TECHNICAL PAPER # 74
 
                     UNDERSTANDING SMALL-SCALE
                         IRRIGATION SYSTEMS
 
 
                                 By
                           John A. Chapman
 
                         Technical Reviewers
                            Claude H. Pair
                            Mohammad Sediq
                          Karl R. Klingelhofer
 
 
                              Published By
   
 
                                  VITA
 
                     1600 Wilson Boulevard, Suite 500
                       Arlington, Virginia 22209 USA
                  Tel:  703/276-1800 * Fax:   703/243-1865
                           Internet:  pr-info@vita.org
 
               Understanding Small-Scale Irrigation Systems
                              ISBN:   0-86619-317-0
               [C] 1991, Volunteers in Technical Assistance
                          
 
                                PREFACE
 
This paper is one of a series published by Volunteers in Technical Assistance to provide an
introduction to specific state-of-the-art technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help people choose technologies that are suitable
to their situations.  They are not intended to provide construction or implementation details.  People
are urged to contact VITA or a similar organization for further information and technical assistance
if they find that a particular technology seems to meet their needs.
 
The papers in the series were written, reviewed, and illustrated almost entirely by VITA Volunteer
technical experts on a purely voluntary basis.   Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately 5,000 hours of their time.  VITA staff
included Patrice Matthews and Suzanne Brooks handling typesetting and layout, and Margaret Crouch
as senior editor and project manager.   VITA Volunteer Dr. R. R. Ronkin, retired from the National
Science Foundation, lent his invaluable perspective, as a volunteer, to the compilation of technical
reviews, conversations with contributing writers, editing, and in a variety of other ways.
 
John Chapman is an agricultural engineer employed with a large irrigation equipment manufacturer.
Claude Pair, retired after more than 40 years with the U.S.  Department of Agriculture, is an expert
on sprinkler irrigation with experience throughout Asia.  Karl Klingelhofer is also an agricultural
engineer with extensive experience in the Far East and Central America.  All three have been VITA
Volunteers for many years.  Eng. Mohammad Sediq is the former President of Public Works for the
government of Afghanistan and presently heads VITA's Agricultural Rehabilitation Program for that
country.
 
VITA is a private, nonprofit organization that supports people working on technical problems in
developing countries.  VITA offers information and assistance aimed at helping individuals and groups
to select and implement technologies appropriate to their situations.  VITA maintains an international
Inquiry Service, a specialized documentation center, and a computerized roster of volunteer technical
consultants; manages long-term field projects; and publishes a variety of technical manuals and
papers.
 
               UNDERSTANDING SMALL-SCALE IRRIGATION SYSTEMS
 
                     by VITA Volunteer John A. Chapman
 
1.  THE IMPORTANCE OF IRRIGATION
 
Irrigation is the practice of supplying needed water to cropland to produce plant growth.  It may
be used to combat occasional drought or to make arid lands productive.  Cropland may be irrigated
before planting or as the crops are growing.   Clearly, a decision to irrigate requires knowledge of
the needs of crop plants and of local, natural conditions affecting water supply and loss.
 
Irrigation has been conducted for thousands of years.   In some areas of the world, the only interruptions
have been due to war or plague.   Where irrigation was needed and not possible, land has
become wasteland and irrigation systems have been abandoned.  Some societies that depended
heavily on irrigation did not survive poor irrigation system design.  From these experiences it is
clear that irrigated agriculture can be sustained and a properly designed irrigation system may be
needed to support a society for a long period.
 
2.  COMPONENTS OF AN IRRIGATION SYSTEM
 
The scope of irrigation is not limited to the application of water to the soil.  In a larger sense, it
deals with all aspects of water supply and use, from the watershed to the farms.  It includes the
design and construction of such works as dams, weirs, and water now regulators for storage or
diversion of water, as well as subsoil drainage, soil reclamation, and the economics of the relationships
among water, soil, and crop plants.   This paper emphasizes practices of applying water to the
soil.
 
Irrigation projects may be large or small, but scale does not affect the principles of operation.  The
important components or ingredients of an irrigation project are as follows:  the characteristics of
the soil, the kinds of crops to be grown, the water to be used, the kinds of irrigation methods, and
project management.
 
The Soil
 
The design of an effective irrigation project requires an understanding of soil characteristics.  The
soil is the main source of plant nutrients.   Moreover, its structural features enable it to hold the
plant roots in position and allow the plant to stand erect.  The problems that occur with the soil are
usually related to its chemical or structural features.   In places where there are long periods of
heavy rainfall (more than 100 cm per year), for example, soils are usually acidic.  This happens
because falling rain is slightly acidic and in passing through the soil dissolves some of the water-soluble
nutrients, carrying (leaching) them below the root zone of the plants.  Leaching of important
nutrients is harmful to plant growth, but can be corrected by applying fertilizers to restore
the soil to a more productive state.
 
