply knowledge and skills gathered in the respective working areas.
1.2 Major factors considered in planning an irrigation system
Irrigation development should be based on convincing evidence of beneficial outcomes and
sustainability of the system. Feasibility studies provide a means for assessing developmental
options for investment, in this case, irrigation or controlled drainage project. A feasibility study
for irrigation development assesses the physical aspects of land, water and climate, and evaluates
crop production potential and cropping programmes within the context of the physical aspects.
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The same study reviews and analyses 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 assessed. In summary, the
feasibility study should provide options for the client with recommendations for the best option
combining technical feasibility, financial and economical viability, social desirability and
environmental sustainability.
For irrigation projects, the feasibility study should cover the following components; (i) climate
and natural resources, (ii) agriculture/agronomic issues, (iii) credit and marketing, (iv)
engineering and infrastructural components, (v) social/political issues, (vi) organization and
management aspects, (vii) health and environmental aspects, and (viii) economic and financial
analysis of the project. These components are presented briefly as follows:
Meteorological data
Normally, 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 is an important factor in crop production. Different crops have different requirements in
terms of temperature, humidity and light. Also, occurrence of frost and hail stones at certain
times may exclude a number of crops from the cropping programme. Generally, 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.
Topography
The topography of the land when combined with the soil characteristics will provide the means
of assessing the irrigability of the land and selection of the most suitable areas for irrigation. In
this respect, soil and topographic surveys provide the means for this assessment.
Water resources
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, hydro-geological 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.
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
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responsible for regulating abstraction (Field and Collier, 1998). 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.
Existing production systems
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. The
existing agricultural practices are assessed to analyze the without-project situation. Data are
gathered from baseline socio-economic surveys. The data are aggregated to reflect the average
production cost and gross margins and incorporated in the financial and economic analysis. The
same surveys provide information on the availability of family labour in both rainfed and
irrigation farming and assess the need for hired labour.
Land tenure
Land tenure under smallholder agriculture varies across countries in Sub-Saharan Africa. In some
countries, smallholders have communal rights to land, while in others, smallholders own the land
which may have title deeds. 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.
Credit and marketing
In most cases, 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
investments. Consequently, there is need for credit and/or subsidies for farmers to cope. It is
therefore necessary that the feasibility study reviews potential options and makes
recommendations under the prevailing socio-economic circumstances in the scheme. The choice
of crops to be grown and the cropping patterns influence the field layout and irrigation method.
However, the choices of crops as well as the cropping programmes are influenced by their
market potentials. Therefore, an assessment of the existing markets, transport system/road
infrastructure, including their potential for development, should be made. On the output side,
market prices, transport costs and farm-gate prices must be predicted, as relates to expected
increased volume of production. Processing and storage facilities should be included and
planned as part of the overall marketing strategy.
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Engineering infrastructure
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, on-farm
irrigation works and drainage. For these and other engineering works, preliminary designs are
made and cost estimates prepared. In addition, water allocation and availability should be
calculated while the selection of the water application equipment e.g. sprinklers, drip lines as well
as drainage facilities should be designed.
Social aspects
The project’s objectives and expectations can not be realized unless farmers’ considerations on
benefits and costs, feasibility and desirability and their priorities in life match those of the
project. 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 of the community. 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. Sometimes, irrigation development can bring about cultural shocks to a
community. It may mean more work since compared to rainfed systems, farmers work for a few
months in a year while irrigated crop production may require almost daily attention throughout
the year. Thus, irrigated agriculture could mean less time for social aspects of society. The ability
of a community to adjust to these and other changes are critical and should be discussed with the
farmers.
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. This requires the services of
professionals to coordinate and supervise the works of all involved in the planning and
implementation of the project. The same professionals, through established procedures, should
be responsible for the selection of the contractor and sub-contractors. Sometimes the selection
of inexperienced contractors on the basis of a cheaper offer does not always cost less. Delays
from one contractor can have far reaching effects on other contractors and on the project
delivery as a whole.
Organization and management
An analysis of the structures and competences of the agencies or persons responsible for the
organization and management of the project is usually done. Constraints should be expected and
should be factored into the planning, construction and operation of projects. This calls for the
presence or establishment of competent agencies to manage the planning and implementation of
the project.
The organization of operation and management
Irrigation development, especially in sub-Saharan Africa, can be expensive. It is therefore
necessary for investments to be utilized productively and as soon as possible. Thus, provisions
should be made from the feasibility study stage onwards for the needed trained engineers,
agronomists, technicians and social workers and time considerations. Equally important is the
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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 in-field part of
the system, and long-term sustainability of the project.
Extension services
The training of farmers and the adoption of new farming practices is handled by extension
workers. However, most extension agents in sub-Saharan Africa are not familiar with the
intricate requirements of irrigated crop production, as it comes with new crops and handling
systems. Hence the need to assess the level of extension know-how and provide for the training
needs of the extension staff. While the success of achieving the desirable results will greatly
depend on the adaptability of farmers, this means developing and implementing appropriate
training packages for them. Establishment of on-farm research, demonstrations, farmer field
schools and provision of advisory services with back up from specialists should be considered.
Health and environmental impact Assessment
Very often, the health and environmental aspects of irrigation development are not given the
deserved attention in feasibility studies. Water-related diseases affect the health of 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 ecosystems at large.
Justification for irrigation development
The financial analysis evaluates the project’s capability to repay the investment and the operation
costs of the project. In other 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. For more details the reader is referred to Module
Maintenance
The maintenance of the existing water control structures and protecting them against heavy
water flows is very important. The strength and capacity of irrigation and drainage structures
should be based on designs that have taken into consideration the magnitude of run-off flows,
using peak storm/runoff rates whose return period is calculated through detailed analysis of
available climate data as well as biophysical and hydrological characteristics of the catchment. As
is observed by Withers and Vipond (1974), irrigation is a reliable way of achieving food security.
However, it has failed in certain regions of the world either because of lack of knowledge in
appropriate water management, or inability of the existing technology and know-how to handle
problems resulting from erroneous irrigation practices. Draining the excess water from
marshlands requires technical know-how, failure of which would lead to overall project failure in
the long-run. In this context, managers and engineers concerned with design, construction and
operation of the irrigation schemes should be trained in the design of irrigation as well as on
drainage and the hydrological processes governing the flows of water. In addition, it is important
that the techniques of matching water supply rate with crop water requirements are understood
to enable the design of efficient water distribution networks.
1.3 Hydrologic cycle
The hydrological cycle (Figure 1) is the global circulation of water, and it occurs above, on the
surface and beneath the earth’s surface. It is influenced by the sun, wind and other global
5
climatic phenomena as well as inland topographic features such as mountains and expanse of
land mass. For instance, the windward and leeward sides of the same mountain experience
completely different climatic conditions. Cloud formation on the windward side is responsible
for the subsequent higher rainfall, lower temperatures, humidity and hence variation in climatic
conditions. Recent scientific research indicates that the hydrologic cycle can be altered by climate
change caused by global warming. However, the total water in the world remains the same, only
changing in form and distribution across the world. Irrigation is affected by the hydrological
cycle, and can contribute to local level hydrological changes.
Figure 1: The Hydrological cycle
1.4 Water availability for irrigation
Although the world is three quarters covered by water, yet less than one percent of this water is
available for agriculture (Table 1). There are considerable variations in water availability, both
within a year and over the years. To be of any value, a constant water supply must be sustained,
with a stated risk of failure. Worldwide, a risk of 20% for agricultural use is generally acceptable,
implying a failure to supply the quoted yield 1 in 5 years. Lower risk factors imply lower yields,
since lower yields can be supplied with less risk than higher yields. Lower yields result in lower
levels of investments in irrigation infrastructure (dam construction, conveyance systems, etc.),
since the area that can be irrigated is less. Higher risks translate into higher yields and this could
act as an incentive in irrigation infrastructure investment, thereby transforming the
socioeconomic status of most people, in particular the beneficiary rural communities.
However, the risk factor should be carefully weighed against the benefits. Conservative (low) risk
factors lead to a lower total utilization of the water as less base flow can be used (rivers) or a
greater proportion of water held in storage to carry over with consequent higher evaporation
losses from dams. It should be noted, however, that the yield at 10% risk gives greater security
against short-term shortages than the yield at 20% risk. Yield versus dam capacity curves can be
constructed for various risk factors. These are asymptotic and there is an optimum yield
6
obtainable for a certain dam capacity and any increase in dam capacity would not result in any
significant increase in the yield. It is thus not cost effective to over-design a dam.
Table 1: Estimate of the water balance of the world
Source: Nace, 1971
1.5 Rivers as sources of irrigation water
Rivers or streams with a regular and certain minimum flow (baseflow) are suitable for irrigation.
Unfortunately, many rivers in Southern Africa have short duration flash floods during the rainy
season and no or very little flow during the dry season. These rivers are not suited for year round
irrigation, unless the water can be stored in a reservoir behind a dam. From Figure 2, the
hydrograph of river A shows that the base flow at 10% risk is 1.0 m-3s-1. Therefore, this flow
could be diverted throughout the year. River B is seasonal and irrigation can only take place
during the rainy season between November and March at a safe abstraction of about 200 ls-1.
However, reservation for other purposes (municipal, industrial, environmental) also has to be
considered.
Figure 2: Examples of streamflow hydrographs
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1.6 Analysis of hydrological data for irrigation
The feasibility of using rivers for irrigation can be determined by a statistical analysis of longterm river flows. For most major rivers, these data are available from the departments or
organizations responsible for hydrological data such as the Ministry of Forestry and Water
Affairs in South Africa or National Water Authorities in other countries. For most small rivers,
flow measurements are not easily available. It is thus difficult to determine the water flow during
the growing seasons. Nevertheless, a clear indication is needed, especially during the latter part of
the dry season when minimum river flow normally coincides with maximum evapotranspiration.
There are ways of obtaining some idea about the flow regime, such as by talking to local
(preferably elder) people, visiting the area during the dry season, analyzing satellite imagery data
(remote sensing) and by carrying out flow measurements with current meters or isotope and salt
dilution methods. Whether data are available or not, one has to come up with a safe water yield,
which in turn determines the possible irrigation area.
Once this is known, one should apply for an appropriate water right or water abstraction permit
from the relevant authority in the country. It is equally important to have knowledge of high
floods in order to properly design diversion structures and flood protection works near the river.
Again, it is useful to talk to the local people, who can often indicate flood marks, for example on
trees. Many rivers carry large amounts of sediments especially during the rainy season. This has
to be verified and, if so, the designs of the headworks have to cater for sediment flushing
arrangements to avoid it entering the canal system. The stability of especially meandering rivers
has to be considered in order to avoid placing headworks in unstable parts of the river.
Hydrologic and climatic data in Irrigation water storage
Where rivers do not provide sufficient baseflow for irrigation, storage structures could be built in
order to balance river flows, not only throughout the year but also over sequences of several
years. Dams and reservoirs are used to store such water. However, sedimentation and
evaporation have direct effects on the storage capacity and should therefore be understood. This
is an example where isolating climatic and hydrological data is not possible.
1.7 Sedimentation
The amount of sedimentation depends on many aspects, including soil type, climate, slopes,
vegetation cover, deforestation, livestock, population pressure and management practices in the
catchment area of a dam. Sedimentation can cause serious problems to dams, particularly small
ones, or weirs, as the reservoirs could fill up rapidly. The source of sediment is the land in the
catchment area of the dam. Sediment that enters the river system is transported either as bed
load or as suspended load. Bed load comprises the larger (sand) particles that are swept along or
close to the riverbed. This type of load accounts for approximately 10% of the total sediment in
the river. Suspended load includes all finer particles like silt and clay. These materials are carried
in suspension and will only settle down when the flow is slowed down, for example in a reservoir
created by a dam (Figure 3). In general, the bed load is deposited first at the tail end of the
reservoir, after which respectively the heavier and lighter suspended materials settle.
Sometimes fine mud settles out on top of the coarser materials at the end of the flood season
since the flow, in most cases, will be very much reduced. The mud is relatively impermeable,
which can cause impermeable layers with no free movement of water between the layers, thus
resulting in the river completely drying up during the dry seasons. There are cases where small
reservoirs behind dams and weirs are filled with sand and alluvium, which would still allow
8
abstraction of water, as approximately 30% of the reservoir volume remains filled with water. In
such cases, abstraction can be done through sand abstraction.
Figure 3: Sedimentation in a reservoir created by a dam
A series of screens or slotted pipes are buried below the water table in the sand and attached to a
pump, which pumps the water from the sand. It should be noted, however, that dams are not
constructed to be used for sand abstraction. Sand abstraction schemes are mostly carried out in
riverbeds with significant amounts of sand or alluvium. The reservoir trap efficiency is a measure
of the proportion of the total volume of sediment that is deposited in a reservoir to that which
enters the reservoir. The total volume of sediment entering a reservoir each year will be the
product of the sediment concentration in the water, the mean annual runoff and the catchment
area:
Calculating the sedimentation of a reservoir
Sediment concentration (SC):
This depends on how well preserved and conserved the catchment area is. Three categories are
often used, namely sediment concentrations of 3,000 mg/l (3 kg/m3), 5,000 mg/l (5 kg/m3) and
10,000 mg/l (10 kg/m3).
Catchment area (CA):
This is the total land area contributing runoff into the reservoir (km2).
Mean annual runoff (MAR):
This is the average net runoff, expressed as a depth of water over the dam’s catchment area
(mm).
