Guidelines for Designing and Evaluating Surface Irrigation Systems
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Guidelines for designing and evaluating surfaceirrigation systems
Table of Contents
FAO IRRIGATION AND DRAINAGE PAPER 45
by W.R. Walker
Professor and Head
Department of Agricultural and Irrigation Engineering
Utah State University
Logan, Utah, USA(Consultant to FAO)
FAO
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
Rome, 1989
The designations employed and the presentation of material in this publication do not imply the
expression of any opinion whatsoever on the part of the Food and Agriculture Organization of the United
Nations concerning the legal status of any country, territory, city or area or of its authorities, or concerning
the delimitation of its frontiers or boundaries.
M-56ISBN 92-5-102879-6
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system,
or transmitted in any form or by any means, electronic, mechanical, photocopying or
otherwise, without the prior permission of the copyright owner. Applications for such
permission, with a statement of the purpose and extent of the reproduction, should be
addressed to the Director, Publications Division, Food and Agriculture Organization of the
United Nations, Via delle Terme di Caracalla, 00100 Rome, Italy.
FAO 1989
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This electronic document has been scanned using optical character recognition (OCR)
software and careful manual recorrection. Even if the quality of digitalisation is high, the FAO
declines all responsibility for any discrepancies that may exist between the present document
and its original printed version.
Table of Contents
Preface
Acknowledgements
1. The practice of irrigation
1.1 The perspective and objectives of irrigation
1.2 Irrigation methods and their selection
1.2.1 Compatibility
1.2.2 Economics
1.2.3 Topographical characteristics
1.2.4 Soils1.2.5 Water supply
1.2.6 Crops
1.2.7 Social influences
1.2.8 External influences
1.2.9 Summary
1.3 Advantages and disadvantages of surface irrigation
1.3.1 Advantages
1.3.2 Disadvantages
2. Surface irrigation systems
2.1 Introduction to surface irrigation
2.1.1 Definition
2.1.2 Scope of the guide
2.1.3 Evolution of the practice
2.2 Surface irrigation methods
2.2.1 Basin irrigation
2.2.2 Border irrigation
2.2.3 Furrow irrigation
2.2.4 Uncontrolled flooding
2.3 Requirements for optimal performance
2.3.1 Inlet discharge control
2.3.2 Wastewater recovery and reuse
2.4 Surface irrigation structures
2.4.1 Diversion structures
2.4.2 Conveyance, distribution and management structures
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2.4.3 Field distribution systems
3. Field measurements
3.1 Field topography and configuration
3.2 Determining water requirements
3.2.1 Evapotranspiration and drainage requirements
3.2.2 Soil moisture principles
3.2.3 Soil moisture measurements
3.2.4 An example problem on soil moisture
3.3 Infiltration
3.3.1 Infiltration functions
3.3.2 Typical infiltration relationships
3.3.3 Measuring infiltration
3.3.4 An example infiltrometer test
3.4 Flow measurement
3.4.1 Cutthroat flumes
3.4.2 Example of cutthroat flume calibration3.4.3 Rectangular thin-plate weirs
3.4.4 Example of rectangular sharp crested weir analysis
3.4.5 V-notch weirs
3.5 Field evaluation
3.5.1 Advance phase
3.5.2 Ponding phase or wetting
3.5.3 Depletion phase
3.5.4 Recession phase
4. Evaluation of field data
4.1 Objectives of evaluation
4.1.1 Field data
4.2 Performance measures
4.2.1 Application uniformity
4.2.2 Application efficiency
4.2.3 Water requirement efficiency
4.2.4 Deep percolation ratio
4.2.5 Tailwater ratio4.2.6 Integration measures of performance
4.3 Intermediate analysis of field data
4.3.1 Inflow-outflow
4.3.2 Advance and recession
4.3.3 Flow geometry
4.3.4 Field infiltration
4.4 System evaluation
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4.4.1 Furrow irrigation evaluation procedure
4.4.2 Border irrigation evaluation
4.4.3 Basin irrigation evaluation
4.5 General alternatives for improvement
4.6 An example furrow irrigation evaluation
4.6.1 Field infiltration characteristics
4.6.2 Evaluation of system performance
4.6.3 Measures to improve performance
5. Surface irrigation design
5.1 Objective and scope of design
5.2 The basic design process
5.2.1 Preliminary design
5.2.2 Detailed design
5.3 Computation of advance and intake opportunity time
5.3.1 Common design computations
5.4 Furrow irrigation flow rates, cutoff times, and field layouts
5.4.1 Furrow design procedure for systems without cutback or reuse
5.4.2 Design procedure for furrow cutback systems
5.4.3 Design of furrow systems with tailwater reuse
5.4.4 Furrow irrigation design examples
5.5 Border irrigation design
5.5.1 Design of open-end border systems
5.5.2 Design of blocked-end borders
5.5.3 An open-end border design example5.5.4 A blocked-end border design example
5.6 Basin irrigation design
5.6.1 An example of basin design
5.7 Summary
6. Land levelling
6.1 The importance of land preparations
6.2 Small-scale land levelling6.3 Traditional engineering approach
6.3.1 Initial considerations
6.3.2 Engineering phase
6.3.3 Adjusting for the cut/fill ratio
6.3.4 Some practical problems
6.3.5 An example problem
6.4 Laser land levelling
7. Future developments
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7.1 Background
7.2 Surge flow
7.2.1 Effects of surging on infiltration
7.2.2 Effects of surging on surface flow hydraulics
7.2.3 Surge flow systems
7.3 Cablegation
7.4 Adaptive control systems
7.5 Water supply management
References
Appendix I - Fortran 77 surface irrigation design program
FAO irrigation and drainage papers
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ace
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Preface
This guide is intended to serve the needs of the irrigation technician for the evaluation of
surface irrigation systems. The scope is focussed at the farm level. A limited series of
graphical and tabular aids is given to relieve the user of some burden of computation.
Unfortunately, the number of variables associated with surface irrigation prevents this from
being completely practical. There are also two matters of philosophical nature that have led to
the approach presented herein. First, the irrigation technician and engineer must understand
the fundamental interactions characterizing surface flow in order to evaluate, improve, design
and manage effectively. This suggests a mathematical presentation which briefly andconcisely portrays these interrelationships. This guide omits nearly all theoretical development
and presents the most basic mathematical description. Nevertheless, the complexity of the
problem still requires an extensive mathematical analysis, even at this basic level. The
expertise required of the technician is that of at least a secondary education and the engineer
whose training needs to be at approximately the BSc level. The second philosophical aspect is
the belief that irrigation engineering practices are moving steadily toward a computerized
methodology. The interactions referred to above require large enough computational
commitments that they are only feasibly evaluated with hand-held programmable calculators
or microcomputers. As a result, the procedures outlined herein have been presented so they
can be applied directly via computer. A diskette copy of this program source and executable
codes for IBM PC and compatible microcomputers is available from FAO.
Some of the material used to develop this paper is included in more theoretical texts of the
writer's. Occasionally, direct quotes and figures have been extracted without citation in order
to minimize the diversions encountered by the reader. When the work of others has been
used, more careful attention to the detail of the citation has been given. Surface irrigation is a
complex subject which many have investigated and written about. The purpose of this guide
was not to review the technical literature exhaustively and many valuable works are not cited,
but it is hoped that the essence of surface irrigation evaluation and design practice has been
captured.