Soils that have not been subjected to long periods of rainfall are often alkaline (basic).  The reason
is that the basic soil constituents have not been leached, so that the soil may retain high concentrations
of the basic components of the rocks from which it is derived.  A high concentration of
sodium, for example, can seriously disrupt the chemical balance needed for plant growth.  Minor
chemical imbalances can sometimes be corrected by additions to the soil, but major imbalances
may be repairable only at prohibitive cost.
 
Soil structure relates to the size of the soil particles that make up the soil and the manner in which
these particles are arranged.   Coarse, sandy soils have low water-holding capacity (4 centimeters or
less of water in a one-meter layer of soil) and need to be irrigated frequently to grow most crops.
A soil with a high clay content can be highly productive and may hold a considerable quantity of
water that is available to the plant (16 cm or more per m of soil).  This type of soil will require less
frequent irrigation cycles and larger quantities of water can be applied at each irrigation.
 
Some soils tend to become compacted.   Compaction reduces the pore space in the soil and makes it
difficult for the plant roots to penetrate it.   Compaction also retards penetration of water that is
applied to the surface.  It can usually be corrected by mechanical tillage, which may need to be
repeated on a regular seasonal basis.
 
The Plants
 
The plant species that is to be grown may dictate the type of irrigation project that needs to be
installed.  Most plants have a variable water requirement during their life cycle.   At the time of
planting, the seed needs only enough moisture for germination.  Initially, the amount needed may
be only about twice the weight of the seed.   However, as the seed starts putting out shoots and
roots, the water demand increases.   When the plant reaches its full flowering and fruiting stage, it
usually has its highest water demand.   It then requires less water until maturity.  At fruit maturity,
the plant may die (maize, wheat, etc.) and require no more water, or it may go dormant and only
need enough water to sustain it until the next reproductive cycle (fruit trees).
 
The Water
 
Quantity and Quality of Water.   The amount of water needed in the peak use periods varies with climatic
and geographic conditions.  An approximate rule is that the plant will extract 0.75 cm of
water from the soil each day.   That is, if the crop field is completely covered with plant growth,
the entire field will have water extracted from it equivalent to a layer of water 0.75 cm deep.  This
estimate, along with others that are more exact, predicts the minimal water requirement that must
be considered when the irrigation project is designed.
 
The quality of the water is also important.   Some waters have such a large content of soluble saits
that they cannot be used.  Rough guidelines for estimating water quality are as follows:   Rain water
that falls directly on the soil is almost always good water.  Water that has drained from a field
where it was previously used to irrigate another crop should be tested.  The taste of water is not a
reliable indication of quality; samples of the water should be analyzed at a competent water laboratory.
 
Naturally occurring water always contains some dissolved material.  Water pumped from the
ground or from drainage probably contains salts.   When this is applied to the soil, it picks up additional
soluble salts.  The water is then extracted from the soil by the plant.   The plant probably does
not utilize much of the dissolved salts.   but filters these out at the root.   The clean water is then
used by the plant to create new growth, or may be evaporated into the atmosphere.  Salts remain
behind in the soil.  If they are not removed, they can accumulate to a level that renders the soil
unfit for crop production.
 
Because of the prospect of salt accumulation, some experts recommend that soil that is to be irrigated
must also be properly drained.   For some projects, this recommendation is correct.   However,
management schemes that leach the salts to a level below the root zone are just as effective as
drainage in keeping the salts under control.   Such controlled leaching is usually applied with irrigation
schemes employing sprinkler and drip-irrigation technologies.
 
Surface Water.  The source of water should be reliable.  Unfortunately, most sources of surface water
are in greatest supply at early stages in the life of the crop plant.  As the plant gets larger, it
needs more water, but by that time the water supply is often diminished in flow
or availability.
 
Water is transported from the source to the field by some form of conveyance structure.  Structures
may be open furrows (ditches, channels), closed conduits (pipes), or lined furrows.  They are often
expensive and and labor intensive to build.   Some of them require labor-intensive maintenance.
 
Water supplied by a stream can often be delivered to a field using only the assistance of gravity.  A
common method is to construct a small diversion dam across the stream.  Most of the water will
flow over the dam and continue to flow downstream.   A small part of the water will be diverted
into a furrow where it flows in the same direction as the stream, but declines in elevation more
slowly than the stream.  After some distance, the stream level will be much lower than the water in
the diversion furrow flowing in the same general direction.  At that point, water from the diversion
can be directed to the field for use.   The structure and conveyance should be protected from
floods, wild burrowing animals, and vegetation that may cause damage.
 
Ground Water.  A reliable source of good-quality ground water may be useful for irrigation.   Here
are the questions that need to be answered:   Does it provide enough water to meet the demand of
the crop?  Is the quality of the water suitable for the application?   Are the costs of getting the
water and maintaining the source affordable in the context of the project?
 