The mean annual inflow into the reservoir (MAI) is expressed as follows:
MAI = CA x MAR
Where:
o MAI = Mean annual inflow into the reservoir (m3)
o CA = Catchment area behind the dam (m2)
o MAR = Mean annual runoff (m)
The trap efficiency is related to the gross storage ratio, which is expressed as follows:
SRg = DC/MAI
9
Where:
o SRg = Gross storage ratio
o DC = Gross dam capacity (m3)
o MAI = Mean annual inflow into the reservoir (m3)
For large dams with a gross storage ratio of at least 0.10, the trap-efficiency is 100%, as it is
assumed that all the sediment will be settled. For very small dams, there will be almost
continuous spilling and only the bed load will settle, thus the trap efficiency will be 10%.
The dam yield (Q) is defined as the volume of water in m3 that can be drawn from a reservoir
behind a dam for use each year, at the designated risk level. The following parameters are used in
the estimation of dam yield:
2
o Dam catchment area CA (km )
o Mean annual runoff MAR (mm)
3
o Gross mean annual inflow into the reservoir MAI: the product of CA and MAR (m )
Example: Calculation of Trap efficiency
Given:
– Catchment area (CA)
= 148 km2
– Mean annual runoff (MAR)
= 40 mm
– Gross dam capacity (DC)
= 1 700 000 m3
– Sediment concentration (SC)
= 5 000 mg/l or 5 kg/m3
– Density of deposited sediments (d)
= 1 550 kg/m3
What is the volume of the reservoir that is lost yearly to sedimentation?
- Gross mean annual reservoir inflow (MAI) = (148 x 106) x (40 x 10-3) = 5 920 000 m3
Gross ration (SRg) = 1700,000 ÷ 5,920,000 = 0.29
Hence
Trap efficiency = 100%, since the storage ratio > 0.1
The deposit of sediment in an average year in kg will be equal to the gross mean annual inflow in
m3 multiplied by the sediment concentration. Thus, the mass of sediments in the inflowing river
water per year is:
SM = (5.92 x 106 m3) x (5 kg/m3) = 29.6 x 106 kg
The volume occupied by the sediment per year is:
SV = 29.6 X 106 ÷ 1550 = 19100 M3
This is the volume of reservoir or water lost to sedimentation yearly.
Coefficient of variation (CV) is a mathematical measure of the variability of runoff from year
to year. It is the ratio of standard deviation of annual inflow to the mean annual inflow. A low
CV indicates regular inflow and high chances of meeting a particular yield and, conversely, a high
CV implies that the chances of meeting a particular yield are less. CV can be expressed in % or in
decimals
Maximum reservoir surface area A: the surface area of reservoir when water is at full supply
level (ha)
Net storage ratio SRn: the ratio of live storage capacity U to gross mean annual inflow
SRn = U / MAI
Where:
SRn = Live storage ratio
U = Live storage capacity (m3)
MAI = Mean annual inflow into the reservoir (m3)
10
The live storage capacity is defined as:
U = DC - DS - SA
Where:
U = Live storage capacity (m3)
DC = Gross dam capacity (m3)
DS = Dead water storage below the outlet level (water which can not be abstracted) (m3)
SA = Sediment allowance over a chosen period (m3)
1.8 Use of analogue and digital maps
Topomaps represent the physical features of the ground on paper and are characterized by
contour lines, drainage systems, highlands and lowlands. Layers of digitized maps that represents
specific features and topology on the ground are used in geographic information systems (GIS)
or AutoCAD programs. These provide interactive digital ground modelling soft ware for
planning, design monitoring and evaluation of projects. The catchment area (watershed/basin)
and the maximum reservoir surface area can usually be determined (mapped out) from maps
with contour lines at a scale of 1:50,000 for example. The storage capacity of the dam could also
be determined from such maps, although a reservoir survey often has to be carried out to obtain
more accurate data on the storage capacity. Inflow characteristics consist of the MAR and CV of
the annual runoff. In most countries, estimates of MAR and CV are given for each subcatchment area or hydrological sub-zone.
The evaporation index is defined as follows:
El = E x A x 104 ÷ U
Where:
EI = Evaporation Index
E = Evaporation over the dry months (m)
A = Reservoir surface area (ha)
U = Live storage capacity in (m3)
The method described in this example does not apply to situations where SRn is above 0.5.
Lowering groundwater levels
Aquifers adjoining rivers or other surface water sources or with rivers running through them can
potentially be recharged from the surface water. This can be established by the use of isotopes.
In such cases, a reduction in the groundwater levels induces recharge from the surface water
source, if the surface water body is hydraulically linked to the aquifer. It is good management
practice to draw down the groundwater table during dry periods. The reduction in the water
table would be temporary and rapid recovery could be expected during normal rainy seasons.
1.9 Conjunctive use of surface water and groundwater
Conjunctive use involves the coordinated and planned utilization of both surface and
groundwater resources to meet water requirements in a manner where water is conserved. In a
conjunctive scheme, during periods of above normal rainfall, surface water is utilized to the
maximum extent possible and, where feasible, artificially recharged (pumped into aquifers
through wells known as injection wells) into the aquifer to augment groundwater storage and
raise groundwater levels (care should be taken to not the raise the levels to the crop root zone).
Conversely, during drought periods the limited surface water resources will be supplemented by
pumping groundwater, thereby lowering the water levels. However, the cost of setting up such a
scheme could be prohibitive for most African countries.
11
2 CLIMATE, HYDROLOGY AND GEOMORPHOLOGY
2.1 Climatic, hydrologic and geomorphologic characteristics
The most important climatic data used in irrigation planning include; rainfall, maximum and
minimum temperatures, maximum and minimum relative humidity, wind and sunshine hours.
Climate is an important factor in crop production. 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. Analysis of climatic data with respect to crop
production is needed before a cropping programme can be prepared. The reliability of estimated
crop water requirements relies heavily on the availability of accurate meteorological data.
Climatic data analyses are used to:
o Explain the relationships between hydrological processes and geomorphic characteristics
of the landscape
o Show how the climatic variables influence the crop water needs
o Enable practical calculations of important variables
o Determine water flow measurements in the field.
2.2 Hydrological processes and geomorphic characteristics
An irrigation manager or operator should appreciate the fact that an irrigation system extends
from the top of the watershed to the farm and onto the drainage system. Geomorphic
characteristics of the catchment are expressed in terms of landscape patterns that influence the
flow of water. The watershed yielding the irrigation water, the streams conveying the water, the
management and distribution of water, and the drainage problems arising from irrigation practice
are concerns which can be handled by understanding hydrological characteristics of the irrigated
area. Based on these characteristics, both flood flow and dry flow can be predicted. Since
irrigated areas are in most cases low-lying, the flooding regimes and dry weather flow through
lateral or subsurface water movement should be assessed. This assessment involves the analysis
of geomorphic data such as altitude, slope, relief intensity, and vegetation. These variations can
be studied using air photo interpretation or high resolution remotely sensed data. Thematic
maps, e.g. digital elevation models, can be used to determine spatial distribution of landform and
for deriving soil and land use patterns of the area, which are important factors influencing the
water flow patterns on the landscape. Different landforms have different physical, chemical and
biological characteristics that influence the flow patterns (Muya et al, 2009). For instance,
volcanic footridges generate higher rate of overland flow than plains and plateaus with flat to
very gently sloping topography.
An understanding of irrigation water requirements starts by linking the water supply with the
hydrological processes influenced by geomorphic characteristics of the landscape (Figure 4). The
hydrological processes involve evaporation, transpiration and water flows taking place in the
surface and sub-surface parts of the landscape. The important geomorphic characteristics of
landscape that influence these flows are slope length, slope steepness, surface conditions such as
sealing and crusting, subsurface soil texture, structure as well as soil water holding capacity.
These factors determine the partitioning of water between infiltration and run-off. Infiltration is
the movement of water into the soil surface. Run-off is the concentrated overland flow, and it is
the difference between the infiltrated rain water and the total rain water falling onto the soil
surface.
12
Rainwater supply
(all units mm)
Short-rain
Evap 225 +
Runoff 135 +
Drainage 10 +
Soil water 80 +
= 450mm
450
Long-rain
Yearly rain =1368
225
650
574 crop
100
200
135 (30%)
+ 80
10 (2%)
(18%)o
Figure 3: Managing rainwater
Soil
water
Rule
of balance
thumb
Estimated soil
water storage:
1 fifth of
rainfall
+1 1 0350
11
Lateral flows into the bottomlands
Tracing rainwater
Evaporation 50%
SWater 20%
Run-off 30%
31
Figure 4: Illustration of the partitioning of rainwater in relation to irrigation
2.3 Rainfall, Runoff and Water balance
An important component the water available for irrigation is the hydrological cycle and the
consequent water balance. Evaporation, transpiration and surface run-off are the most important
components of the hydrological cycle. Water quality in valley bottoms is affected by the quality
of water flowing from cultivated areas. Water quality is reduced when materials from the upper
parts of the landscape are transported into the bottomlands through run-off. Some of this water
can be intercepted and used for crop production in rainfed agriculture. Depending on the scale,
agricultural production systems and management can be defined in terms of experimental plots,
farmers’ fields, watershed, river basins or the entire geographical region. In a broader
geographical scale, the individual land management strategies are transformed into collective
responsibility of managing land resources and protecting the watershed from land degradation.
At all levels, the sustainability of the use of land resources is determined by the interactions
between the biophysical, social, policy and economic factors.
These interactions may result into negative or positive energy balance that degenerates or
sustains human life. The soil water balance is a part of the energy balance. It is based upon the
physical principle of conservation of mass, which states that any change in the amount of water
stored within the soil is equal to the difference between water inputs and outputs. Inputs are
generally limited to rainfall and run-on. Outputs include drainage from the root zone,
evaporation from the soil surface and plant transpiration and surface run-off. The water balance
in agro-ecological systems can, therefore, be defined by the following equation:
S = (P+Rn)-(D+E+R+T).
13
Where:
S = water stored within the soil
P = Precipitation
Rn = Run-on
D = Depth of soil root zone
E = Evaporation from soil surface
R = Runoff
T = Transpiration by plants
Since rainfall is beyond human control, optimizing the soil water utilization requires that R, S and
Rn be managed, so that there is sufficient water for the crops. Water management in irrigated
systems also should be aimed at optimizing the use of rain water should appreciate the
hydrological processes taking place on the soil surface (Figure 5). The run-on from the upper
catchment area may be intercepted through well constructed and stable conservation structures,
while the run-off, the so-called blue water can be used for irrigation in the lower catchment area.
Part of the rain water used by the crop through evapotranspiration (green water) is converted
into biomass. The water moving down the soil profile through deep percolation is contributing
to both ground water recharge and lateral flows.
Figure 5: Water balance and irrigation water (Source: Barron et al. 1999)
The soil acts as a reservoir for water because it holds and stores water against the force of
gravity, and the stored water can be used for irrigation. Therefore, information on the
distribution of major soils in the project area and their characteristics is important, as shown by
the example from Kenya, in developing the watershed management strategies for protecting the
irrigated area against high volumes of run-off, which can destroy irrigation infrastructure.
2.4 Influence of climatic variables on crop water requirements
An analysis of climatic data for irrigation involves the determination of reference crop water
requirement, also known as reference evapotranspiration (ETo). It is defined as the sum of water
loss through evaporation and through transpiration of a healthy crop, growing in a large field,
where water, nutrients, pest and diseases are not limiting crop growth. The climatic data normally
analyzed for this purpose includes temperature, radiation, air humidity and wind speed. The
actual crop water requirement is determined by multiplying ETo with a coefficient of a given
crop. It is important to note that the crop water needs change with growth stages and the
designed rate of water application must meet the changing water demand in the right quantity
14
and time. Crop water requirements are calculated based on the guidelines provided by FAO
(1977, 1986). In these guidelines, crop water requirement is defined as the depth of water needed
to meet the water loss through evapotranspiration of (ETcrop) of a disease-free crop, growing in
large fields under non-restricting soil conditions including soil water and fertility, and achieving
full production potential under the given growing environment. There are three methods of
estimating the ETo, namely: Blaney-Criddle, Penman and radiation methods. Each of the three
methods is used where applicable.
2.5 Methods of determining reference crop evapotranspiration (ETo)
There are several methods to determine the ETo. They are either: experimental, using an
evaporation pan, or theoretical, using measured climatic data. Many of them have been
determined and tested locally. If such local formulae are not available one of the general
theoretical methods has to be used. The most commonly used theoretical method is the
modified Penman method which is described in detail in FAO Irrigation and Drainage Paper 24.
This method, however, is rather complicated and beyond the scope of this manual. The methods
presented here include the only the Pan evaporation, Blaney-Criddle and radiation methods.
2.5.1 Pan Evaporation Method
Many different types of evaporation pans are being used. The best known pans are the Class A
evaporation pan (circular pan) as shown in Figure 6, or the Sunken Colorado pan (square pan).
Figure 6: Class A Evaporation method (circular pan)
The E pan is multiplied by a pan coefficient, K pan, to obtain the ETo.