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nowledgements
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Acknowledgements
This work has been Undertaken under the supervision of Dr. Abdullah Arar, Senior Regional
Officer, Land and Water Development Division, FAO. His continual support and careful
attention to the details involved in producing a document such as this are very much
appreciated. Numerous other staff of the FAO have also contributed to this work through their
reviews, editorial oversight, and publication.
In the last decade or so, the methodology of surface irrigation engineering has moved from the
empirical to the quantitative. This has been accomplished by the concerted efforts ofnumerous researchers and practitioners, some of whom are acknowledged in the
REFERENCES. However, many others have made substantial contributions. Of these,
perhaps the graduate students at the universities where surface irrigation technology has been
extended have been the most unheralded. To those who have worked with the author, special
thanks.
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he practice of irrigation
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1. The practice of irrigation
1.1 The perspective and objectives of irrigation
1.2 Irrigation methods and their selection
1.3 Advantages and disadvantages of surface irrigation
1.1 The perspective and objectives of irrigation
A reliable and suitable irrigation water supply can result in vast improvements in agricultural
production and assure the economic vitality of the region. Many civilizations have been
dependent on irrigated agriculture to provide the basis of their society and enhance the
security of their people. Some have estimated that as little as 15-20 percent of the worldwide
total cultivated area is irrigated. Judging from irrigated and non-irrigated yields in some areas,
this relatively small fraction of agriculture may be contributing as much as 3040 percent of
gross agricultural output.
Effective agronomic practices are essential components of irrigated systems. Management of
the soil fertility, cropping selection and rotation, and pest control may make as much
incremental difference in yield as the irrigation water itself. Irrigation implies drainage, soil
reclamation, and erosion control. When any of these factors are ignored through either a lack
of understanding or planning, agricultural productivity will decline. History is absolutely certain
on this point.
Irrigated agriculture faces a number of difficult problems in the future. One of the major
concerns is the generally poor efficiency with which water resources have been used for
irrigation. A relatively safe estimate is that 40 percent or more of the water diverted for
irrigation is wasted at the farm level through either deep percolation or surface runoff. These
losses may not be lost when one views water use in the regional context, since return flows
become part of the usable resource elsewhere. However, these losses often represent
foregone opportunities for water because they delay the arrival of water at downstream
diversions and because they almost universally produce poorer quality water. One of the moreevident problems in the future is the growth of alternative demands for water such as urban
and industrial needs. These uses place a higher value on water resources and therefore tend
to focus attention on wasteful practices. Irrigation science in the future will undoubtedly face
the problem of maximizing efficiency.
Irrigation in arid areas of the world provides two essential agricultural requirements: (1) a
moisture supply for plant growth which also transports essential nutrients; and (2) a flow of
water to leach or dilute salts in the soil. Irrigation also benefits croplands through cooling the
soil and the atmosphere to create a more favourable environment for plant growth.
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The method, frequency and duration of irrigations have significant effects on crop yield and
farm productivity. For example, annual crops may not germinate when the surface is inundated
causing a crust to form over the seed bed. After emergence, inadequate soil moisture can
often reduce yields, particularly if the stress occurs during critical periods. Even though the
most important objective of irrigation is to maintain the soil moisture reservoir, how this is
accomplished is an important consideration. The technology of irrigation is more complex than
many appreciate. It is important that the scope of irrigation science not be limited to diversion
and conveyance systems, nor solely to the irrigated field, nor only to the drainage pathways.
Irrigation is a system extending across many technical and non-technical disciplines. It only
works efficiently and continually when all the components are integrated smoothly.
1.2 Irrigation methods and their selection
1.2.1 Compatibility
1.2.2 Economics
1.2.3 Topographical characteristics
1.2.4 Soils
1.2.5 Water supply
1.2.6 Crops
1.2.7 Social influences1.2.8 External influences
1.2.9 Summary
There are three broad classes of irrigation systems: (1) pressurized distribution; (2) gravity
flow distribution; and (3) drainage flow distribution. The pressurized systems include sprinkler,
trickle, and the array of similar systems in which water is conveyed to and distributed over the
farmland through pressurized pipe networks. There are many individual system configurations
identified by unique features (centre-pivot sprinkler systems). Gravity flow systems convey
and distribute water at the field level by a free surface, overland flow regime. These surface
irrigation methods are also subdivided according to configuration and operational
characteristics. Irrigation by control of the drainage system, subirrigation, is not common but isinteresting conceptually. Relatively large volumes of applied irrigation water percolate through
the root zone and become a drainage or groundwater flow. By controlling the flow at critical
points, it is possible to raise the level of the groundwater to within reach of the crop roots.
These individual irrigation systems have a variety of advantages and particular applications
which are beyond the scope of this paper. Suffice it to say that one should be familiar with
each in order to satisfy best the needs of irrigation projects likely to be of interest during their
formulation.
Irrigation systems are often designed to maximize efficiencies and minimize labour and capital
requirements. The most effective management practices are dependent on the type of
irrigation system and its design. For example, management can be influenced by the use of
automation, the control of or the capture and reuse of runoff, field soil and topographical
variations and the existence and location of flow measurement and water control structures.
Questions that are common to all irrigation systems are when to irrigate, how much to apply,
and can the efficiency be improved. A large number of considerations must be taken into
account in the selection of an irrigation system. These will vary from location to location, crop
to crop, year to year, and farmer to farmer. In general these considerations will include the
compatibility of the system with other farm operations, economic feasibility, topographic and
soil properties, crop characteristics, and social constraints (Walker and Skogerboe, 1987).
1.2.1 Compatibility
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The irrigation system for a field or a farm must function alongside other farm operations such
as land preparation, cultivation, and harvesting. The use of the large mechanized equipment
requires longer and wider fields. The irrigation systems must not interfere with these
operations and may need to be portable or function primarily outside the crop boundaries (i.e.
surface irrigation systems). Smaller equipment or animal-powered cultivating equipment is
more suitable for small fields and more permanent irrigation facilities.
1.2.2 Economics
The type of irrigation system selected is an important economic decision. Some types of
pressurized systems have high capital and operating costs but may utilize minimal labour and
conserve water. Their use tends toward high value cropping patterns. Other systems are
relatively less expensive to construct and operate but have high labour requirements. Some
systems are limited by the type of soil or the topography found on a field. The costs of
maintenance and expected life of the rehabilitation along with an array of annual costs like
energy, water, depreciation, land preparation, maintenance, labour and taxes should be
included in the selection of an irrigation system.
1.2.3 Topographical characteristics
Topography is a major factor affecting irrigation, particularly surface irrigation. Of generalconcern are the location and elevation of the water supply relative to the field boundaries, the
area and configuration of the fields, and access by roads, utility lines (gas, electricity, water,
etc.), and migrating herds whether wild or domestic. Field slope and its uniformity are two of
the most important topographical factors. Surface systems, for instance, require uniform
grades in the 0-5 percent range.
1.2.4 Soils
The soil's moisture-holding capacity, intake rate and depth are the principal criteria affecting
the type of system selected. Sandy soils typically have high intake rates and low soil moisture
storage capacities and may require an entirely different irrigation strategy than the deep claysoil with low infiltration rates but high moisture-storage capacities. Sandy soil requires more
frequent, smaller applications of water whereas clay soils can be irrigated less frequently and
to a larger depth. Other important soil properties influence the type of irrigation system to use.