If all of these questions are answered by "yes," then ground water may be the best source of supply.
In an area where little is known about the water-bearing underground layer from which the
water is to be pumped, it may be necessary to drill several test wells to locate the best site for a
well.  After the test well is installed, it should be test pumped for up to 24 hours to ensure that it
will sustain an adequate flow.
 
The drilling or digging of a well should be done by someone who is familiar with constructing
wells of the same size and capacity in the same area.   Several techniques are used in making wells.
Each is suited to a particular application.   The equipment for well construction can be as simple as
a shovel or as complex as a reverse rotary drilling rig.  Try to secure locally available equipment
that is suitable for the type of well needed.
 
2.  IRRIGATION TECHNIQUES
 
The commonly used techniques for distributing irrigation water within a field are flood ("surface")
and furrow irrigation, sprinkler irrigation, and low volume irrigation; each has its advantages.
 
Flood and Furrow Irrigation
 
This method is the oldest form of irrigation; it involves the direct discharge of water at low pressure
from a conveyance structure (lateral furrow) to the land.  The distribution of water over the
soil is achieved by gravity.   This technique is not generally as efficient as others because water
percolates farthest into the soil at the point where it is first discharged to the land.
 
The efficiency can be increased with reuse pits and pumps, or surge irrigation.  In every case a
uniform and level or gently sloping field is required.   Diversion dams on streams, diversion furrows,
and flood distribution of water may be involved.   Most fields require some earth work to
make them level enough to be used.   Once installed, these systems require.   little capital investment.
Their operation can be labor intensive, but labor costs can be reduced by methods described below.
Knowledge of operation requires both experience and education.
 
Siphon tubes can be used to bring water into a lateral furrow and onto a field.  The water first
flows into a lateral furrow at the high end of the field.  The water level is maintained fairly close
to the top of the furrow.  Small plastic or aluminum tubes that have been bent into a partial "U"
shape have one end placed in the water.   The other end is placed in a furrow that slopes downward
across the field.  Siphon action then moves the water from the upper furrow into the one below.
The tubes can be of various sizes; one common size is 2.5 cm.  If more water is needed in a furrow,
more tubes or a larger tube can be used.
 
<Figure 1>

19p04.gif (486x486)


 
Gated pipe is used on some farms.   With this system the water is pumped into a pipeline and conveyed
to the field.  At the field there are pipes that have openings in them at intervals between the
rows of crops.
 
Perhaps the oldest form of water distribution is the small lateral furrow, opened and closed by the
irrigator who uses a shovel to break down the wall of the furrow so the water can run onto the
field.  Although this primitive technology is rarely efficient, it works in certain locations.
 
Sprinkler Irrigation
 
When water is delivered to the field under pressure, it can be deposited on the land in many different
ways.  One of these ways is the automatic distribution by sprinklers.   The water is discharged
into the air and falls to the ground in a fine mist, similar to the fall of gentle rain.  The discharge
pressure at the nozzle of the sprinkler device is usually between 1.5 and 5 atmospheres (atm).  This
type of irrigation requires more energy than flood irrigation, but is more versatile since it can be
used on steep slopes.  Moreover, one can easily irrigate by frequent light applications.   No more
water should be supplied than the root zone of the plant can retain.  The capital equipment costs
are comparatively high, but are somewhat offset because the cost of land preparation (for example,
levelling) is less compared to flood irrigation.
 
Large, mobile devices can be used to fully automate large land areas, among them central pivot,
linear, and reel units.  Central pivot units supply the water to the center of the field, at which
point the water flows into a long pipeline supported above the ground at up to 50- to 60-meter
intervals on mobile carts.  The carts move the pipeline about the pivot point where the water is
introduced.  The pipeline moves like the minute hand of a clock around the field.   These units are
able to cover small fields of 4 to 5 hectares (ha) and large fields of over 200 ha.  They traverse
slopes of up to 25 or 30 percent.   With proper design, the units can be almost fully automated; one
properly trained person can easily irrigate over 1000 ha without help.
 
Rectangular fields of sufficient size are often irrigated with linear units.  These are, in effect,
made from the components of the central pivot unit.   They travel back and forth and can irrigate
the entire field as they move.   They can irrigate fields with slopes of 5 percent and can be automated,
but require about twice as much labor as the central pivot units.
 
<Figure 2>

19p05.gif (486x486)


 
A reel irrigator is mounted on a skid, or a trailer, that is attached to a hose that supplies the water.
As water is applied, the hose is coiled up on the hose reel; the hose and reel are
quite heavy and require a stable roadway.   The units also may require water pressure between 5
and 10 atm.  As a consequence of the pressure requirement,  reel units are generally considered
high consumers of energy.
 