ETo = K pan × E pan
Where:
ETo = reference crop evapotranspiration
K pan: = pan coefficient
2.5.2 Blaney-Criddle Method
If no measured data on pan evaporation are available locally, a theoretical method (e.g. the
Blaney-Criddle method) to calculate the reference crop evapotranspiration ETo can be used. The
Blaney-Criddle method is simple, using measured data on temperature only. It should be noted,
however, that this method is not very accurate; it provides a rough estimate or "order of
magnitude" only. Especially under "extreme" climatic conditions the Blaney-Criddle method is
15
inaccurate: in windy, dry, sunny areas, the ETo is underestimated (up to some 60 percent), while
in calm, humid, clouded areas, the ETo is overestimated (up to some 40 percent).
The Blaney-Cridle method is expressed as follows:
ETo = C{P(0.46T + 8)} mm/day
Where:
Eto=
T=
P=
C=
Reference evapotranspiration in mm/day for the month
Mean daily temperature for the month considered.
Mean daily percentage of total annual daytime hours
Adjustment factor
The adjustment factor depends on the minimum relative humidity, sunshine hours and daytime
wind estimates. The climatic data required for the calculation of the ETo are obtained at the
meteorological stations.
Using the Blaney-Criddle formula
Step 1: Determination of the mean daily temperature: T mean
The Blaney-Criddle method always refers to mean monthly values, both for the temperature and
the ETo. If, for example, it is found that T mean in March is 28°C, it means that during the
whole month of March the mean daily temperature is 28°C.
If in a local meteorological station the daily minimum and maximum temperatures are measured,
the mean daily temperature is calculated as follows:
Step 2: Determination of the mean daily percentage of annual daytime hours. To be able to
determine the p value it is essential to know the approximate latitude of the area: the number of
degrees north or south of the equator. The p value for the month of March can be determined
for an area at latitude 45° South.
Step 3: Calculate ETo, using the formula: ETo = p (0.46 T mean + 8)
For example, when p = 0.29 and T mean = 21.5°C then ETo is calculated as follows:
ETo = 0.29 (0.46 × 21.5 + 8) = 0.29 (9.89 + 8) = 0.29 × 17.89 = 5.2 mm/day
3.1.4 Calculation Example Blaney-Criddle
Given
Latitude - 35° North
Mean T max in April = 29.5°C
Mean T min in April = 19.4°C
Question
Determine for the month April, the mean ETo in mm/day using the Blaney-Criddle method
Solution
Formula: ETo = p (0.46 T mean + 8)
16
Step 1: Determine Tmean.
Step 2: Determine p
Given that Latitude is 35° North, Month: April; estimated value of p = 0.29
Step 3: Calculating ETo
ETo = 0.29 (0.46 × 24.5 + 8) = 5.6 mm/day
Thus the mean reference crop evapotranspiration, ETo =5.6 mm/day in the month of April.
Exercise on the relationships between rainfall and flows
1
2
3
4
How much water would be collected on a flat slab measuring 40 by 20 metres from rainfall
of 25 mm?
If this water was supplied through a pipe at a flow rate of 50 litres per min for how long
would the water flow?
In order to supply 27 mm of irrigation water to a field measuring 40 x 40 metres within 10
hours what should be the flow rate assuming there are no losses in the system?
How much water should be diverted from a river to irrigate 3000 ha of land if the irrigation
requirement is 6 mm per day and if the system operates for 12 hours per day? Assume an
overall efficiency of 45%.
2.5.3 Radiation Method
The radiation method is used where available climatic data include measured air temperature,
sunshine, cloudiness or radiation, but not measured wind and humidity, and it is expressed as
follows:
ETo = C(W.Rs) mm/day
Where:
ETo = Reference evapotranspiration
Rs = Solar radiation in equivalent of evaporation in mm/day.
W = Weighting factor which depend on the temperature and altitude
C =Adjustment factor which depends on mean humidity and day time conditions.
The effects of the crop characteristics on the crop water requirements are calculated, using crop
coefficient provided by FAO (1998). These effects will determine the actual crop water
requirement, determine the consumptive water use, depending on the prevailing weather
conditions and soil moisture availability. For a given climate, crop growth and development
stage, the maximum evapotranspiration (ETm) in mm/day of the day considered as follows:
ETm =
k ETo
Where ETm is the maximum evapotranspiration and kc is the crop coefficient.
The areas where measured data on temperature, humidity, wind and sunshine duration or
radiation are available, use of Penman method is recommended. This method is expressed in the
following equation:
ETo = C[W.Rn+(1-W).f(u).(ea-ed)]
17
Where:
ETo = Reference crop evapotranspiration
C= Adjustment factor to compensate for the effect of day and night weather conditions.
W = Temperature-related weighting factors.
Rn = Net radiation evaporation in equivalent mm/day
f(u) =Wind-related factor
(ea-ed) = Difference between the saturation vapour pressure at mean air tempearture and the
mean actual vapour pressure of the air, both in mmbars
Evaporation rate can be measured directly using evaporation pan and the results can be used to
estimate ETo.
18
3
DESIGN OF IRRIGATION SYSTEMS
3.1 Data requirements and preparations
The design of an irrigation scheme is an engineering undertaking, requiring use of reliable and
accurate data as well as other information sources. Such data are collected from direct
measurements or obtained from records. Information can also be gathered from observations
and interviews with important stakeholders such as farmers, extension workers, local leaders and
those engaged in the value chain. The proposed design and agricultural enterprise should ensure
optimum productivity of available land, water supplies, labour and capital. Generally, feasibility
studies are conducted accompanied by field surveys. Several factors are considered during
project identification, design, and operation with relevant information gathered at each stage
(Table 2).
Table 2: Information required in the design of an irrigation scheme
Activity
Data application
Inventory of the resources
Present hydrological budget
Identification of irrigable area
Choice of production system
Method of water delivery
o
Project
Identification
o
o
o
o
o
Project Design
o
o
o
o
o
Project size
Layout of the distribution systems
Cropping patterns
Water supply scheduling.
Capacity of the engineering
o
o
o
Irrigation interval and time
Size of the stream flow
Irrigation cycle
o
A review of irrigation efficiency and
water supply rate to the field
Analysis of the production of different
crops per unit water applied
Monitor the field water balance
o
o
Daily water supply rate
Actual consumptive water use
per crop
Production in kg per mm of the
irrigation water.
Project
Operation
o
o
Data required
o
o
Average month and peak water
supply requirements
Physical, hydraulic and chemical
soil quality attributes.
3.1 Procedures for calculating irrigation water requirements
Irrigation water requirement is normally determined through calculation, based on climatic data.
The reference evapotranspiration together with crop coefficient assist to estimate the crop water
requirement.
Data analyses include:
1.
2.
3.
4.
Selection of the reference evapotranspiration (ETo)
Determining the crop water requirement.
Calculate the irrigation interval and timing, and
Determine the water losses in the irrigation system (seepage from canals and other losses
in the field).
Major variables determined
o ETo selected based on the climatic characteristics
Crop water needs=kc x ETo
o Effective rain: dependable rain that can be expected in each month in 3 out of 4 years or 4
out of 5 years
19
o Irrigation requirement in mm/day (In) = ETcrop-Pe-Ge-Wb
where,
ETcrop is crop water needs,
Pe is effective rain,
Ge is ground water contribution, and
Wb is the field water balance (Wb).
o Irrigation requirement in m3/month= (A x In)/efficiency (Ea)
o Irrigation interval (d) = (p.Sa.D)/(In.Ea)
Ea=irrigation efficiency
D=Rooting depth in m
p=% of the available soil water permitting unrestricted evapotranspiration (ET)
Sa=total available water in mm/m depth of soil.
For the design of the canal capacity and distribution system (Figure 7), the appropriate irrigation
method and design quantity of the flow should be known. The data required for this design
include (i) irrigation efficiency, (ii) the available water holding capacity of the soil in mm/m, (iii)
the fraction of the available water permitting unobstructed evapotranspiration, (iv) rooting
depth, and (v) area to be irrigated.
The stream flow into each field is given by the following equation:
q=10{(p.Sa).D.A}/{(Ea.t)}
Where:
Q = Stream size in m3/s
p = the fraction of the available water permitting unobstructed ET
Sa = Available soil moisture holding capacity mm/m
D = Rooting depth in m
A = Area of each field
Irrigation
field 1
Irrigation
field 2
Irrigation
field 3
Irrigation
field 4
Irrigation
field 5
Irrigation
field 6
Irrigation
field 7
Irrigation
field 9
Irrigation
field 10
Irrigation
field 11
Irrigation
field 12
Irrigation
field 13
Irrigation
field 14
Irrigation
field 15
Irrigation
field 16
Irrigation
field 8
Figure 7: Illustration of water distribution system through irrigated fields
20
Selection of the reference evapotranspiration (ETo) is based on the climatic characteristics of the
project area (Table 3).
Table 3: Daily evapotranspiration for different climatic zones
Evapotranspiration in mm/day at various daily temperatures (oC)
Climatic zone
Desert/arid
Semi-arid
Sub-humid
Humid
Low (<15oC)
4-6
4-5
3-4
1-2
Medium (15- 25oC)
7-8
6-7
5-6
3-4
High (> 25oC)
9-1o
8-9
7.8
5-6
Source: Muchangi et al., 2005
Exercise on calculation of irrigation water requirement
1. If the coefficient a given crop is 0.3 what is the crop water needs if the reference
evapotranspiration is 8.9 mm/day?
2. If effective rain is 1.0 mm in a given month of the growing season, ground water
contribution is 0.1mm and the field water balance at the time of irrigation is 1.2 mm,
what is the irrigation water requirement?
3. If the area of each field is 6 m2 and the quantity of water supplied is 20 litres, calculate
the gross water application depth in mm.
Exercise on calculation of water losses in canals and irrigated fields
This section was to help the trainees to understand how to estimate the irrigation efficiency.
Understanding the efficiency with which the system operates is important in irrigation water
management because it provides data on the basis of which to make necessary changes.
Given the following:
Field efficiency: 65%
Water course conveyance efficiency: 85%
Irrigation interval: 14 days
94% of the losses through seepage in the canals
95% losses through deep percolation in the irrigated fields
1. Calculate the water losses in the canal and irrigated fields.
2. Calculate the rate at which the water table rises, following the seepage and percolation.
3.2 Effect of growth stage on crop water needs
A fully grown maize crop will need more water than a maize crop which has just been planted.
As discussed before, the crop water need or crop evapotranspiration consists of transpiration by
the plant and evaporation from the soil and plant surface. When the plants are very small the
evaporation will be more important than the transpiration. When the plants are fully grown the
transpiration becomes more important than the evaporation.
The evapotranspiration or crop water need during the initial stages can be estimated at 50
percent of the crop water requirement when the crop is fully developed.
During the crop development stage, the crop water need gradually increases from 50 percent of
the maximum crop water need to the maximum crop water need. The maximum crop water
need is reached at the beginning of the grain filling stages during crop development. With respect
21
to the late season stage, which is the period during which the crop ripens and is harvested, a
distinction can be made between two groups of crops:
Fresh harvested crops: such as lettuce, cabbage, etc. With these crops the crop water need
remains the same during the late season stage as it was during the mid-season stage. The crops
are harvested fresh and thus need water up to the last moment.
Dry harvested crops: such as cotton, maize (for grain production), sunflower, etc. During the
late season stage these crops are allowed to dry out and sometimes even die. Thus, their water
needs during the late season stage are minimal. If the crop is indeed allowed to die, the water
needs are only some 25 percent of the crop water need during the mid-season or peak period.
Generally, no irrigation is given to these crops during the late season stage.
3.3 Irrigation efficiencies
There is increasing demand on water resources, which emanates from growing human
population and the need for more irrigated crops. This means that there is increasing
competition for the use of water for agricultural, industrial, domestic and ecosystem purposes.
This calls for more efficient use of finite water resources in order to minimize conflict between
users and sectors. Planners are concerned optimizing the use of water while selecting irrigation
systems, based on their efficiencies. In the process of applying irrigation water to crops, water
losses occur. These losses have to be taken into account when calculating the gross irrigation
requirements of an irrigation project. This can be done through the use of an efficiency factor,
which has to be estimated during planning. Different types of irrigation systems have different
levels of efficiency. The higher the irrigation efficiency, the larger the area that can be irrigated
from a given water source. This results in reduced leaching of nutrients, less damage to the soil
and better environmental management. The water so saved can be used for other productive
purposes.
Worked example
The following are some data used to design an irrigation system:
Area
15 ha
Soil type
Clay loam Sandy mixture
Available soil moisture (= FC - PWP)
130 mm/m
Design root zone depth (RZD)
0.70 m for maize
Allowable soil moisture depletion
(P)
0.50
Assumed field application efficiency (Ea)
0.50
Assumed field canal/ efficiency (Eb)
0.9
Assumed conveyance efficiency (Ec)
0.9
Farm irrigation efficiency (Ef = Eb x Ea)
0.45
Distribution system efficiency (Ed) (= Ec x Eb)
0.8
Overall irrigation efficiency (Ep) (= Ec x Eb x Ea) 0.41
Peak ETcrop
6.0 mm/day
Step 1
Calculate the net and gross depths of water application for this irrigation project
dgross = dnet ÷ E
dnet = (FC - PWP) x RZD x P
Where:
dnet = Net depth of water application per irrigation for the selected crop (mm)
FC = Soil moisture at field capacity mm/m)
PWP = Soil moisture at the permanent wilting point (mm/m)
RZD = The depth of soil that the roots exploit effectively (m)
22
P = The allowable portion of available moisture permitted for depletion by the crop before the
next irrigation
dnet = 130 mm/m x 0.70 m x 0.50 = 45.5 mm
The gross depths of water application at field and at overall level would be:
dgross = 45.5 ÷ 50 = 91.0 mm at field level
dgross = 45.5 ÷ 0.41 = 111.0 mm at overall level
Step 2
Calculate the irrigation frequency/interval and determine the irrigation cycle for this scheme
The irrigation frequency is equal to: IF = dnet ÷ ETcrop
Where:
IF
= Irrigation frequency (days)
Dnet
= Net depth of water application (mm)
ETcrop = Crop evapotranspiration (mm/day)
If = 45.5 ÷ 6 = 7.5 days
The system should be designed to provide 45.5 mm every 7.5 days. For practical purposes,
fractions of days are not used for irrigation frequency purposes. Hence, the irrigation
frequency/interval in our example should be 7 days, with a corresponding dnet of:
dnet = 7 x 6.0 = 42 mm for an If of 7 days
The dgross at field and overall level will be 42 ÷ 0.5 = 84.0 mm and 42 ÷0.41 = 102.4 mm
respectively.