The physical, biological and chemical interactions of soil and water influence the hydraulic
characteristics and filth. The mix of silt in a soil influences crusting and erodibility and should
be considered in each design. The soil influences crusting and erodibility and should be
considered in each design. The distribution of soils may vary widely over a field and may be
an important limitation on some methods of applying irrigation water.
1.2.5 Water supply
The quality and quantity of the source of water can have a significant impact on the irrigation
practices. Crop water demands are continuous during the growing season. The soil moisture
reservoir transforms this continuous demand into a periodic one which the irrigation system
can service. A water supply with a relatively small discharge is best utilized in an irrigation
system which incorporates frequent applications. The depths applied per irrigation would tend
to be smaller under these systems than under systems having a large discharge which is
available less frequently. The quality of water affects decisions similarly. Salinity is generally
the most significant problem but other elements like boron or selenium can be important. A
poor quality water supply must be utilized more frequently and in larger amounts than one of
good quality.
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1.2.6 Crops
The yields of many crops may be as much affected by how water is applied as the quantity
delivered. Irrigation systems create different environmental conditions such as humidity,
temperature, and soil aeration. They affect the plant differently by wetting different parts of the
plant thereby introducing various undesirable consequences like leaf burn, fruit spotting and
deformation, crown rot, etc. Rice, on the other hand, thrives under ponded conditions. Some
crops have high economic value and allow the application of more capital-intensive practices.
Deep-rooted crops are more amenable to low-frequency, high-application rate systems than
shallow-rooted crops.
1.2.7 Social influences
Beyond the confines of the individual field, irrigation is a community enterprise. Individuals,
groups of individuals, and often the state must join together to construct, operate and maintain
the irrigation system as a whole. Within a typical irrigation system there are three levels of
community organization. There is the individual or small informal group of individuals
participating in the system at the field and tertiary level of conveyance and distribution. There
are the farmer collectives which form in structures as simple as informal organizations or as
complex as irrigation districts. These assume, in addition to operation and maintenance,
responsibility for allocation and conflict resolution. And then there is the state organizationresponsible for the water distribution and use at the project level.
Irrigation system designers should be aware that perhaps the most important goal of the
irrigation community at all levels is the assurance of equity among its members. Thus the
operation, if not always the structure, of the irrigation system will tend to mirror the community
view of sharing and allocation.
Irrigation often means a technological intervention in the agricultural system even if irrigation
has been practiced locally for generations. New technologies mean new operation and
maintenance practices. If the community is not sufficiently adaptable to change, some
irrigation systems will not succeed.
1.2.8 External influences
Conditions outside the sphere of agriculture affect and even dictate the type of system
selected. For example, national policies regarding foreign exchange, strengthening specific
sectors of the local economy, or sufficiency in particular industries may lead to specific
irrigation systems being utilized. Key components in the manufacture or importation of system
elements may not be available or cannot be efficiently serviced. Since many irrigation projects
are financed by outside donors and lenders, specific system configurations may be precluded
because of international policies and attitudes.
1.2.9 Summary
The preceding discussion of factors affecting the choice of irrigation systems at the farm level
is not meant to be exhaustive. The designer, evaluator, or manager of irrigation systems
should be aware of the broader setting in which irrigated agriculture functions. Ignorance has
led to many more failures or inadequacies than has poor judgement or poor training.
As the remainder of this guide deals with specific surface irrigation issues, one needs to be
reminded that much of the engineering practice is art rather than science. Experience is often
a more valuable resource than computational skill, but both are needed. It is a poor
engineering practice that leaves perfectly feasible alternatives just beyond one's perspective.
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1.3 Advantages and disadvantages of surface irrigation
1.3.1 Advantages
1.3.2 Disadvantages
The term 'surface irrigation' refers to a broad class of irrigation methods in which water is
distributed over the field by overland flow. A flow is introduced at one edge of the field and
covers the field gradually. The rate of coverage (advance) is dependent almost entirely on thedifferences between the discharge onto the field and the accumulating infiltration into the soil.
Secondary factors include field slope, surface roughness, and the geometry or shape of the
flow cross-section.
The practice of surface irrigation is thousands of years old. It collectively represents perhaps
as much as 95 percent of common irrigation activity today. The first water supplies were
developed from stream or river flows onto the adjacent flood plain through simple check-dams
and a canal to distribute water to various locations where farmers could then allocate a portion
of the flow to their fields. The low-lying soils served by these diversions were typically high in
clay and silt content and tended to be most fertile. The land slope was normally small because
of the structure of the flood plain itself.
With the advent of modern equipment for moving earth and pumping water, surface irrigation
systems were extended to upland areas and lands quite separate from the flood plain of local
rivers and streams. These lands tend to have more variable soils and topographies, are
usually better drained, and may be naturally less fertile. Thus, these lands usually require
greater attention to design and operation.
1.3.1 Advantages
Surface irrigation offers a number of important advantages at both the farm and project level.
Because it is so widely utilized, local irrigators generally have at least minimal understanding
of how to operate and maintain the system. In addition, surface systems are often moreacceptable to agriculturalists who appreciate the effects of water shortage on crop yields since
it appears easier to apply the depths required to refill the root zone.
The second advantage of surface irrigation is that these systems can be developed at the
farm level with minimal capital investment. The control and regulation structures are simple,
durable and easily constructed with inexpensive and readily-available materials like wood,
concrete, brick and mortar, etc. Further, the essential structural elements are located at the
edges of the fields which facilitates operation and maintenance activities. The major capital
expense of the surface system is generally associated with land grading, but if the topography
is not too undulating, these costs are not great. Recent developments in surface irrigation
technology have largely overcome the irrigation efficiency advantage of sprinkler and tricklesystems. An array of automating devices roughly equates labour requirements. The major
trade-off between surface and pressurized methods lies in the relative costs of land levelling
for effective gravity distribution and energy for pressurization. Energy requirements for surface
irrigation systems come from gravity. This is a significant advantage in today's economy.
Another advantage of surface systems is that they are less affected by climatic and water
quality characteristics. Even moderate winds can seriously reduce the effectiveness of
sprinkler systems. Sediments and other debris reduce the effectiveness of trickle systems but
may actually aid the performance of the surface systems. Salinity is less of a problem under
surface irrigation than either of these pressurized systems.
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There are other advantages specific to individual regions that might be mentioned. Surface
systems are better able to utilize water supplies that are available less frequently, more
uncertain, and more variable in rate and duration. The gravity flow system is a highly flexible,
relatively easily-managed method of irrigation.
1.3.2 Disadvantages
There is one disadvantage of surface irrigation that confronts every designer and irrigator. The
soil which must be used to convey the water over the field has properties that are highly varied
both spatially and temporally. They become almost undefinable except immediately precedingthe watering or during it. This creates an engineering problem in which at least two of the
primary design variables, discharge and time of application, must be estimated not only at the
field layout stage but also judged by the irrigator prior to the initiation of every surface irrigation
event. Thus while it is possible for the new generation of surface irrigation methods to be
attractive alternatives to sprinkler and trickle systems, their associated design and
management practices are much more difficult to define and implement.