Low-Volume Irrigation
 
Low-volume irrigation (also called "drip irrigation") is a relatively recent technique developed for
areas with low water supply.   The water is delivered to the field under a pressure of 1 to 2 atm.  It
is then distributed through small plastic tubes and is discharged to the soil through small holes
(emitters) very close to the plant, either above or below ground.  The rate of discharge is low and
may be only a steady drip rather than a stream.   This technique probably uses water more efficiently
than any other.
 
Drip irrigation equipment is fairly expensive to install.  Its use usually requires filtered water lest
the emitters become clogged; algal growth also can plug them.  The soil tends to become saline
where the wet and dry zones of the soil meet.   Even with these problems, however.   the advantages
of drip irrigation are evident, and the method is often preferred for tree crops.
 
<Figure 3>

19p06.gif (486x486)


 
3.  DESIGNING AND MANAGING THE PROJECT
 
Irrigation projects must be designed on a site-specific basis.  Topography, soil type, soil depth,
water supply, climatic conditions, and kinds of crops grown all differ from site to site.  Here are
the factors to be considered in order to design a project:
 
  *    the amount of water the soil holds in its root zone that can be available for the plant to
      use;
 
  *    the amount of water the plants need to produce crops; and
 
  *    the amount of water that is expected as rainfall during the growing season.
 
  *    water quality:   the amounts of dissolved materials in the water in relation to the needs of
      the crop.
 
  *    respurces available to install and maintain the system.
 
Assuming the soil to be saturated before planting the crop, then by adding the rainfall during the
growing season, one predicts approximately how much water is available to grow the crop.  Subtract
this from the water needed by the crop to establish the approximate amount of water to be
supplied by irrigation.  Alternatively, an irrigation demand can be planned to supply all of the
water needed by the crop.  This will enable the plants to survive an infrequent drought.
 
After determining the amount of water needed by the crop, one then defines the source of water
and ensures that it is adequate to meet the demand.
 
The next step is to determine how to distribute the water on the fields.  A rough topographic map
should be made of each site to be irrigated.   The elevation and location of the water supply relative
to the site should be determined if it is not known.   Soil and water analysis should be made to determine
their suitability for irrigation.   Finally, a competent irrigation designer should review any
plans.  This may seem rather restrictive or expensive,
but decisions on irrigation are not trivial, short-term matters.  They involve a major commitment
of resources.
 
Starting with a careful design, one can often construct a complete irrigation project with local
unskilled labor and locally available materials.   For example, if the project is very small, a diversion
dam can be built by placing rocks in a stream.   Hand shovelling can construct a diversion
canal, distribution laterals, or a field distribution system.  The designer needs to know what materials
and skills are available at the site.
 
It is essential to discuss the project early in the planning phase with those that must use and maintain
it.  Later, the users must be trained in its proper seasonal shut-down and maintenance.
Most irrigation systems are drained, repaired, and cleaned after the crop has been harvested.  This
is the opportunity to remove debris, repair leaks, and make improvements without affecting crop
production.  Most irrigation systems need off-season maintenance.   Design of the project should
include operation, the maintenance schedule, and training.
 
When the water conveyance system is used by several people, serious problems can arise in distributing
the water on a timely basis to all of them.   Those in control should ensure that priorities
and rules are set forth that all or the users understand and which can be enforced.
 
                         REFERENCES
 
There are many good books for those who wish to review design
techniques and have more detailed data.   Some of these are used by engineers and soil scientists
and may seem complex, but the principles are relatively simple.  These books should aid in understanding
terms used in this discussion and help to satisfy interests that go beyond it.
 
R. M. Hagan, H.R. House, and T. W. Edminster (eds.), Irrigation of Agricultural Lands.  Madison,
Wisconsin:  American Society of Agronomy, 1967.
 
V.E. Hansen and G.E. Stringham, Irrigation Principles and Practices, 4th ed. New York:  Wiley,
1980. Also:  O.W. Israelsin and G.E. Stringham, Irrigation Principles and Practices [in Arabic], 4th
ed. New York:  Wiley, 1984.
 
Claude H. Pair (ed.), Irrigation, 5th ed. Arlington, Virginia:
The Irrigation Association, 1983.
 
 
Glenn O. Schwab et al. (eds.), Soil and Water Conservation Engineering, 2nd ed. New York:  Wiley,
1981.
 
Peter Stern. Small Scale irrigation.   London:  Intermediate Technology Publications, 1979.   This is
an excellent source of information for the non-expert for designing and installing small-scale
irrigation systems.
 
See also:
 
Village Technology Handbook, Margaret Crouch and Len Doak, eds., Arlington, Virginia:  Volunteers
in Technical Assistance, 1988.   The water and agriculture sections of this how-to guide contain
much valuable information on the construction of a variety of land-leveling implements,
water supply and diversion, and simple pumps.
 
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                         TECHNICAL PAPER # 66