Calculate the adjusted allowable depletion for the 7 days (instead of 7.5 days) from the
relationship:
dnet = (FC - PWP) x RZD x P
Where:
dnet = Net depth of water application per irrigation for the selected crop (mm)
FC = Soil moisture at field capacity mm/m)
PWP = Soil moisture at the permanent wilting point (mm/m)
RZD = The depth of soil that the roots exploit effectively (m)
P = The allowable portion of available moisture permitted for depletion by the crop
before the next irrigation
P = dnet ÷ (FC – PWP) X RZD = 42 ÷(130 x 0.7) = 0.46
The Irrigation Cycle (IC) is then fixed at 6 days.
Calculate the system capacity (Q)
Considering and irrigation duration of 10 hours per day
System capacity refers to the discharge that has to be abstracted from the headwork during a
given period per day and it is used for the design of the headwork and the conveyance system. It
is determined by the following equation:
Q = V/T
Where:
Q = Discharge (m3/hr or l/sec)
V = Volume of water to be abstracted per day (m3 or l)
T = Irrigation duration per day (hr or sec)
V = 10 x A. x dgross hence
A = 15 /6 = 2.5 ha
The volume of water to be abstracted per day is equal to:
V = 10 x 2.5 x 102.4 = 2 560 m3/day
23
The system capacity, assuming 10 hours of irrigation per day, will be equal to:
Q = 2 560/10 = 256 m3/hr or 71.1 l/s.
If, however, this results in large conveyance dimensions, a night storage reservoir could be
introduced so that abstraction from the head works could be continuous (24 hours/day) at peak
demand. In that case, the conveyance system capacity would be 71.1x(10/24)=29.6 l/s.
1. Net depth of water application
dnet is 45.5 mm
2. Calculated irrigation frequency
If is 7.5 days
3. Adjusted irrigation frequency
If is 7 days
4. Adjusted net depth of water application
dnet is 42.0 mm
5. Adjusted allowable depletion
P is 46%
6. Proposed irrigation cycle
IC is 6 days
7. Gross depth of water application (overall level) dgross is 102.4 mm
8. Gross depth of water application (field level)
dgross is 84.0 mm
9. Area to be irrigated per day
A is 2.5 ha
10. Volume of water to be abstracted daily for 10 hours (V)
2,560 m3/day
11. System capacity with irrigation duration of 10 hours day (Q) 256.0 m3/hr or 71.1 l/s
12. System capacity for 24 hrs conveyance and night storage Q 106.6 m3/hr or 29.6 l/s.
3.4 Assessing soil parameters
It is necessary to identify, quantity and monitor the relevant soil quality indicators under different
soil moisture regimes. The soil parameters to assess include; soil bulk density, soil pH, soil
strength and aggregate stability, drainable porosity and pore size distribution, heavy metals such
as boron and aluminium as well as the nutrient balances.
3.5 Flow measurement in canals
The total flow in a canal can be determined using various devises such as a current meter, a float
or a measuring flume. It can also be calculated from empirical equations based on measurements
made at various cross-sections. The continuity equation can be applied as flows:
Q=AV, where
Q = the stream flow in m3/s,
A = the cross-sectional area in m2 , and
V = the velocity in m/s.
To make a float for measuring flow in a river or canal, a plastic bottle filled with water for about
a quarter way full can be used. The bottle is placed at the centre of the stream, and allowed to
move with the flowing water to a distance of 10 m. The cross sectional area of the channel is
calculated by multiplying the average width of the channel by the depth, assuming a rectangular
channel, or more accurately using the actual shape of the canal. For trapezoidal and parabolic
channels, further measurements of bottom and top width are required. The flow velocity is
determined by dividing 10 m by the time taken by the float to move that distance. Important
precautions to observe include:
o The float must be weighted, so as to sink to some depth, so that representative currents
could be captured in the flow measurement.
o Several depths need to be taken for the calculation of the average depth because the
velocity of the flow varies with the depth.
o Measurements are made at several sections of the channel because the gradient of the
channel may not be constant throughout its length.
24
3.6 Design of canals
The canal dimensions and longitudinal slope, whether for irrigation or drainage, can be
calculated through trial and error with the Manning formula. This formula is derived from the
continuity equation and the equation for unsteady flow. These equations have been simplified by
assuming steady uniform flow in the canal, assuming that the canal is long with constant crosssection and slope (Figure 8). Canal design makes use of the Continuity equation (described
above) or using Mannings equation as follows:
Q = Km x As x R2/3 x S1/2
or
Q = 1/n x As x R2/3 x S1/2
Where:
Q = Discharge (m3/sec)
Km = Manning roughness coefficient (m1/3/sec)
n = Roughness coefficient; Km = 1/n or n =1/Km (sec/m1/3)
As = Wetted cross-sectional area (m2)
P = Wetted perimeter (m)
R = Hydraulic radius (m) (R=As/P)
S = Canal gradient or longitudinal slope of the canal.
Figure 8 Cross sectional view of a canal
Figure 9: Canal parameter for different cross-sectional shapes
25
Different canal parameters. As and P, and thus R in the Manning formula, can be expressed in d,
b and X (Figure 9).
Where:
d = Water depth (m)
b = Bed width (m)
X = Side slope = horizontal divided by vertical
For a trapezoidal canal, As is the sum of a rectangle and two triangles.
The cross-sectional area of a rectangle is:
Area of rectangle = (b x d)
The cross-sectional area of a Although the trapezoidal canal shape is very common, other canal
shapes, including V-shaped, U-shaped, semicircular shaped and rectangular shaped canals, can
also be designed.
Area of triangle = 1/2(base x height) =1/2 (Xd x d)
Thus, the wetted cross-sectional area As of the trapezoidal canal is:
As = b x d + 2(1/2 x Xd x d ) = b x d + Xd2 = d(b + Xd)
The wetted perimeter is the sum of the bed width b and the two sides from the water level to the
bottom. The length of a side, considering the formula c2 = a2 + b2, is:
(d2 + d2X2)
Thus the wetted perimeter for the trapezoidal canal section is:
P = b + {2(d2 + (dX)2}1/2 = b + 2d(1 + X2)1/2
The hydraulic radius R is:
R=d(b+Xd)/(b+2d(1+X2)1/2
Factors affecting canal discharge
Canal gradient or longitudinal slope of the canal. The steeper the gradient, the faster the water
will flow and the greater the discharge will be. This is substantiated by the Continuity Equation.
Velocity increases with an increase in gradient or longitudinal slope. It therefore follows that a
canal with a steeper gradient but with the same cross-section can discharge more water than a
canal with a smaller gradient.
The recommended maximum slope is 1:300 (that is 1 m drop per 300 m canal length), which is
equal to 0.33%. Steeper slopes could result in such high velocities that the flow would be supercritical. It would then be difficult, for example, to siphon water out of the canal, since an
obstruction in a canal where super-critical flow occurs tends to cause a lot of turbulence, which
could result in the overtopping of the canal. This is due to the change from the super-critical
state to the sub-critical state. The state of flow could be checked using the Froude Number. The
Froude Number (Fr) is given by:
Fr=V/(gxI)1/2
Where:
V = Water velocity (m/sec)
g = Gravitational force (9.81m/sec2)
I = Hydraulic depth of an open canal, defined as the wetted cross-sectional area divided by the
width of the free water surface (m)
Fr = 1 for critical flow
Fr > 1 for super-critical flow
Fr < 1 for sub-critical flow
Canal roughness
The canal roughness, as depicted by the Manning roughness coefficient, influences the amount
of water that passes through a canal. Unlined canals with silt deposits and weed growth and lined
canals with a rough finish tend to slow down the water velocity, thus reducing the discharge
26
compared to that of a clean canal with a smooth finish. Canals that slow down the movement of
water have a low Km or a high n. It should be understood that the higher the roughness
coefficient Km, or the lower n, the higher the ability of the canal to transport water, hence the
smaller the required cross-sectional area for a given discharge.
The roughness coefficient depends on:
o The roughness of the canal bed and sides
o The shape of the canal
o Canal irregularity and alignment
o Obstruction in the canal
o Proposed maintenance activities
Manning coefficients (Km) are often assumed relatively high during the design phase compared
to what they actually will be during scheme operation due to deterioration of the canals (Table
4). The result is an increased wetted cross-sectional area of the canal during scheme operation
with the danger of overtopping the canal banks. This in turn means that the canal discharge has
to be reduced to below the design discharge, in order to avoid overtopping. There is therefore a
need for regular and proper maintenance of canals.
Table 4: Manning coefficient for different canal surfaces
Range of roughness coefficient
Km(=1/n) in
m1/3/sec
n(=1/Km) in
sec/m1/3
Metal, wood, plastic, cement, precast concrete
65-100
0.010-0.015
Concrete canal and canal structures
66-85
0.012-0.016
Rough concrete lining
40-60
0.017-0.025
Masonry
30-40
0.025-0.035
Corrugated pipe structures
40-45
0.023-0.025
Clean, recently completed
50-65
0.016-0.020
Clean, after weathering
40-55
0.018-0.025
With short grass, few weeds
35-45
0.022-0.027
No vegetation
35-45
0.023-0.030
Grass, some weeds
30-40
0.025-0.033
Dense weeds or aquatic plants in deep channels
25-35
0.030-0.040
Dense weeds, as high as flow depth
8-20
0.050-0.120
Clean bottom, brush on sides
10-25
0.040-0.080
Type of surface
Pipes, precast and lined canals
Earthen canals, straight and uniform
Earthen canals, winding and sluggish
Canals, not maintained, weeds & brush
Source: Euroconsult (1989)
27
Canal shape
Canals with the same cross-sectional area, longitudinal slope and roughness, but with different
shapes, may carry different discharges because of different wetted perimeters and hydraulic radii.
The most efficient geometry is when the wetted perimeter is minimal for a given discharge.
Under these circumstances, the cross-sectional area for a given discharge will also be minimal.
The optimum canal shape, hydraulically, also tends to be the cheapest to construct as the amount
of surface lining material required will be minimized. The semi-circle is the canal section that has
the lowest wetted perimeter for a given cross sectional area, but semicircular canals are difficult
to construct. The closest canal section to a semi-circle is the trapezoid (Figure 10). This is a quite
common cross-section as it is relatively easy to construct.
Figure 10: Sample calculations of hydraulic parameters for different canal shapes
Canals with narrower beds and higher water depths have a smaller wetted perimeter, and thus a
higher discharge, than canals with larger beds and lower water depths, for the same crosssectional area. This is due to the fact that the hydraulic radius R (= As/P) increases if the wetted
perimeter decreases, while keeping the wetted cross-sectional area the same.
Side slope
The side slope (X = horizontal/vertical) should be selected depending on the type of canal, soil
type and the expected vegetation cover on the slopes.
Earthen canals
If the side slopes are very steep (low X) there is high risk of banks collapsing, especially after
heavy rainfall. Therefore, a compromise has to be reached between loss of land (due to larger
width of canal surface) and bank safety. Table 5 (Withers and Vipond, 1974) gives suggested side
slopes for canals in different soil types.
28
Table 5: Typical canal side slopes
Side Slope X
(horizontal/vertical)
Soil type
Stiff clay or earth with concrete lining
Heavy, firm clay or earth for small ditches
Earth with stone lining or earth for large canals
Fine clay, clay loam
Sandy clay or loose sandy earth
Fine sand or sandy loam
Coarse sand
1 to 2
1 to 1.5
1
1.5 to 2
2
2 to 3
1.5 to 3
Concrete-lined canals
There are no strict rules for the side slopes of concrete lined canals. A major consideration is
ease of construction, thus the side slope should not be too steep .side slopes of around 60º
should be easy to construct. Bed width / water depth ratio for trapezoidal canals. The
recommended bed width/water depth (b/d) ratios for earthen trapezoidal canals are (Withers
and Vipond, 1974):
Water depth
Small (d<0.75 m
Medium (d = 0.75-1.50 m)
Large (d>1.50 m)
b/d ratio
1 (clay) – 2(sand)
2(clay) – 3(sand)
>3
The bed width should be wide enough to allow easy cleaning. A bed width of 0.20-0.25 m is
considered to be the minimum, as this still allows the cleaning of the canal with small tools such
as a shovel. Lined trapezoidal canals could have similar b/d ratios as given above.