Although they need not be, surface irrigation systems are typically less efficient in applying
water than either sprinkler or trickle systems. Many are situated on lower lands with heavier
soils and, therefore, tend to be more affected by waterlogging and soil salinity if adequate
drainage is not provided. The need to use the field surface as a conveyance and distributionfacility requires that fields be well graded if possible. Land levelling costs can be high so the
surface irrigation practice tends to be limited to land already having small, even slopes.
Surface systems tend to be labour-intensive. This labour need not be overly skilled. But to
achieve high efficiencies the irrigation practices imposed by the irrigator must be carefully
implemented. The progress of the water over the field must be monitored in larger fields and
good judgement is required to terminate the inflow at the appropriate time. A consequence of
poor judgement or design is poor efficiency.
One sometimes important disadvantage of surface irrigation methods is the difficulty in
applying light, frequent irrigations early and late in the growing season of several crops. For
example, in heavy calcareous soils where crust formation after the first irrigation and prior tothe germination of crops, a light irrigation to soften the crust would improve yields
substantially. Under surface irrigation systems this may be unfeasible or impractical as either
the supply to the field is not readily available or the minimum depths applied would be too
great.
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Produced by: Natural ResourcesManagement and Environment Department
Title: Guidelines for designing and evaluatin surface irrigation systems...
More details
2. Surface irrigation systems
2.1 Introduction to surface irrigation
2.2 Surface irrigation methods
2.3 Requirements for optimal performance
2.4 Surface irrigation structures
2.1 Introduction to surface irrigation
2.1.1 Definition
Surface irrigation has evolved into an extensive array of configurations which can be broadly
classified as: (1) basin irrigation; (2) border irrigation; (3) furrow irrigation; and (4) uncontrolled
flooding. As noted previously, there are two features that distinguish a surface irrigation
system: (a) the flow has a free surface responding to the gravitational gradient; and (b) the on-
field means of conveyance and distribution is the field surface itself.
A surface irrigation event is composed of four phases as illustrated graphically in Figure 1.
When water is applied to the field, it 'advances' across the surface until the water extends overthe entire area. It may or may not directly wet the entire surface, but all of the flow paths have
been completed. Then the irrigation water either runs off the field or begins to pond on its
surface. The interval between the end of the advance and when the inflow is cut off is called
the wetting or ponding phase. The volume of water on the surface begins to decline after the
water is no longer being applied. It either drains from the surface (runoff) or infiltrates into the
soil. For the purposes of describing the hydraulics of the surface flows, the drainage period is
segregated into the depletion phase (vertical recession) and the recession phase (horizontal
recession). Depletion is the interval between cut off and the appearance of the first bare soil
under the water. Recession begins at that point and continues until the surface is drained.
Figure 1. Time-space trajectory of water during a surface irrigation showing
its advance, wetting, depletion and recession phases.
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The time and space references shown in Figure 1 are relatively standard. Time is cumulative
since the beginning of the irrigation, distance is referenced to the point water enters the field.
The advance and recession curves are therefore trajectories of the leading and receding
edges of the surface flows and the period defined between the two curves at any distance is
the time water is on the surface and therefore also the time water is infiltrating into the soil.
It is useful to note here that in observing surface irrigation one may not always observe a
ponding, depletion or recession phase. In basins, for example, the post-cut off period may onlyinvolve a depletion phase as the water infiltrates vertically over the entire field. Likewise, in the
irrigation of paddy rice, an irrigation very often adds to the ponded water in the basin so there
is neither advance nor recession - only wetting or ponding phase and part of the depletion
phase. In furrow systems, the volume of water in the furrow is very often a small part of the
total supply for the field and it drains rapidly. For practical purposes, there may not be a
depletion phase and recession can be ignored. Thus, surface irrigation may appear in several
configurations and operate under several regimes.
2.1.2 Scope of the guide
The surface irrigation system is one component of a much larger network of facilities diverting
and delivering water to farmlands. Figure 2 illustrates the 'irrigation system' and some of itsfeatures. It may be divided into the following four component systems: (1) water supply; (2)
water conveyance or delivery; (3) water use; and (4) drainage. For the complete system to
work well, each must work conjunctively toward the common goal of promoting maximum on-
farm production. Historically, the elements of an irrigation system have not functioned well as a
system and the result has too often been very low project irrigation efficiencies.
The focus of surface irrigation engineering is at the water use level, the individual irrigated
field. For design and evaluation purposes, these guidelines will note elements of the
conveyance and distribution system, especially those near the field such as flow measurement
and control, but will leave detailed treatment to other technical sources.
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Figure 2. Typical irrigation system components (redrafted from USDA-SCS,
1967)
2.1.3 Evolution of the practice
Although surface irrigation is thousands of years old, the most significant advances have been
made within the last decade. In the developed and industrialized countries, land holdings have
become as much as 10-20 times as large, and the number of farm families has dropped
sharply. Very large mechanized farming equipment has replaced animal-powered planting,cultivating and harvesting operations. The precision of preparing the field for planting has
improved by an order of magnitude with the advent of the laser-controlled land grading
equipment. Similarly, the irrigation works themselves are better constructed because of the
application of high technology equipment.
The changes in the lesser-developed and developing countries are less dramatic. In the
lesser-developed countries, trends toward land consolidation, mechanization, and more
elaborate system design and operation are much less apparent. Most of these farmers own
and operate farms of 1-10 hectares, irrigate with 20-40 litres per second and rely on either
small mechanized equipment or animal-powered farming implements.
Probably the most interesting evolution in surface irrigation so far as this guide is concerned isthe development and application of microcomputers and programmable calculators to the
design and operation of surface irrigation systems. In the late 1970s, a high-speed
microcomputer technology began to emerge that could solve the basic equations describing
the overland flow of water quickly and inexpensively. At about the same time, researchers like
Strelkoff and Katapodes (1977) made major contributions with efficient and accurate
numerical solutions to these equations. Today in the graduate and undergraduate study of
surface irrigation engineering, microcomputer and programmable calculator utilization is, or
should be, common practice.
Microcomputers and programmable calculators provide several features for today's irrigation
engineers and technicians. They allow a much more comprehensive treatment of the vital
hydraulic processes occurring both on the surface and beneath it. One can find optimal
designs and management practices for a multitude of conditions because designs historically
requiring days of effort are now made in seconds. The effectiveness of existing practices or
proposed ones can be predicted, even to the extent that control systems operating, sensing
and adjusting on a real-time basis are possible.
2.2 Surface irrigation methods
2.2.1 Basin irrigation
2.2.2 Border irrigation
2.2.3 Furrow irrigation2.2.4 Uncontrolled flooding
The classification of surface methods is perhaps somewhat arbitrary in technical literature.
This has been compounded by the fact that a single method is often referred to with different
names. In this guide, surface methods are classified by the slope, the size and shape of the
field, the end conditions, and how water flows into and over the field.
Each surface system has unique advantages and disadvantages depending on such factors
as were listed earlier like: (1) initial cost; (2) size and shape of fields; (3) soil characteristics;
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(4) nature and availability of the water supply; (5) climate; (6) cropping patterns; (7) social
preferences and structures; (8) historical experiences; and (9) influences external to the
surface irrigation system.
2.2.1 Basin irrigation
Basin irrigation is the most common form of surface irrigation, particularly in regions with
layouts of small fields. If a field is level in all directions, is encompassed by a dyke to prevent
runoff, and provides an undirected flow of water onto the field, it is herein called a basin. A
basin is typically square in shape but exists in all sorts of irregular and rectangularconfigurations. It may be furrowed or corrugated, have raised beds for the benefit of certain
crops, but as long as the inflow is undirected and uncontrolled into these field modifications, it
remains a basin. Two typical examples are shown in Figure 3, which illustrate the most
common basin irrigation concept: water is added to the basin through a gap in the perimeter
dyke or adjacent ditch.
Figure 3. Typical irrigated basins (from Walker and Skogerboe, 1987)
a. large basin in the USA
b. paddy basin in Asia
There are few crops and soils not amenable to basin irrigation, but it is generally favoured by
moderate to slow intake soils, deep-rooted and closely spaced crops. Crops which are
sensitive to flooding and soils which form a hard crust following an irrigation can be basin
irrigated by adding furrowing or using raised bed planting. Reclamation of salt-affected soils is
easily accomplished with basin irrigation and provision for drainage of surface runoff is
unnecessary. Of course it is always possible to encounter a heavy rainfall or mistake the cut-
off time thereby having too much water in the basin. Consequently, some means of
emergency surface drainage is good design practice. Basins can be served with less
command area and field watercourses than can border and furrow systems because their
level nature allows water applications from anywhere along the basin perimeter. Automation is
easily applied.
Basin irrigation has a number of limitations, two of which, already mentioned, are associated
with soil crusting and crops that cannot accommodate inundation. Precision land levelling is
very important to achieving high uniformities and efficiencies. Many basins are so small that
precision equipment cannot work effectively. The perimeter dykes need to be well maintained
to eliminate breaching and waste, and must be higher for basins than other surface irrigation
methods. To reach maximum levels of efficiency, the flow per unit width must be as high as
possible without causing erosion of the soil. When an irrigation project has been designed for
either small basins or furrows and borders, the capacity of control and outlet structures may
not be large enough to improve basins.
2.2.2 Border irrigation
Border irrigation can be viewed as an extension of basin irrigation to sloping, long rectangular
or contoured field shapes, with free draining conditions at the lower end. Figure 4 illustrates a
typical border configuration in which a field is divided into sloping borders. Water is applied to
individual borders from small hand-dug checks from the field head ditch. When the water is
shut off, it recedes from the upper end to the lower end. Sloping borders are suitable for nearly
any crop except those that require prolonged ponding. Soils can be efficiently irrigated which
have moderately low to moderately high intake rates but, as with basins, should not form
dense crusts unless provisions are made to furrow or construct raised borders for the crops.
The stream size per unit width must be large, particularly following a major tillage operation,
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although not so large for basins owing to the effects of slope. The precision of the field
topography is also critical, but the extended lengths permit better levelling through the use of
farm machinery.
Figure 4. Typical border irrigated field
2.2.3 Furrow irrigation
Furrow irrigation avoids flooding the entire field surface by channelling the flow along the
primary direction of the field using 'furrows,' 'creases,' or 'corrugations'. Water infiltratesthrough the wetted perimeter and spreads vertically and horizontally to refill the soil reservoir.
Furrows are often employed in basins and borders to reduce the effects of topographical
variation and crusting. The distinctive feature of furrow irrigation is that the flow into each
furrow is independently set and controlled as opposed to furrowed borders and basins where
the flow is set and controlled on a border by border or basin by basin basis.
Furrows provide better on-farm water management flexibility under many surface irrigation
conditions. The discharge per unit width of the field is substantially reduced and topographical
variations can be more severe. A smaller wetted area reduces evaporation losses. Furrows
provide the irrigator more opportunity to manage irrigations toward higher efficiencies as field
conditions change for each irrigation throughout a season. This is not to say, however, that
furrow irrigation enjoys higher application efficiencies than borders and basins.
There are several disadvantages with furrow irrigation. These may include: (1) an
accumulation of salinity between furrows; (2) an increased level of tailwater losses; (3) the
difficulty of moving farm equipment across the furrows; (4) the added expense and time to
make extra tillage practice (furrow construction); (5) an increase in the erosive potential of the
flow; (6) a higher commitment of labour to operate efficiently; and (7) generally furrow systems
are more difficult to automate, particularly with regard to regulating an equal discharge in each
furrow. Figure 5 shows two typical furrow irrigated conditions.
Figure 5. Furrow irrigation configurations (after USDA-SCS, 1967)
(a) graded furrow irrigation system
(b) contour furrows
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2.2.4 Uncontrolled flooding
There are many cases where croplands are irrigated without regard to efficiency or uniformity.
These are generally situations where the value of the crop is very small or the field is used for
grazing or recreation purposes. Small land holdings are generally not subject to the array of
surface irrigation practices of the large commercial farming systems. Also in this category are
the surface irrigation systems like check-basins which irrigate individual trees in an orchard,
for example. While these systems represent significant percentages in some areas, they will
not be discussed in detail in this paper. The evaluation methods can be applied if desired, but
the design techniques are not generally applicable nor need they be since the irrigationpractices tend to be minimally managed.
2.3 Requirements for optimal performance
2.3.1 Inlet discharge control
2.3.2 Wastewater recovery and reuse
There is substantial field evidence that surface irrigation systems can apply water to croplands
uniformly and efficiently, but it is the general observation that most such systems operate well
below their potential. A very large number of causes of poor surface irrigation performancehave been outlined in the technical literature. They range from inadequate design and
management at the farm level to inadequate operation of the upstream water supply facilities.
However, in looking for a root cause, one most often retreats to the fact that infiltration
changes a great deal from irrigation to irrigation, from soil to soil, and is neither predictable nor
effectively manageable. The infiltration rates are an unknown variable in irrigation practice.
In those cases where high levels of uniformity and efficiency are being achieved, irrigators
utilize one or more of the following practices: (1) precise and careful field preparation; (2)
irrigation scheduling; (3) regulation of inflow discharges; and (4) tailwater runoff restrictions,
reduction, or reuse. Land preparation is largely a land grading problem which will be discussed
in Section 5. Irrigation scheduling is a theme covered separately by several publications such
as the FAO Irrigation and Drainage Paper 24 (Rev) by Doorenbos and Pruitt (FAO, 1977). Theattention here then is focused on inflow regulation and tailwater control.
2.3.1 Inlet discharge control
Surface irrigation systems have two principal sources of inefficiency, deep percolation and
surface runoff or tailwater The remedies are competitive. To minimize deep percolation the
advance phase should be completed as quickly as possible so that the intake opportunity time
over the field will be uniform and then cut the inflow off when enough water has been added to
refill the root zone. This can be accomplished with a high, but non-erosive, discharge onto the
field. However, this practice increases the tailwater problem because the flow at the
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downstream end must be maintained until a sufficient depth has infiltrated. The higher inflow
reaches the end of the field sooner but it increases both the duration and the magnitude of the
runoff.
There are three options available to solve this problem, at least partially: (1) dyke the
downstream end to prevent runoff as in basin irrigation; (2) reduce the inflow discharge to a
rate more closely approximating the cumulative infiltration along the field following the advance
phase, a practice termed 'cutback'; or (3) select a discharge which minimizes the sum of deep
percolation and tailwater losses, i.e., optimize the field inflow regime. Examples of these
alternative practices are discussed and illustrated in Section 5. In this configuration, the headditch is divided into a series of level bays which are differentiated by a small change in
elevation. Water levels are regulated in two bays simultaneously so that the lower bay has
sufficient head to produce an advance phase flow in the furrows while in the upper bay the
head is only sufficient to produce the cutback flow. Thus, the system operates by moving the
check-dam from bay to bay along the upper end of the field.