Maximum water velocities
The maximum permissible non-erosive water velocity in earthen canals should be such that on
the one hand the canal bed does not erode and that on the other hand the water flows at a selfcleaning velocity with minimum or no deposition. A heavy clay soil will allow higher velocities
without eroding than will a light sandy soil. A guide to the permissible velocities for different
soils is presented in Table 6. The maximum velocity ranges for earthen canals on different types
of soil.
Table 6: Typical maximum flow velocities
Soil type
Sand
Sandy loam
Clay loam
Clay
Gravel
Rock
Maximum flow
velocity (m/s)
0.3 – 0.7
0.5 – 0.7
0.6 – 0.9
0.9 – 1.5
0.9 – 1.5
1.2 – 1.8
Source: Withers and Vipond, (1974)
29
Lined canals can manage a higher range of velocities, as erosion is not an issue. However, for
easier management of water, the permissible velocity should be critical or sub-critical.
Freeboard
Freeboard (F) is the vertical distance between the top of the canal bank and the water surface at
design discharge. It gives safety against canal overtopping because of waves in canals or
accidental raising of the water level, which may be a result of closed gates.
The freeboard can be calculated using Equation
F = C x h1/2
Where:
C = 0.8 for discharges of up to 0.5 m3/s up to 1.35 for discharges in excess of 80 m3/s
h = Water depth (m)
Fr=V/(gxI)1/2
Where: I=As/(b+2d)=0.18/(0.30+2x0.30)=0.20 m
For lined canals, F ranges from 0.40 m for discharges less than 0.5 m3/s up to 1.20 m for
discharges of 50 m3/s or more. For very small lined canals, with discharges of less than 0.5 m3/s,
the freeboard depths could be reduced to between 0.05-0.30 m.
30
4 IRRIGATION METHODS AND THEIR APPLICABILITY
There are several irrigation and drainage techniques but the design criteria has to take into
consideration the soil and environmental conditions under which they function. For this,
adequate data must be available and used to select the appropriate irrigation and drainage system
and to ensure sustainable use of water resources. This chapter therefore:
o Highlights the common irrigation techniques, their characteristics and factors that affect
selection of method/system.
o Illustrate the design criteria for different irrigation methods.
o Discuss the irrigation practices for improved water use efficiency for application in rice
production and the challenges associated with crop production, especially when new
crop varieties.
4.1 Factors considered in selection of irrigation system
It is important to select the appropriate irrigation system. There are many factors to consider
before selecting a particular irrigation system (Figure 11). These include water resources,
topography, soil type, climate, type of crops to be grown, availability and cost of capital and
labour, type and appropriateness of a particular irrigation technology to farmers and its
associated energy requirements, water use efficiencies, as well as socio-economic, health and
environmental aspects. For irrigation schemes, it is also necessary to include the mmanagement
and operation skills required, costs of land preparation, grading and leveling, the availability and
quantity of water supply as well as socio-political and cultural considerations.
Figure 11: Variety in surface and ground water abstraction
4.2 Choice of irrigation method
There are several types of irrigation techniques which perform differently under various
conditions (Table 7). The selection an irrigation system for a given area takes into consideration
suitability of the land, water and socio-economic factors based on the types of irrigation systems
commonly used. Based on the method of applying water to the land, there are four broad classes
of irrigation systems:
(1) Surface irrigation systems,
31
(2) Sprinkler irrigation systems,
(3) Localized irrigation systems and
(4) Sub-surface irrigation systems.
Surface irrigation systems apply water to the land by an overland water flow regime. Within this
group are the furrow, borderstrip and basin irrigation systems. The border irrigation method is
wider than furrow and longer than the basin and its selection criteria is similar to furrow and
basin. In sprinkler irrigation systems, water is conveyed and distributed through pressurized pipe
networks before being sprayed onto the land to mimic natural rainfall. There are several sprinkler
irrigation systems, which can broadly be divided into set systems and continuous move systems.
In localized irrigation systems, a pipe distribution network is used to distribute and deliver
filtered water (and fertilizer) to a predetermined point. The three main categories of localized
irrigation methods are drip, spray and bubbler. More recently, drip irrigation systems have been
developed whereby the laterals are buried in the root zone of the crop. Sub-surface irrigation
systems rely on the raising or lowering of the water table in order to affect groundwater flow to
the root zone. As such, sub-irrigation utilizes drainage flow systems.
Table 7: Irrigation methods for different conditions (Withers and Vipond, 1974)
Irrigation
methods
Suitable
crops
Suitable
soils
Slope
Water availability
Advantages
Watering
Bucket
Horticulture
Most soil
types
Relatively
flat
Applicable where
water supply is
unlimited
Basin
Row field
crops and
horticultural
crops
Fine
textured
soils
Relatively
flat
Requires large
quantities of water
and size depends on
water availability
Furrow
Row field
crops and
horticulture
Most soil
types
Very gently
sloping with
slopes less
than 2%
Requires large
quantity of water
Permits
irrigation of
large fields
Sprinkler
Most crops
except rice
Light
textured
and highly
permeable
soils
A wide range
of slopes
Applies in areas
with limited water
supply
Highly efficient
and can be
used in area
with limited
water supply
Drip
Trees and
low density
crops
A wide
range of
soil types
A wide range
of slopes
Applied in areas
with limited water
supply
Efficient water
use
Inexpensive,
requires little
land grading/
leveling
Good control
of large flows.
Can be used to
leach excess
salts
Disadvantages
Labour
intensive
May require
land grading
and leveling,
hence costly
Difficult to
achieve
uniform water
application
High
installation cost
and requires
skill in
management
and operation
High initial
cost
4.3 Surface irrigation systems
Surface irrigation systems are based on the principle of moving water over the surface of the
land in order to wet it, either partially or completely. They can be subdivided into furrow,
border-strip and basin irrigation. The choice of which type of surface irrigation method to apply
depends on various crop type, water availability, soil properties and land levelling (Table 8). In an
irrigation scheme, the layout from the water source up to field level, such as canals and drains,
can be similar for each system. Low irrigation efficiencies are usually associated with poor land
levelling, wrong stream size and change in soil type along the irrigated area both vertically and
horizontally. According to FAO (1989), 95% of the irrigated area in the world is under surface
32
irrigation. Some of the major advantages of surface irrigation systems over other systems are that
they are easy to operate and maintain with skilled labour, they are not affected by windy
conditions and, with the exception of furrow irrigation, they are good for the leaching of the
salts from the root zone. Generally, surface irrigation is associated with low energy costs.
Surface irrigation systems have several disadvantages, though. They are less efficient in water
application than sprinkler or localized irrigation systems. The spatial and temporal variability of
soil characteristics, such as infiltration rate and texture, make water management practices
difficult to define and implement. It is also difficult to apply light, frequent irrigation required
early and late in the cropping season. Another disadvantage can be the high labour demand, as
compared to sprinkler and localized irrigation systems, in situations where labour is not
abundant.
Table 8: Selection criteria for surface irrigation methods based on soil type
Soil type
Sand
Loam
Clay
Crop rooting
depth
Shallow
Medium
Net irrigation depth
per application (mm)
20-30
30-40
Deep
40-50
Shallow
30-40
Medium
40-50
Deep
50-60
Shallow
40-50
Medium
Deep
50-60
60-70
Surface irrigation method
Short furrows
Medium furrows, short borders
Long furrows, medium borders,
small basins
Medium furrows, short borders
Long furrows, medium borders,
small basins
Long borders, medium basins
Long furrows, medium borders,
small basins
Long borders, medium basins
Large basins
Source: Jansen, 1983
4.4 Borderstrip irrigation
Border irrigation involves irrigating strips of land having a small slope but which are levelled to
allow uniform water distribution (Figures 12, and 13). Borderstrips can vary in size ranging from
3-30 m in width and 60-800 m in length. They are separated by parallel dykes or border ridges
(levées). Normally water is let onto the borderstrip from the canal through intakes, which can be
constructed with gates on the wall of the canal or, when unlined canals are used, by temporarily
making an opening in the canal wall. The latter is not recommended since it weakens the walls of
the canal, leading to easy breakage. Other means used for the same purpose is the insertion of
short PVC pipes into the canal through the wall. The short pipes are usually equipped with an
end cup, which is removed when irrigation is practiced.
33
Figure 12: Border strip irrigation layout (Source: Kay, 1986)
Figure 13: Cross-sectional view of border strip irrigation
To determine the borderstrip length, the following factors need to be considered (Table 9):
o Soil type
o Stream size
o Irrigation depth
o Land slope
o Field size and shape
o Cultivation practices.
Table 9: Typical borderstrip dimensions as related to soil type, slope and irrigation depth
Soil
type
Slope
(%)
0.25
Coarse
1.00
2.00
Depth applied
(mm)
50
100
150
50
100
150
50
100
150
Flow Borderstrip Borderstrip
(l/s)
width (m) length (m)
240
15
150
210
15
250
180
15
400
80
12
100
70
12
150
70
12
250
35
10
60
30
10
100
30
10
200
34
Soil
type
Slope
(%)
0.25
Medium
1.00
2.00
Fine
0.25
1.00
2.00
Depth applied
(mm)
50
100
150
50
100
150
50
100
150
50
100
150
50
100
150
50
100
150
Source: Withers and Vipond, 1974
Flow Borderstrip Borderstrip
(l/s)
width (m) length (m)
210
15
250
180
15
400
100
15
400
70
12
150
70
12
300
70
12
400
30
10
100
30
10
200
30
10
300
120
15
400
70
15
400
40
15
400
70
12
400
35
12
400
20
12
400
30
10
400
30
10
400
20
10
400
For the practical management purpose and for the attainment of reasonable irrigation efficiency,
the borderstrip should not go above the critical. The size of border also depends on soil
infiltration properties as indicated in Table 10.
Table10: Critical widths and lengths of borderstrips by soil infiltration rates
Soil type
Sand
Infiltration rate greater
than 25 mm/h
Loam
Infiltration rate 10 to 25
mm/h
Clay
Infiltration rate less than
10 mm/h
Border strip
slope (%)
0.2-0.4
0.4-0.6
0.6-1.0
0.2-0.4
0.4-0.6
0.6-1.0
0.2-0.4
0.4-0.6
0.6-1.0
Source: Withers and Vipond, 1974
Unit flow per
metre width
(l/s)
10-15
8-10
5-8
5-7
4-6
2-4
3-4
2-3
1-2
Borderstrip
width (m)
Borderstrip
length (m)
12-30
9-12
6-9
12-30
9-12
6
12-30
6-12
6
60-90
80-90
75
90-250
90-180
90
180-300
90-300
90
4.5 Basin irrigation
A basin is a level cultivated piece of land surrounded by earthen bunds and totally flooded during
irrigation (Figure 14). Basin irrigation is the most common type of surface irrigation in
developing countries and is particularly used in rice cultivation, where the fields are submerged.
However, basin irrigation is equally suitable for other crops like cereals, fruit trees and pastures –
as long as water logging conditions do not last for too long. Ideally, the water logging should not
last longer than 24-48 hours. Basin irrigation is practiced where land is relatively flat to very
gently sloping with soils with relatively low water permeability. It needs a substantial quantity of
water which can be either pumped or diverted from a river or a reservoir. It can also work with
35
borehole water. It is also used for the leaching of salts by deep percolation in the reclamation of
saline soils.
Figure 14: Layout of basin irrigation system (Source: FAO, 1985)
Flooding should be done using a large stream size that advances quickly in order for water to
spread rapidly over the basin (Figure 15). The advance time should not exceed a quarter of the
contact time, so as to reduce difference in contact time on the different sections of the basin. It
may be used on a wide variety of soil textures, though fine-textured soils are preferred. As the
area near the water inlet is always longer in contact with the water, there will be some percolation
losses, assuming the entire root zone depth is filled at the bottom of the field. Coarse sands are
not generally recommended for basin irrigation as high percolation losses are expected at the
areas close to water intake.
36
Figure 15: Orientation of feeder canals for basin irrigation
Basin size
The size of basin is critical in the design of this irrigation method and, as for furrow and
borderstrip irrigation, depends on the following factors: soil type, stream size, irrigation depth,
field size and shape, land slope, and farming practices. The width of the basin is determined by
the dominant slope of the area being irrigated detailed surveys, soil description and analysis.
There should be detailed description of different parts of the irrigated land so as to establish a
range of slopes and soil types. Not only the selection of the appropriate size of the basin is
important, but also application of the required design criteria and methods of operation. Failure
to follow the required procedures may lead to inefficient function of the basin. Tables 11, 12 and
13 (Withers and Vipond, 1974) indicate the general criteria for selecting a basin size.