Two very recent additions to the efforts to control surface irrigation systems more effectively
are the 'Surge Flow' system (Figure 6) developed at Utah State University, USA and the
'Cablegation' system developed at the US Department of Agriculture's Snake River Water
Conservation Research Center in Kimberly, Idaho, USA. These systems will be dealt with in
more detail in a later section.
Figure 6. One of the innovations in surface irrigation, the Surge Flow system
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2.3.2 Wastewater recovery and reuse
The tailwater deep percolation trade-off can also be solved by collecting and recycling the
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runoff to improve surface irrigation performance. Reuse systems have not been widely
employed historically because water and energy have been inexpensive. Even today it is often
more economical to regulate the inflow rather than to collect and pump the runoff back to the
head of the field or to another field, tailwater reuse systems are more cost-effective when the
water can be added to the flow serving lower fields and thereby saving the cost of pumping.
2.4 Surface irrigation structures
2.4.1 Diversion structures2.4.2 Conveyance, distribution and management structures
2.4.3 Field distribution systems
Surface irrigation systems are supported by a number of on- and off-farm structures which
control and manage the flow and its energy. In order to facilitate efficient surface irrigation,
these structures should be easily and cheaply constructed as well as easy to manage and
maintain. Each should be standardized for mass production and fabrication in the field by
farmers and technicians.
It is not the intent of this guide to be comprehensive with regard to the selection and design of
these structures since other sources are available, but it is worthwhile to note some of thesestructures by way of presenting a larger view of surface irrigation.
The structural elements of a surface system perform several important functions which
include: (1) turning the flow to a field on and off; (2) conveying and distributing the flow among
fields; (3) water measurement, sediment and debris removal, water level stabilization; and (4)
distribution of water onto the field.
2.4.1 Diversion structures
Most surface irrigation systems derive their water supplies from canal systems operated by
public or semi-public irrigation departments, districts, or companies. Some irrigation water issupplied in piped delivery systems and some directly pumped from groundwater. Diversion
structures perform several tasks including (1) on-off water control which allows the supply
agency to allocate its supply and protects the fields below the diversion from untimely flooding;
(2) regulation and stabilization of the discharge to the requirements of field channels and
watercourse distribution systems; (3) measurement of flow at the turnout in order to establish
and protect water entitlements; and (4) protection of downstream structures by controlling
sediments and debris as well as dissipating excess kinetic energy in the flow. A typical turnout
structure is shown in Figure 7.
Figure 7. Typical turnout from a canal or lateral (from walker end
Skogerboe, 1987)
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2.4.2 Conveyance, distribution and management structures
Conveying water to the field requires similar structures to those found in major canal networks.
The conveyance itself can be an earthen ditch or lateral, a buried pipe, or a lined ditch. Lined
sections can be elevated as shown in Figure 8, or constructed at surface level. Pipe materials
are usually plastic, steel, concrete, clay, or asbestos cement, or they may be as simple as a
wooden or bamboo construction. Lining materials include slip-form cast-in-place, or
prefabricated concrete (Figure 9), shotcrete or gunite, asphalt, surface and buried plastic or
rubber membranes, and compacted earth.
Figure 8. Elevated concrete channel in Iran
Figure 9. Slip-form concrete lining in the USA
The management of water in the field channels involves flow measurement, sediment and
debris removal, divisions, checks, drop-energy dissipators, and water level regulators. Some of
the more common flow control structures for open channels are shown in Figure 10.
Associated with these are various flow measuring devices like weirs, flumes, and orifices. The
designs of these structures have been standardized since they are small in size and capacity.
Designs for flow measurement and drop-energy dissipator structures need more attention and
construction must be more precise since their hydraulic responses are quite sensitive to their
dimensions.
Figure 10. On-farm water management structures (from Skogerboe et al.,
1971)
a. a simple drop structure
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b. a typical check-divider
2.4.3 Field distribution systems
After the water reaches the field ready to be irrigated, it is distributed onto the field by a variety
of means, both simple and elaborately constructed. Most fields have a head ditch or pipeline
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running along the upper side of the field from which the flow is distributed onto the field.
In a field irrigated from a head ditch, the spreading of water over the field depends somewhat
on the method of surface irrigation. For borders and basins, open or piped cutlets as illustrated
in Figure 11 are generally used. Furrow systems use outlets which can be directed to each
furrow.
Figure 11. Head ditch outlets for borders and basins (after Kraatz and
Mahajan, FAO, 1975)
Figure 12 shows a system in which siphon tubes are used as a means of serving each furrow.
Field distribution and spreading can also be through portable pipelines running along the
surfaces or permanent pipelines running underground. Basins and borders usually receive
water through buried pipes serving one or more gated risers within each basin or border. A
typical riser outlet, known as an alfalfa valve, is shown in Figure 13. The most common piped
method of furrow irrigation uses plastic or aluminium gated pipe like that shown in Figure 14.
The gated pipe may be connected to the main water supply via a piped distribution network
with a riser assembly like the one shown in Figure 13, directly to a canal turnout, or through an
open channel to a piped transition.
Figure 12. Siphons for furrow irrigation
Figure 13. An alfalfa valve riser
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Figure 14. Gated pipe for furrows
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Produced by: Natural ResourcesManagement and Environment Department
Title: Guidelines for designing and evaluatin surface irrigation systems...
More details
3. Field measurements
3.1 Field topography and configuration
3.2 Determining water requirements
3.3 Infiltration
3.4 Flow measurement
3.5 Field evaluation
The evaluation of surface irrigation at the field level is an important aspect of both
management and design. Field measurements are necessary to characterize the irrigation
system in terms of its most important parameters, to identify problems in its function, and to
develop alternative means for improving the system.
System characterization necessitates a series of basic field measurements before, during, and
after the irrigation. The objectives of the evaluation will dictate whether the field measurements
are comprehensive or are simplified for special purposes. In some cases, there are alternative
methodologies and equipment for accomplishing the same ends. The selection provided
herein is based on a limited selection found to be most useful during numerous field
evaluations and, in some measure, the practicality in the international sense.
Five classes of field measurements are presented: (1) field topography and configuration; (2)
water requirements; (3) infiltration; (4) flow measurement; and (5) irrigation phases.
3.1 Field topography and configuration
All field evaluations should include a relatively simple assessment of the field topography and
layout. These measurements are well enough known that only their brief mention is required.
There is first of all the field's primary elevations. This information requires that a surveying
instrument be used to measure elevations of the principal field boundaries (including dykes if
present), the elevation of the water supply inlet (an invert and likely maximum water surface
elevation), and the elevations of the surface and subsurface drainage system if possible.These measurements need not be comprehensive nor as formalized as one would expect for
a land levelling project.
The field topography and geometry should be measured. This requires placing a simple
reference grid on the field, usually by staking, and then surveying the elevations of the field
surface at the grid points to establish slope and slope variations. Usually one to three lines of
stakes placed 20-30 metres apart or such that 5-10 points are measured along the expected
flow line will be sufficient. For example, a border or basin would require at most three stake
lines, a furrow system as little as one, depending on the uniformity of the topography. The
survey should establish the distance of each grid point from the field inlet as well as the field
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dimensions (length of the field in the primary direction of water movement as well as field
width). There are important items of information that should be available from the survey: (1)
the field slope and its uniformity in the direction of flow and normal to it; (2) the slope and area
of the field; and (3) a reference system in the field establishing distance and elevation
changes.
It is also worthwhile at this stage of the evaluation to record the location and extent of major
soil types (this may require sampling and some laboratory analyses). The cropping pattern
should be determined and, if a crop is on the field at the time of the evaluation, any obvious
differences in growth and vigour should be noted. Similarly, the cultivation practices should berecorded.
3.2 Determining water requirements
3.2.1 Evapotranspiration and drainage requirements
3.2.2 Soil moisture principles
3.2.3 Soil moisture measurements
3.2.4 An example problem on soil moisture
The irrigation system may not be designed to supply the total amount of moisture required forcrop growth. In some cases, precipitation or upward flow from a water table may contribute
substantially towards fulfilling crop water requirements. It is also unrealistic to expect that
irrigation can be practiced without losses due to deep percolation, or tailwater runoff. The
fraction of the water that is used should be maximized, but this fraction cannot be 100 percent
without other serious problems developing such as a salt build-up in the crop root zone.
The dependency on irrigation in an area requires some analyses of the water balance. Water
balance may have three perspectives. The first is the balance of agricultural demands within a
watershed as depicted in Figure 15. The outcome of such an analysis establishes the safe
yield of water from various sources and thereby indicates the area of a project, the priorities
among projects, and the configuration of the large systemic components of the project. Anevaluation at the field level presumes that this information is available, and it should be
generally understood in as much as the limits of on-farm irrigation may be dictated by the
magnitude and distribution of the total water supply.
Figure 15. The perspective of water balance at the river basin level (from
Walker, 1978)
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The second water balance perspective, illustrated in Figure 16, is the water balance within the
farm or command area. An individual field is generally irrigated in concert with others in the
command or farm through sharing the water delivered through a canal turnout or a well. Fields
also typically share drainage channels. Water balance at the farm or command area level is
established on a field's access to water, its priority, timing and duration. Again, a field
evaluation presumes that these factors have been formulated and can be determined. Figure
17 illustrates the perspective of water balance at the field level.
Figure 16. A perspective of the on-farm water balance
Figure 17. The perspective of water balance at the field level
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The water balance within the confines of a field is a useful concept for characterizing,
evaluating or monitoring any surface irrigation system. In using this aspect of water balance,
an important consideration is the time frame in which the computations are made, i.e. whether
the balance will use annual data, seasonal data, or data describing a single irrigation event. If
a mean annual water balance is computed, then it becomes reasonable that the change in
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root zone soil moisture storage could be assumed as zero. In some irrigated areas,
precipitation events are so light that the net rainfall can be reasonably assumed to equal the
measured precipitation. Under other circumstances, various other terms can be neglected. In
fact, the time base and field conditions are often selected to eliminate as many of the
parameters as possible in order to study the behaviour of single parameters.
One of the more important is crop evapotranspiration. The upward movement of groundwater
to the root zone can usually be ignored if the water table is at least a metre below the root
zone. Then if the soil moisture is measured before and after a period when there is no
precipitation or irrigation, the depletion from the root zone is a viable estimate of crop wateruse.
There are two particularly important components in the field water balance which impact
design and evaluation. The first is the irrigation requirement of the crop, or its
evapotranspiration and leaching needs. This is a design parameter and will be briefly
described here, but a detailed treatment is left to the FAO Irrigation and Drainage Paper 24,
Crop Water Requirements, by Doorenbos and Pruitt (FAO, 1977). The second important
component deals with field evaluation and concerns the nature of moisture content changes in
the soil profile.
3.2.1 Evapotranspiration and drainage requirements
Evapotranspiration, ET, is dependent upon climatic conditions, crop variety and stage of
growth, soil moisture depletion, and various physical and chemical properties of the soil. A two
step procedure is generally followed in estimating ET: (1) the seasonal distribution of
reference crop "potential evapotranspiration", Etp, which can be computed with standard
formulae; and (2) the Etpis adjusted for crop variety and stage of growth. Other factors like
moisture stress can be ignored for the purposes of design computations.
There are perhaps twenty commonly used methods for calculating evapotranspiration, ranging
in complexity from the Blaney-Criddle Method using primarily mean monthly temperature to
more complete equations such as the Penman Method requiring radiation, temperature, wind
velocity, humidity and other factors comprising the net energy balance at the crop canopy.
The actual crop water demand depends on its stage of development and variety. Generally it
is estimated by multiplying Etpby a crop growth stage coefficient, kCO. Values of kCOhave
been published by Jensen (1973), Kincaid and Heermann (1974) and Doorenbos and Pruitt
(FAO, 1977) for a wide range of crops grown worldwide.
Some irrigation water should be applied in excess of the storage capacity of the soil to leach
salts from the rooting region, although this does not have to be achieved during each irrigation
event. It can usually be applied on an annual basis. As a matter of practicality, the normally
occurring deep percolation under most surface irrigation systems exceeds the leaching
fraction necessary for salt balance, particularly for the first and second irrigations each seasonwhen deep percolation losses are typically greatest. In addition, precipitation helps leach salts
throughout the year. Nevertheless some irrigated areas maintain a salt balance in the root
zone with excess leaching during only years of plentiful water supplies, which may occur as
infrequently as every three to eight years.
3.2.2 Soil moisture principles
Important soil characteristics in irrigated agriculture include: (1) the water-holding or storage
capacity of the soil; (2) the permeability of the soil to the flow of water and air; (3) the physical
features of the soil like the organic matter content, depth, texture and structure; and (4) the
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soil's chemical properties such as the concentration of soluble salts, nutrients and trace
elements.
The total available water, TAW, for plant use in the root zone is commonly defined as the
range of soil moisture held at a negative apparent pressure of 0.1 to 0.33 bar (a soil moisture
level called 'field capacity') and 15 bars (called the 'permanent wilting point'). The TAWwill
vary from 25 cm/m for silty loams to as low as 6 cm/m for sandy soils. Some typical values ofTAW, field capacity, permanent wetting point and miscellaneous features have been given in
various texts. A typical summary is shown in Figure 18.
Figure 18. Relationships between soil types and total available soil
moisture holding capacity, field capacity and wilting point (from Walker and
Skogerboe, 1987)
Other important soil parameters include its porosity, f, its volumetric moisture content, q; its
saturation, S; its dry weight moisture fraction, W; its bulk density, gb; and its specific weight, gs. The relationships among these parameters are as follows.
The porosity, f, of the soil is the ratio of the total volume of void or pore space, Vp,to the total
soil volume V:
f= Vp/V(1)
The volumetric water content, q, is the ratio of water volume in the soil, VW, to the total
volume, V:
q
= Vb/V (2)
The saturation, S, is the portion of the pore space filled with water:
S= VW/Vp(3)
These terms are further related as follows:
q= S* f(4)
When a sample of field soil is collected and oven-dried, the soil moisture is reported as a dry
weight fraction, W:
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(5)
To convert a dry weight soil moisture fraction into volumetric moisture content, the dry weight
fraction is multiplied by the bulk density, gb; and divided by specific weight of water, gwwhich
can be assumed to have a value of unity. Thus:
q= gbW/gw(6)
The gbis defined as the specific weight of the soil particles, gs, multiplied by the particle
volume or one-minus the porosity:
gb= gb* (1 - f) (7)
The volumetric moisture contents at field capacity, qfc, and permanent wilting point, qwp, then
are defined as follows:
qfc= gbWfc/gw(8)
qwp= gbWwp/gw(9)
where Wfcand Wwpare the dry weight moisture fractions at each point.