Table 11: Criteria for basin size determination
Criteria
Soil type
Stream size
Irrigation depth
Land slope
Filed preparation
Large basin size
Sandy
Small
Small
Steep
Hand or animal traction
Small basin size
Clay
Large
Large
Gentle or flat
Mechanized
37
Table 12: Basin area for different stream sizes
Stream Size
(m3/s)
5
10
15
30
60
90
Basin area (m2)
Sand
35
65
100
200
400
600
Sandy loam
100
200
300
600
1200
1800
Clay loam
200
400
600
1200
2400
3600
Clay
350
650
1000
2000
4000
6000
Table 13: Sample values of maximum basin widths
Slope (%)
0.2
0.3
0.4
0.5
0.8
1.0
1.2
1.5
2.0
3.0
4.0
5.0
Maximum basin width (m)
Average
Range
45
35-55
37
30-45
32
25-40
28
20-30
25
15-30
22
15-25
20
10-20
17
5-15
13
5-10
10
3-8
7
2-6
5
1-4
4.6 Furrow irrigation
Furrow irrigation is another common form of surface irrigation method. It is applicable where
the soil is relatively very permeable A furrow irrigation system consists of parallel channels
(furrows) separated by slightly raised beds or ridges whose shape, spacing and length depend
mainly on the crops to be grown and the types of soils. Siphons are mostly used to take water
from the field canal to the furrows. The functions and efficiency of furrow irrigation are also
governed by the soil type. This is because the soil physical characteristics determine the shape of
the wetting front within the soil profile. As shown in Figure 16, furrow widths vary from 250400 mm, depths from 150-300 mm and spacings can vary from 0.75-1.0 m, depending on soil
type, crops and stream size to be applied to the furrow. Coarse soils require closely-spaced
furrows in order to achieve lateral water flow in the root zone. Figure 17 show the general
wetting patterns of sand and clay. There is more lateral water flow in clay than in sand. Typical
furrow lengths vary from about 60 m on coarse textured soils to 500 m on fine textured soils,
depending on the land slope, stream size and irrigation depth. The minimum and maximum
slopes for furrows should be 0.05% and 2% respectively in areas of low rainfall intensity. In
areas where there is a risk of erosion due to intensive rainfall, the maximum slope should be
limited to 0.3%. Most field crops, except very closely spaced crops such as wheat, as well as
38
orchards and vineyards can be irrigated using furrows. However, with this type of irrigation there
is a risk of localized salinization in the ridges.
Figure 16: Sketch of furrow irrigation and cross-section
Figure 17: Typical soil moisture distribution in a sandy soil (left) and a clay soil (right)
(Source: Kay, 1986)
Like with basin irrigation system, the dimensions of furrows can be designed using the guidelines
of the factors shown in Table 14. These factors must be measured and assessed for different
project sites.
39
Table 14: Furrow length as related to soil type, slope, stream size and irrigation depth
Furrow
slope %
Max. stream
size (l/s)
0.05
0.10
0.20
0.30
0.50
1.00
1.50
2.00
3.0
3.0
2.5
2.0
1.2
0.6
0.5
0.3
Average irrigation depth (mm) for various furrow lengths (m)
Clay
75 m
300
340
370
400
400
280
250
220
150 m
400
440
470
500
500
400
340
270
Loam
50 m
120
120
180
220
280
280
250
220
100 m
270
340
370
400
370
300
280
280
Sand
150 m
400
440
470
500
470
370
340
300
50 m
60
90
120
150
120
90
80
60
75 m
90
120
190
220
190
150
120
90
100 m
150
190
250
280
250
220
190
180
4.6 Sprinkler system
Sprinkler irrigation is a method of applying irrigation water to the soil that tries to mimic natural
rainfall. It is generally an overhead irrigation technique that operates under high pressure, and
utilizes piped water. Sprinkler irrigation comprises a set of equipment which include sprinklers,
risers, laterals, sub-mains, main pipelines, pumping plants and boosters, operational control
equipment and other accessories required for efficient water application. In some cases, sprinkler
systems may be pressurized by gravity and therefore pumping plants may not be required. The
mainline is the pipe that delivers water from the pump to the laterals. The risers then convey the
irrigation water from the laterals through a valve to the sprinkler where it is sprinkled over the
crops (Figure 18). The sprinkler forms the most important component of the system. The
planning and design of irrigation systems should aim at maximizing the returns and minimizing
both the initial capital outlay and the costs per unit volume of water used, thus contributing both
directly and indirectly to the overall reduction of the production costs and the increase of
returns. In other words, planning and design is a process of optimizing resources. Sprinkler
irrigation requires skill and is fairly expensive to install and operate. The laterals can be moved
from one irrigated field to another depending on the planned irrigation cycle.
Figure 18: Sprinkler irrigation method
40
Types of sprinkler irrigation systems
1. Semi-portable sprinkler irrigation system, suitable for an individual small farm
2. Semi-permanent sprinkler irrigation system for a smallholder scheme (system for several
small fields)
3. Drag-hose sprinkler irrigation system for a smallholder scheme (system for several small
fields)
4. Hose-drag travelling irrigator for individual farm
5. Hose-pull travelling irrigator for individual farm
6. Solid-set system which is permanent, mostly used in large farms.
The outputs of the designs are alternative irrigation system options for possible adoption. Once
the components of each system are selected, a bill of quantities will be drawn up for each case in
order to estimate the cost of the project.
4.7 Localized Irrigation systems
Localized irrigation is a system for supplying filtered water (and fertilizer) directly onto or into
the soil. The water is distributed under low pressure through a pipe network, in a pre-determined
pattern, and applied as a small discharge to each plant or adjacent to it. There are three main
categories of localized irrigation:
ƒ drip irrigation, where drip emitters are used to apply water slowly to the soil surface
ƒ spray irrigation, where water is sprayed to the soil near individual trees
ƒ bubbler irrigation, where a small stream is applied to flood small basins or the soil
adjacent to individual trees
A localized irrigation system consists of the head of the system that filters and controls the
supply of water and fertilizers to the network, the plastic buried pipes that supply the water to
the laterals, the polyethylene laterals, usually 16-20 mm in diameter, that supply the water to the
emitters, and the emitters that discharge the water to the pre-determined points and at predetermined flows.
Localized irrigation system is a capital-intensive system with built in management that requires
very little but skilled labour. The main advantage of localized irrigation is its potential to reduce
water requirements and achieve a very high efficiency, while at the same time increasing crop
yield and quality. The system has been successfully used on tree and vegetable crops, and high
yields attributed to it. Localized irrigation provides the means for very frequent irrigation, daily if
needs be. Hence it is particularly suitable for light shallow soils, irrespective of slope, and for
shallow-rooted crops. It has also proved suitable for most row crops. The main disadvantages of
localized irrigation systems are their high capital cost, a susceptibility to clogging and a tendency
to build up localized salinity, especially in low rainfall areas. As such, this category of system
requires careful management for its maintenance.
4.8 Drip Irrigation
Drip irrigation is the slow application of water to the soil through mechanical devices called
emitters, located at selected points along the delivery line (Figure 19). Water is conveyed under
pressure through a pipe system to the fields where it drips slowly onto the soil through emitters
(drippers), which are located close to the plants. For low pressure, drip is appropriate overhead
irrigation method.
41
Figure 19: Schematic representation of drip irrigation in the field
4.9 Bucket irrigation
Bucket irrigation is still a popular method among small scale farmers in Africa. It is affordable
and easy to apply in areas where labour is abundant and gardens are small. Though relatively
inefficient, it can be organized to apply the appropriate amounts of water. Different sizes of the
buckets can be used to apply different quantities of water which will penetrate to different
application depths for a specified soil types. Table 15 shows rough guidelines on appropriate
water application using bucket irrigation. It also requires knowing the different soil types in an
irrigated area so that correct irrigation water can be applied to suit specific conditions and crops.
Table 15: Water application depths for bucket irrigation using 15 and 20 litre cans
Bucket size
Quantity of
water(litres)
15 litre-Bucket
20 litre-Bucket
15
30
45
60
20
40
60
80
Irrigated Area
(m2)
3
6
5
26
10
5
15
7.5
20
10
7
3
13
7
20
10
27
13
42
4.10 Pitcher Irrigation
Pitcher Irrigation or ‘Pot Irrigation’ is a traditional, low-volume irrigation technology that uses
baked clay pots buried adjacent the roots of the crop to be irrigated (Mati, 2007). Such pots are
made by women in the traditional way, but the clay is mixed with saw-dust to create porosity
when the pot is fired during curing. The pot is filled with water and covered with a clay slab or
polythene paper, to reduce evaporation losses. Water seeps slowly through the porous sides of
the pot (Figure 20). The minute hairs of nearby plants pull the water out from the pots. The
method encourages deeper rooting and reduced evaporation. The method is commonly used for
fruit-tree crop production.
Figure 20: Illustration of pitcher pot irrigation
43
5. DRAINAGE TECHNIQUES
5.1 The need for drainage
The problem of drainage arises due to excess waterlogging in a spatial area, which can be surface
or sub-surface. These are caused by excess rainwater and high water table or poor drainage in
irrigated lands. The consequences of impeded drainage are normally poor aeration of the crop
root zone and increased salinization.
Identifying the site for drainage can be guided by the following:
o Drainage on demand by the residents,
o Surveillance/reconnaissance surveys, and
o A study of existing maps, data and literature.
Other factors considered include land tenure/ownership, legislation, and viability. In planning
field drainage system, the following factors are considered include the causes of waterlogging, the
extent of drainage required, the feasibility of the system, e.g. where to dispose drained water, and
the choice of the system.
Drainage is needed in order to:
o Maintain the soil structure
o Maintain aeration of the root-zone, since most agricultural crops require a well aerated
root-zone free of saturation by water; a notable exception is rice
o Assure accessibility to the fields for cultivation and harvesting purposes
o Drain away accumulated salts from the root zone
A drainage system can be surface, sub-surface or a combination of the two.
5.2 Factors that affect drainage
Climate
Irrigation schemes in an arid regions require different drainage systems compared to those in
humid areas. Arid climates are characterized by high-intensity, short-duration rainfall and high
evaporation losses throughout the year. The aim of drainage in this case is to dispose off excess
surface runoff, emanating from the high-intensity precipitation, and to control the water table so
as to prevent the accumulation of salts in the root zone, due to high evapotranspiration. Surface
drainage system is most appropriate in this case. In a humid climates, the removal of excess
surface and subsurface water originating from rainfall is the paramount. Both surface and
subsurface drains are commonly used in humid areas.
Soil type and profile
The rate at which water moves through the soil determines the ease of drainage. Therefore, the
physical properties of the soil have to be examined for the design of a subsurface drainage
system. Sandy soils are easier to drain than heavy clay soils. Capillary rise is the upward
movement of water from the water table. It is inversely proportional to the soil pore diameter.
The capillary rise in a clay soil is thus higher than in a sandy soil. In soils with layered profiles,
drainage problems may arise when an impermeable clay layer exists near the surface.
Water quantity
The quantity of water flowing through the soil can be calculated by means of Darcy’s law:
Q=kxAxI
Q = Flow quantity (m3/sec)
k = Hydraulic conductivity (m/sec)
A = Cross-sectional area of the soil through which the water moves (m2)
44
i = Hydraulic gradient
The hydraulic conductivity, or the soils’ ability to transmit water, is an important factor in
drainage flow. Procedures for field measurements of hydraulic conductivity are discussed below.
Irrigation practice
The type of irrigation practice has a bearing on the amount of water applied to the soil and the
rate at which it is removed. For example, poor water management practices result in excess water
being applied to the soil, just as heavy mechanical traffic results in a soil with poor drainage
properties due to compaction.
5.3 Design of agricultural drainage systems
Good water management of an irrigation scheme not only requires proper water application but
also a proper drainage system. Agricultural drainage can be defined as the removal of excess
surface water and/or the lowering of the groundwater table to below the root zone in order to
improve plant growth. The common sources of the excess water that have to be drained are
precipitation, over irrigation and the extra water needed for the flushing away of salts from the
root zone. Furthermore, an irrigation scheme should be adequately protected from drainage
water coming from adjacent areas.
Information required for the drainage design includes:
o Rainfall characteristics of the area (intensity and duration).
o Topography
o Soil properties such as depth, infiltration, permeability and hydraulic conductivity
o Leaching requirements
o Drainage outlets
o Drainage coefficient, i.e. depth drained in area per day.
o Sources of excess water – precipitation, irrigation water, overland flow, artesian aquifers.
o Drainage system applicable, i.e. random, parallel or cross slope.
o Whether gravity or pumped outlets.
The drainage system parameters required for the design include:
o The system type and layout – depth, spacing and materials.
o Design capacity – channel/tile discharge.
o Channel shape – side slopes.
o Roughness coefficient.
o Permissible flow velocity, depending on the soil type, i.e. whether clay, sandy loam, fine
sand etc
o Energy/slope reduction – use of checks and drops
o Degree of erosion.
A procedure for designing the drainage system is given in Figure 21, in which important
considerations include water discharge, surface roughness, side slopes of the canal and the
gradient of the flow.
45
Figure 21: Flow chart indicating drainage design
Determining hydraulic conductivity
Hydraulic conductivity is an important variable that depicts how well water moves in the soil. It
depends on the actual soil conditions. In clear sands it can range from 1-1,000 m/day, while in
clays it can range from 0.001-1 m/day. Several methods for field measurement of hydraulic
conductivity have been established. One of the best-known field methods for use when a high
water table is present is Hooghoudt’s single soil auger hole method. A vertical auger hole is
drilled to the water table and then drilled a further 1-1.5 m depth or until an impermeable layer
or a layer with a very low permeability is reached. The water level in the hole is lowered by
pumping or by using buckets. The rate of recharge of the water table is then timed. For the
calculation of the hydraulic conductivity the following formula has been established:
k={(3600a2/(d+10a)+(2-y/d)x y)}x ΔH/Δt
Where:
k = Hydraulic conductivity (m/day)
a = Radius of the auger hole (m)
d = Depth of the auger hole below the static ground water table (m)
ΔH = Rise in groundwater table over a time (t) (cm)
Δt = Time of measurement of rise in groundwater table (sec)
y = Average distance from the static groundwater table to the groundwater table during
the measurement:
y = 0.5 x (y1 + y2) (m)
Worked example
An auger hole with a radius of 4 cm is dug to a depth of 1.26 m below the static groundwater table. The rise of
the groundwater table, measured over 50 seconds, is 5.6 cm. The distance from the static groundwater table to the
46
groundwater table is 0.312 m at the start of the measurement and 0.256 m at the end of the measurement. What
is the hydraulic conductivity?
a = 0.04 m
d = 1.26 m
ΔH = 5.6 cm
Δt = 50 sec
y = 0.5 x (0.312 + 0.256) = 0.284 m
Substituting the above data in Equation gives:
k={(3600 x 0.042)/(1.26+10x0.04) + (2-(0.0284/1.26)x0.284)}x5 .6/50
If the water table is at great depth, the inverted auger hole method can be used to determine the
hydraulic conductivity. The hole is filled with water and the rate of fall of the water level is
measured. Refilling has to continue until a steady rate of fall is measured. This value is used for
determination of k, which can be found from graphs.
5.4 Surface drainage
When irrigation or rainfall water cannot fully infiltrate into the soil over a certain period of time
or cannot move freely over the soil surface to an outlet, ponding or waterlogging occurs.
Grading or smoothening the land surface so as to remove low-lying areas in which water can
settle can partly solve this problem. The excess water can be discharged through an open surface
drain systems. Drains of less than 0.50 m deep can be V-shaped. In order to prevent erosion of
the banks, field drains often have flat side slopes, which in turn allow the passage of equipment.
The side slopes could be 1:3 or flatter. Larger field drains and most higher order drains usually
have a trapezoidal cross-section.
The water level in the drain at design capacity should ideally allow free drainage of water from
the fields. The design of drain dimensions should be based on a peak discharge. It is, of course,
impractical to attempt to provide drainage for the maximum rainfall that would likely occur
within the lifetime of a scheme. It is also not necessary for the drains to instantly clear the peak
runoff from the selected rainfall because almost all plants can tolerate some degree of
waterlogging for a short period. Therefore, drains must be designed to remove the total volume
of runoff within a certain period. If, for example, 12 mm of water (= 120 m3/ha) is to be drained
in 24 hours, the design steady drainage flow of approximately 1.4 1/sec per ha (= (120 x
103)/(24 x 60 x 60)) should be employed in the design of the drain. If rainfall data are available,
the design drainage flow, also called the drainage coefficient, can be calculated more precisely for
a particular area. The following method is usually followed for flat lands. The starting point is a
rainfall-duration curve, an example of which is shown in the Figure 22.
47
Figure 22: Rainfall- Duration curve
This curve is made up of data that are generally available from meteorological stations. The curve
connects, for a certain frequency or return period, the rainfall with the period of successive days
in which that rain is falling. Often a return period of 5 years is assumed in the calculation. It
describes the rainfall which falls in X successive days as being exceeded once every 5 years. For
design purposes involving agricultural surface drainage systems X is often chosen to be 5 days.
Thus, it follows that the rainfall falling in 5 days is 85 mm. This equals a drainage flow
(coefficient) of 1.97 l/sec per ha (= (85 x 10 x 103)/(5 x 24 x 60 x 60)).
The design discharge can be calculated, using the following equation:
Q = (q x A)/1000
Where:
Q = Design discharge (m3/sec)
q = Drainage flow (coefficient) (l/sec per ha)
A = Drainage area (ha)
It would seem contradictory to take 5 days rainfall, when the short duration storms are usually
much more intensive. However, this high intensity rainfall usually falls on a restricted area, while
the 5 days rainfall is assumed to fall on the whole drainage area under consideration. It appears
from practice that a drain designed for a 5 days rainfall is, in general, also suited to cope with the
discharge from a short duration storm. Thus, the above scenario is not necessarily true in small
irrigation schemes, especially on sloping lands (with slopes exceeding 0.5%), which may cover an
area that could entirely be affected by an intense short duration rainfall. The design discharge
could then be calculated with empirical formulas, two of the most common ones being:
o The rational formula
o The curve number method
The rational formula is the easier of the two and generally gives satisfactory results. It is also
widely used and will be the one explained below. The formula reads:
Q = (C x I x A)/360
Where:
Q = Design discharge (m3/sec)
C = Runoff coefficient
I = Mean rainfall intensity over a period equal to the time of concentration (mm/hr–)
A = Drainage area (ha)
48
The time of concentration is defined as the time interval between the beginning of the rain and
the moment when the whole area above the point of the outlet contributes to the runoff. The
time of concentration can be estimated the following formula:
Tc = 0.0195 x K0.77
Where:
Tc = Time of concentration (minutes)
K = L/√S and S = H/L
L = Maximum length of drain (m)
H = Difference in elevation over drain length (m)
The runoff coefficient represents the ratio of runoff volume to rainfall volume. Its value is
directly dependent on the infiltration characteristics of the soil and on the retention
characteristics of the land. The values are presented in Table 16.
Table 16: Guide values for run-off coefficients
Land cover
Forest
Pastures
Arable land
Slope
(%)
0-5
5-10
10-30
0-5
5-10
10-30
0-5
5-10
10-30
Sandy
loam
0.10
0.25
0.30
0.10
0.15
0.20
0.30
0.40
0.50
Silty clay
loam
0.30
0.35
0.50
0.30
0.35
0.50
0.50
0.60
0.70
Clay
0.40
0.50
0.60
0.40
0.55
0.60
0.60
0.70
0.80
Worked Example
An irrigation scheme of 100 ha with sandy loam soils and a general slope of less than 5% has a main drain of
2.5 km long with a difference in elevation of 10 m. What is the time of concentration?
S = H/L = 10/2500 = 0.004 or 0.4%
K = L/√S = 2500/√0.0004 = 39528
Substituting this value of K into Equation
Tc = 0.0195 8 395280.77 = 68 minutes
The rainfall intensity can be obtained from a rainfall duration curve. For short duration rainfall, it
is necessary to make a detailed rainfall duration curve for the first few hours of the rainfall.
In previous example, the 68 minutes rainfall with a return period of 5 years is estimated at 8.5 mm. What is the
design discharge of the drain?
The mean hourly rainfall intensity is (60/68) x 8.5 = 7.5 mm/hour. The runoff coefficient for
sandy loam arable land with a slope of less than 5% is 0.30 (from table)
Thus the design discharge for the scheme is:
Q = (C x I x A)/360 = (0.30 x 7.5 x 100)/360
Q = 0.625 m3/s or 6.26 litres/sec per ha
49
Once the design discharge has been calculated, the dimensions of the drains can be determined
using the Manning Formula. It should be noted that higher order canal design should not only
depend on the design discharge, but also on the need to collect water from all lower order drains.
Therefore, the outlets of the minor drains should preferably be above the design water level of
the collecting channel.
5.5 Subsurface drainage
Subsurface drainage (Figure 23) is used to control the level of groundwater so that the root zone
can be aerated. The natural water table can be so high that without a drainage system it would be
impossible to grow crops. After establishing the irrigation system the groundwater table might
rise into the root zone because of percolation of water to the groundwater table. These situations
may require a subsurface drainage system.
Figure 23: Subsurface drainage systems at field level
A subsurface drainage system at field level can consist of any of the systems shown above:
o Horizontal drainage by open ditches (deep and narrowly-spaced open trenches) or by pipe
drains
o Vertical drainage by tubewells and drainage tiles.
5.5.1 Horizontal subsurface drainage
Open drains can only be justified to control groundwater if the permeability of the soil is very
high and the ditches can consequently be spaced widely enough. Otherwise, the loss in area is
too high and proper farming is difficult because of the resulting small plots, especially where
mechanized equipment has to be used.
Instead of open drains, water table control is usually done using field pipe drains. The pipes are
installed underground (thus there is no loss of cultivable land) to collect and carry away excess
groundwater. This water could be discharged through higher order pipes to the outlet of the area
but, very often, open ditches act as transport channels.
The materials used for pipe drains are:
50
o Clay pipes (water enters mainly through joints)
o Concrete pipes (water enters mainly through joints)
o Plastic pipes (uPVC, PE, water enters through slots)
Plastic pipes are the most preferred choice nowadays, because of lower transport costs and ease
of installation, although this usually involves special machinery. The principal design parameters
for both open trenches and pipe drains are spacing and depth, which are both shown in Figure
24 and explained below.
Figure 24: Subsurface drainage parameters
The most commonly used equation for the design of a subsurface drainage system is the
Hooghoudt Equation, which is:
S2 = (4 x k1 x h2) + (8 x k2 x d x h)
h
Where:
S = Drain spacing (m)
k1 = Hydraulic conductivity of soil above drain level (m/day)
k2 = Hydraulic conductivity of soil below drain level (m/day)
h = Hydraulic head of maximum groundwater table elevation above drainage level (m)
q = Discharge requirement expressed in depth of water removal (m/day)
d = Equivalent depth of substratum below drainage level (m).
It should be noted that the Hooghoudt Equation is a steady state one, which assumes a constant
groundwater table with supply equal to discharge. In reality, the head losses due to horizontal
and radial flow to the pipe should be considered, which would result in complex equations. To
simplify the equation, a reduced depth (d) was introduced to treat the horizontal/radial flow to
drains as being equivalent to flow to a ditch with the impermeable base at a reduced depth,
equivalent to d. The equivalent flow is essentially horizontal and can be described using the
Hooghoudt formula. The average thickness (D) of the equivalent horizontal flow zone can be
estimated as:
D = d + h/2
Nomographs have been prepared to determine the equivalent depth more accurately (Figure 25).
51
Figure 25: Nomograph to determine equivalent sub-stratum depth (Source: FAO, 1985)
Worked Example
A drain pipe of 10 cm diameter should be placed at a depth of 1.80 m below the ground surface. Irrigation water
is applied once every 7 days. The irrigation water losses, recharging the already high groundwater table, amount to
14 mm per 7 days and have to be drained away. An average water table depth, z of 1.20 m below the ground
surface, has to be maintained. k1 and k2 are both 0.8 m/day (uniform soil). The depth to the impermeable layer
D is 5 m. What should be the drain spacing?
q = 14/7 = 2 mm/day or 0.002 m/day and h = 1.80 – 1.20 = 0.60 m
The calculation of the equivalent depth of the sub-stratum d is done through trial and error.
Initially the drain spacing has to be assumed using nomograph. After determining d, the assumed
S should be checked with the calculated S from the Hooghoudt Equation.
Lets assume S = 90 m.
The wetted perimeter of the drain pipe, u, is 0.32 m (= 2 x π x r = 2 x 3.14 x 0.05).
Thus D/u = 5/0.32 = 15.6.
52
From Figure 25 it follows that d = 3.65 m.
This has been determined as follows:
– Draw a line from D = 5 on the right y-axis to D/u = 15.6 on the left y-axis
– Determine the intersection point of the above line with the S = 90 line
– Draw a line from this point to the right y-axis, as shown by the dotted line
– The point where it reaches the right y-axis gives the d value
Substitution of all known parameters in the Hooghoudt Formula (Equation) gives:
S2 = {(4x0.8x0.62) + (8x0.8x3.65x0.6)}/0.002 = 7584 m2
Thus S = 87 m, which means that the assumed drain spacing of 90 m is quite acceptable.
5.5.2 Vertical subsurface drainage
Where soils are of high permeability and are underlain by highly permeable sand and gravel at
shallow depth, it may be possible to control the water table by tube wells located in a broad grid,
for example at one well for every 2 to 4 sq.km. Tube wells minimize the cost of and disturbance
caused by field ditches and pipe drains and they require a more sparse drainage disposal network.
The excess water is pumped out from the tube wells and disposed safely. If the groundwater is
of good quality, it could be re-used for irrigation or other purposes.
5.5.3 Salt problems in irrigated agriculture
Salt problems in the crop root-zone occur mainly in arid countries. Drainage systems installed
for the purpose of salinity control aim at removing salts from the soils so that a salinity level that
would be harmful to plants is not exceeded. Irrigation water sometimes contains salts, but to
varying degrees. When the water is applied to the soil surface, some of it evaporates or is taken
up by the plants, leaving salts behind in the root zone. If the groundwater table is too high, there
will be a continuous capillary rise into the root zone and if the groundwater is saline, a high
concentration of salts will accumulate in the root-zone. Figure 26 demonstrates this
phenomenon.
Leaching is the procedure whereby salts are flushed out from the root-zone by applying excess
water, sufficient in quantity to reduce the salt concentration in the soil to a desired level.
Generally, about 10-30% more irrigation water than is needed by the crops should be applied to
the soil for this purpose. This excess water has to be drained away by the subsurface drainage
system.
Figure 26: Salt accumulation in the root zone showing effect of capillary rise
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6
OPERATION AND MAINTENANCE OF IRRIGATION SCHEMES
6.1 Participatory irrigation development
The sustainability of an irrigation scheme refers to the proper functioning of the infrastructure,
the people, agricultural enterprises, management and social systems in the long run. This
happens if all factors are considered at planning and design of the scheme, but require regular
updating with changing circumstances. Generally, the social and institutional context of irrigation
development has immense bearing on the ultimate performance of irrigation schemes. Over the
years, the process of implementation of irrigation projects, especially those spearheaded by
governments and some donors, followed a top-down approach. However, experience has shown
that if farmers are not involved in all the development stages of a project, they will lose the sense
of ownership and therefore treat that project as alien to them. Consequently, the long-term
performance and sustainability of the scheme is negatively affected. According to FAO (1999),
projects planned with beneficiaries, rather than for them, have proved more sustainable and no
more costly. Problems of farmer participation are rarely encountered in privately owned schemes
or those initiated by farmers. However, for schemes initiated by donors or governments, there is
a need for close consultation between farmers and implementing agencies in all stages of
development. This can be achieved through participatory planning, designing, construction and
management of irrigation schemes. The following factors are taken into consideration:
ƒ The importance of participatory planning in management irrigation systems.