The total available water, TAW is the difference between field capacity and wilting point
moisture contents multiplied by the depth of the root zone, RD(refer to Table 1):
TAW= (qfc- qwp) RD(10)
Table 1 AVERAGE ROOTING DEPTHS FOR COMMONLY GROWN CROPS 1
Crop Root Depth (metres)
Alfalfa 1.5
Almonds 1.8
Apricots 1.8
Artichokes 1.4
Asparagus 1.5
Bananas 0.9
Beans 0.9
Beets 0.8
Broccoli 0.5
Cabbage 0.5
Cantaloupes 1.5
Carrots 0.9
Cauliflower 0.6
Celery 0.4
Cherries 2.0
Citrus 1.4
Corn (maize) 1.3
Cotton 1.2
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Cucumber 1.1
Eggplant 0.9
Figs 1.5
Grains and flax 1.2
Grapes 1.5
Groundnuts. 0.7
Ladino clover 0.6
Lettuce 0.3Melons 1.3
Milo (Sorghum) 1.2
Mustard 1.1
Olives 1.5
Onions 0.3
Palm Trees 0.9
Peaches 1.6
Pears 1.6
Peas 0.8
Peppers 0.9
Pineapple 0.5
Potatoes 0.9
Prunes 1.5
Pumpkins 1.8
Radishes 0.5
Safflower 1.5
Soybeans 1.0
Spinach 0.6
Squash (summer) 0.9
Strawberries 0.5
Sudan grass 1.8
Tomatoes 1.5
Turnips 0.9
Walnuts 2.0
Watermelon 1.2
Summarized from Marr (1967) and Doorenbos and Pruitt (FAO, 1977)
The Soil Moisture Deficit, SMD, is a measure of soil moisture between field capacity andexisting moisture content, qi, multiplied by the root depth:
SMD= (qfc- qi) * RD (11)
A similar term expressing the moisture that is allotted for depletion between irrigations is the
'Management Allowed Deficit', MAD. This is the value of SMDwhere irrigation should be
scheduled and represents the depth of water the irrigation system should apply. Later this will
be referred to as Zreqindicating the 'required depth' of infiltration.
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3.2.3 Soil moisture measurements
The soil moisture status requires periodic measurements in the field, from which one can
project when the next irrigation should occur and what depth of water should be applied.
Conversely, such data can indicate how much has been applied and its uniformity over the
field. As noted in the previous subsections, bulk density, field capacity and the permanent
wilting point are also needed.
There are numerous techniques for evaluating soil moisture. Perhaps the most useful are
gravimetric sampling, the neutron probe and the touch-and-feel method.
i. Gravimetric sampling
Gravimetric sampling involves collecting a soil sample from each 15-30 cm of the soil profile to
a depth at least that of the root penetration. Typical samplers are shown in Figure 19. The soil
sample of approximately 100-200 grammes is placed in an air tight container of known weight
(tare) and then weighed. The sample is then placed in an oven heated to 105 C for 24 hours
with the container cover removed. After drying, the soil and container are again weighed and
the weight of water determined as the before and after readings. The dry weight fraction of
each sample can be calculated using Eq. 5. Knowing the bulk density, one can determine
moisture contents from Eq. 6 and the soil moisture depletion from Eq. 11.
Figure 19. Small equipment used for collecting soil samples from the field
a. sampling auger
b. sampling tube
ii. The neutron Probe
The neutron probe and scaler for making soil moisture measurements are illustrated in Figure
20. The neutron probe is inserted at various depths into an access tube and the count rate is
read from the scaler. The manufacturers of neutron probe equipment furnish a calibration
relating the count rate to volumetric soil moisture content. Field experience suggests thatthese calibrations are not always accurate under a broad range of conditions so it is advisable
for the investigator to develop an individual calibration for each field or soil type. Most
calibration curves are linear, best fit lines of gravimetric data and scaler readings but may in
some cases be slightly curvilinear (van Baval et al., 1963).
Figure 20. A neutron probe and scaler for soil moisture measurements (after
Walker and Skogerboe, 1987)
The volume of soil actually monitored in readings by the neutron probe depends on the
moisture content of the soil, increasing as the soil moisture decreases. The accuracy of soil
moisture determinations near the ground surface is affected by a loss of neutrons into the
atmosphere thereby influencing measurements prior to an irrigation more than afterwards. Asa consequence, soil moisture measurements with a neutron probe are usually unreliable within
10-30 cm of the ground surface.
iii. Touch-and-feel
As a means of developing a rough estimate of soil moisture, the Touch-and-feel method can
be used. A handful of soil is squeezed into a ball. Then the appearance of the squeezed soil
can be compared subjectively to the descriptions listed in Table 2 to arrive at the estimated
depletion level. Merriam (1960) has developed a similar table which gives the moisture
deficiency in depth of water per unit depth of soil. Over the years various investigators have
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compared actual gravimetric sample results to the Touch-and-Feel estimates, finding a great
deal of error depending on the experience of the sampler.
Table 2 GUIDELINES FOR EVALUATING SOIL MOISTURE BY FEEL
Percent
Depletion
Feel or Appearance of Soil
Loamy sands to fine sandy
loams
Fine sandy loams to silt
loams
Silt loams to clay
loam
0 (field
capacity)
no free water on ball* but wet
outline on hand same same
0-25makes ball but breaks easily
and does not feel slick
makes tight ball, ribbons easily,
slightly sticky and slick
easily ribbons slick
feeling
25-50balls with pressure but easily
breaks
pliable ball, not sticky or slick,
ribbons and feels damp
pliable ball, ribbons
easily slightly slick
50-75 will not ball, feels dryballs under pressure but is
powdery and easily breaksslightly balls still pliable
75-100 dry, loose, flows through fingers powdery, dry, crumbleshard, baked, cracked,
crust
* A "Ball" is formed by squeezing a soil sample firmly in one's hand
A "Ribbon" is formed by squeezing soil between one's thumb and forefinger.
iv. Bulk density
Measurements of bulk density are commonly made by carefully collecting a soil sample of
known volume and then drying the sample in an oven to determine the dry weight fraction.
Then the dry weight of the soil, Wbis divided by the known sample volume, V, to determine
bulk density, gb:
gb= WbV(12)
Most methods developed for determining bulk density use a metal cylinder sampler that isdriven into the soil at a desired depth in the profile. Bulk density varies considerably with depth
and over an irrigated field. Thus, it is generally necessary to repeat the measurements in
different places to develop reliable estimates.
v. Field capacity
The most common method of determining field capacity in the laboratory uses a pressure
plate to apply a suction of -1/3 atmosphere to a saturated soil sample. When water is no
longer leaving the soil sample, the soil moisture in the sample is determined gravimetrically
and equated to field capacity.
A field technique for finding field capacity involves irrigating a test plot until the soil profile issaturated to a depth of about one metre. Then the plot is covered to prevent evaporation. The
soil moisture is measured each 24 hours until the changes are very small, at which point the