ƒ Indicators of sustainability of irrigation systems,
ƒ Concepts of operation, function and maintenance of an irrigation system,
ƒ Principles of establishing the organization structure of an irrigation scheme and initiate
preparation of an action plan from each participant,
ƒ Interrelationships of the biophysical, social, economic and policy dimensions in
sustaining an irrigation system.
6.2 Participatory and strategic change process
To be effective, reform should be both participatory and strategic. A reform is participatory when
it includes all stakeholders in the process of assessment, policy making, programme formulation
and implementation (FAO, 1999). A stakeholder is any person or group which has an important
interest in the prospective reforms. Reform is strategic when it deals with fundamental issues and
is forward-looking, politically feasible and integrated with the external environment. Strategic
change is difficult. It requires a methodology and coordination with stakeholders, in order to
mobilize diverse inputs and build consensus. Participatory and strategic reform generally involves
the following elements:
• Representational involvement of stakeholders;
• Setting objectives;
• Assessing management gaps and options for change;
• Developing a shared vision of the future;
• Developing policies and programmes;
• Facilitating teams to work on the process;
• Analysis, negotiation and possibly experimentation;
• Organizational restructuring; and
• Performance assessment.
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6.3 Identification of stakeholders
Stakeholders are individuals, groups or organizations who have an interest in a particular project.
For irrigation projects these are normally farmers, persons displaced by the project, lending
institutions, government, donors, input suppliers, service suppliers and buyers. The purpose of
stakeholder participation in project development is to give planners and the parties involved an
overview of the persons, groups, organizations and institutions involved in or connected with
the project. Participation is expected to result in the incorporation of the interests and
expectations of all parties significant to the project. It will also provide room for the clearing of
potential conflict areas. To encourage inclusive participation of stakeholders in irrigation
involves:
o Identifying the persons, groups and organizations connected with, influencing or
influenced by the project
o Identifying their level of influence on the project, e.g. persons who provide more labour,
local leaders and displaced persons rank higher than middlemen
o Sensitizing and involving them in all decision-making processes and characterize their
influence on the project
o Assuring them and making them feel that they have the power to influence the course of
development and their own welfare. Let them take over and be the prime movers of the
project.
6.4 Participation of farmers in planning and design
To improve on the performance of the irrigation scheme and the productivity of water, it is
important to instil best practice at planning, through detailed analysis of physical as well as social
assets and limitations. This is because implementations of the identified technical solutions
depend on the extent of understanding and addressing socio-economic issues first, followed by
policy and biophysical constraints. Therefore, planners, engineers, technicians, managers and
social workers charged with the responsibility of irrigation should initiate the process of
participatory planning for harmonious working and ultimate improvement of the efficiency of
the whole system. This should include the following:
o The farmers should be involved in the selection of lands to be irrigated and the irrigation
agency should assist farmers by assessing the suitability of those lands
o The communities within the area to be developed should participate in the technical as well
as the environmental impact assessment (EIA)
o Where possible, farmers should provide labour for topographic, soil and socio-economic
surveys. They should, through their committees, decide who should do which activity
o Farmers could provide information on past experience with floods, point out areas with
potential for flooding, and suggest to the planners locations for structures such as water
abstraction from the river, hence preventing the pumping station from being flooded
o The farmers should be fully involved in the selection of crops to be grown and the agency
should guide them on the technical, environmental and socio-economic matters related to
the suitability of such crops including returns on investment, storage, processing and
marketing potential.
o The irrigation agency should facilitate extension and training on various irrigation methods
and enterprises pointing out the advantages and disadvantages of each. The farmers then
should propose the irrigation methods they prefer for the design.
o After completing the designs, the irrigation agency should explain the alternative designs to
farmers and the implications of each vis-à-vis land redistribution, water resources potential,
plot sizes and total area to be irrigated, cropping programmes, labour requirements, capital
costs, operation and maintenance costs, environmental aspects, land use patterns and other
considerations
55
o Finally, the farmers should decide which option to adopt, preferably one which serves the
majority and has least risks.
6.5 Farmer participation in irrigation development
During project identification, stakeholders of an irrigation scheme should be identified first.
Irrigation projects should ideally be developed on farmers’ requests in order to ensure that
development is demand-driven. However, government, donors, NGOs or other agencies may
identify a need for them and sensitize them to initiate the process. In this case it is incumbent
upon the institution spearheading the development to mobilize farmers and other stakeholders
so that they appreciate the benefits of irrigation and will give their go-ahead for the project.
Meetings and continuous dialogue throughout the development process are necessary for the
stakeholders to make contributions as well as to identify and defuse potential conflicts. There
should also be agreements, preferably written and signed, that each party will execute its function
throughout the planning, design, implementation, operation and maintenance of the scheme.
Planners should take into account the fact that new developments tend to alter traditional land
use patterns and are a potential source of conflict. Potential conflict areas should be identified
and addressed from the outset. Therefore, there is a need to actively involve the affected
communities in the decision-making process right from the outset. Appropriate policies should
guide the processes. In order to capture the determinant issues for farmer participation, planners
should understand:
a) The characteristics of the farmer groups they are dealing with:
– Social background, religion and cultural aspects,
– Status of groups in society, formal or informal,
– Organizational and leadership structures, and
– Current constraints and farmers’ priorities.
b) Farmers’ interests, motives and attitudes:
– Needs and aspirations,
– Openly expressed, hidden and vested interests,
– Hopes, expectations and fears related to the project, and
– Attitudes, friendly or hostile, towards implementing agencies and other groups
c) The farmers’ potentials:
– Strengths of groups with regards to skills, resources, knowledge, rights,
– Weaknesses and constraints, e.g. knowledge of benefits of project or otherwise,
– What the group can contribute to or withhold from the project, and
– The role of water user groups and other subsidiary actors in the project.
d) The implications of farmers’ roles and responsibilities on the planning, design and
construction of the project:
– How the project should be designed and implemented in order to address the concerns
and needs of the farmers or farmer groups.
In this respect the use of the participatory rural appraisal (PRA) tool can facilitate the
understanding of existing opportunities and constraints as well as farmers’ perceptions of how
irrigation can be used to remove some of the constraints in crop production. During the same
process, and in order to avoid interference by individuals or groups that may have vested
interests, farmers should identify the stakeholders that will be involved with the participatory
planning. Also, right from the outset not only the advantages but also the responsibilities that
come hand-in-hand with a new scheme should be made clear to all involved.
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6.6 Farmer participation in the implementation of an irrigation scheme
The implementation of an irrigation project involves preparing tender documents for
construction, evaluating the tenders, selecting the contractor and supervising construction. The
farmers should be involved in all these processes, especially if they are contributing part of the
finance, in cash or kind, for the project (Mati, 2008). The irrigation agency should provide
technical information to assist the farmers in reaching decisions. The farmers should contribute
their own labour for certain construction activities, such as trenching, back-filling, pipefitting,
land levelling and concrete mixing. This will also assist them gain some experience needed later
in the maintenance of the project. In this respect it is advisable to use labour-intensive methods,
wherever possible. The supervision of construction still remains the responsibility of the
irrigation agency. Where the farmers contribute money for the project, they should also sign
certificates authorizing payments to service providers.
6.7 Operation and maintenance responsibilities
The responsibilities of operation and maintenance (O&M) of an irrigation scheme should be
clear to all parties from the outset. To assist farmers in selecting a design alternative, planners
should estimate the O&M requirements at the planning stage and discuss them with farmers. If
the irrigation agency is to pay for O&M for a specified time before hand-over to farmers, the
farmers should be organized and prepared for take-over well in advance. While the experience
gained by the farmers during the course of planning and development is a valuable tool for the
O&M of the irrigation scheme, farmers would still require assistance from the irrigation agency
and the extension service in the form of training in the following areas:
Crop production and protection
Irrigation scheduling and in-field water management
Schedule of scheme maintenance
Bookkeeping and accounting
Access to markets and market information
Sustainability of water user groups and other management structures
Such training should be practical, in order to provide the hands-on experience needed and
should take into consideration that the background of most smallholders in Eastern and
Southern Africa is in rainfed crop production (Mati et al, 2008). It is necessary to have
appropriate guidelines, procedures and relevant material for the development of a participatory
training and extension programme for technical staff, extension workers and other stakeholders.
This helps farmers take charge of water management at field and scheme levels. The programme
is particularly relevant to irrigation management transfer programmes, assisting water users
associations in the operation and maintenance of irrigation systems, and in providing guidance
on efficient water use techniques.
6.8 Monitoring and evaluation of smallholder irrigation schemes
Once an irrigation scheme has been implemented, there is need to continuously monitor its
performance, in order to identify constraints and opportunities for improved performance.
There are a number of parameters that can be measured or assessed as performance indicators.
These include; technical system performance, which looks at performance in terms of water use
efficiencies and other related parameters; economic analyses, which evaluates economic and
financial performance; as well as socio-economic analyses, which evaluate the impact of
economic performance on the social well-being of the people. Success resulting from irrigation
57
development as associated with farmer participation, is reflected by the socio-economic benefits
accrued to the beneficiaries.
6.9 Indicators of sustainability
The indicators of the sustainability of an irrigation scheme should be holistic and encompassing
all the important dimensions, since they will be used in monitoring and evaluating the impacts of
irrigation on environment and livelihoods. In carrying out the field study for the development of
the new irrigation scheme or improving the existing ones, it is important to identify these
indicators, based on the observed biophysical, technical and social constraints. These include:
o Biophysical/environmental indicators
o Economic indicators
o Social indicators
o Policy indicators
Most of the biophysical/environmental and economic indicators can be measured directly, while
social and policy indicators are difficult to measure. The main challenge for the irrigating
technicians, engineers, managers and farmers is the need to:
o Identify and map out the biophysical constraints
o Define the critical limits of the indicators below or above which the impacts of irrigation
would be negative
o Define the mechanisms for monitoring the changes in these indicators
Sustainable agricultural productivity and improved water use efficiency requires integrated
approaches that may include (Mati and Penning de Vries, 2005); (i) new knowledge, (ii) training,
(iii) educational levels, iv) investment of financial capital, (v) cost sharing and loans, (vi) markets,
(vii) facilitation, (viii) involving government institutions, (ix) infrastructure (roads, mobile
phones), (x) involvement of youth, (xi) land tenure, and (xii) water availability. In addition,
mapping and differentiating of the irrigated area should be based on detailed survey and
laboratory analysis of soil, in addition to physical and hydraulic characterization. Full water and
fertility management packages for each production system along with irrigation scheduling, based
on the hydraulic conditions of the soil, can lead to sustainable irrigated agriculture.
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7
SUMMARY CONCLUSIONS AND RECOMMENDATIONS
Climatic data are important input-variables into the models used for estimating the capacity and
strength of irrigation and drainage structures. There is also need for biophysical and hydrological
characterization of catchment areas as basis of predicting run-off and water flow patterns.
It is important to differentiate between the irrigation requirement and crop water use and where
to apply each. It is necessary to take into account the ground water contribution, effective rain
and field water balance, in determining how much water to apply per irrigation shift. This is
particularly important where the ground contribution through capillary rise is significant and
there are no conveyance loses to be accounted for.
The design of irrigation system commences with the calculated crop water requirements over the
specified cropped area, efficiencies are factored in to determine gross water requirement from
the source so as to calculate the design capacity of the channels and the system capacity.
Generally, the design capacity of the drainage channels is based on both the excess irrigation and
both in-situ and flowing rain water (runoff).
It is useful to demonstrate and practically apply measurement of flow/discharge in open
channels in order to estimate flows. However, it is important that measuring gauges are installed
and calibrated at appropriate points within irrigation channels for use with permanent
weirs/structures. Understanding the working principles of stream gauges and how to calibrate
them is important.
It is necessary to learn how to identify the appropriate irrigation and drainage techniques in
projects. The factors to be considered in selecting and the design of these techniques include
climatic characteristics of the area, soil types, slopes, crops to be grown, labour requirements,
management and operational skills required and the cost involved in land preparation, grading
and levelling.
At country scales, the available maps and data on characterized and classified soils and
topography, provides some basis for selecting appropriate irrigation methods for specific soil
types, climatic and topological conditions.
Identification, planning and design of an irrigation system requires data, which are obtained
through a feasibility study. The sustainability of an irrigation system depends on the extent to
which the socio-economic and policy dimensions are integrated in the strategies to sustain soil
quality and productivity for enhanced water use efficiency. These could include:
o Water gauges should be installed and calibrated using existing control structures such as
weirs and drop structures for irrigation water flows within the primary and secondary canals.
o Development of the production units, based on detailed biophysical and socio-economic
characterization and packaging the appropriate soil fertility management strategies and
monitoring the impacts of irrigation on environmental productivity and sustainability.
o Each production unit/scheme should initiate the process of participatory approaches to
development strategies for improvement of water use efficiency in rice production and
irrigation/drainage.
o Multidisciplinary teams should be involved in evaluating the technical, environmental and
socio-economic implications of introducing new crops and high-yielding varieties.
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