CHAPTER FOUR
4. Irrigation application systems
4.1. Introduction
Irrigation water may be applied to the crops by flooding it on the surface, by applying it beneath the
soil surface, by supplying it under pressure or by applying it in drops. The common methods of
irrigation are indicated as follows schematically:
Types of irrigation system
Subsurface
irrigation
Surface
irrigation
Pressurized
irrigation
Sprinkler
Furrow
Drip
Basin
Border
In the surface methods of irrigation, water is applied directly to the soil surface from a channel located
at the upper reach of the field. Water may be distributed to the crops on borders, basin, flood type or
furrow. The efficient conveyance of water from the reservoir up to the farm gate is insured by the
quality of executed conveyance system, comprising of main and branch canals, distributaries,
secondary and tertiary channels. The efficient use of the water reaching the farm gate is however,
dependant on the water application methods used to distribute the water over the entire field.
The suitability of the various irrigation methods, i.e. surface, sprinkler or drip irrigation, depends
mainly on the following factors such as natural conditions, type of crop, type of technology, previous
experience with irrigation, required labor inputs and costs and benefits. What ever the method of
irrigation, it is necessary to design the system for the most efficient use of water by the crop.
An ideal method of irrigation must satisfy the following conditions
1. Since the depth of water required during crop season vary for different stages of growth of
different crops the method should be capable of:
a. Applying uniform small depths for light irrigation say 4 to 6 cm
b. Applying uniform heavy depths of 15 to 20cm
2. The method should be such that loss of water in handling should be minimum
3. The method should not be complicated and costly in implementation
4. It should not create hindrance to mechanized and other farming methods
5. Operation cost of the method should not be excessive
6. It shouldn’t cause soil erosion in the field
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4.2. Surface irrigation design variables and parameters
The term “surface irrigation” refers to a broad class of irrigation methods in which water is distributed
over the field by gravity. A flow is introduced at a high point or along a high edge of the field and
allowed to cover the field by overland flow. The rate of coverage is dependent almost entirely on the
quantitative differences between inlet discharge and the accumulating infiltration. Secondary factors
include field slope and surface roughness. The practice of surface irrigation is thousands of years old
and collectively represents by far the most common irrigation activity today.
Surface irrigation systems can be developed with minimal capital investment, although these
investments can be very large if the water supply and irrigated fields are some distant apart. At the
farm level and even at the conveyance and distribution levels, surface irrigation systems need not
require complicated and expensive equipment. Labor requirements for surface irrigation tend to be
higher than for the pressurized types, but the labor still need not be high unless maximum efficiencies
are sought. Energy costs are substantially lower, but inefficiency may very well reverse this factor.
On the negative side, surface irrigation systems are typically less efficient in applying water than
either sprinkle or drip systems. Since many are situated on lower lands with tighter soil, surface
systems tend to be more affected by water logging and salinity problems. Fields need be well graded.
Land leveling costs are high, so the surface irrigation practice tends to be limited to land already
having small, even slopes. Surface irrigation has evolved into an extensive array of configurations
which can be broadly classified as: (1) basin irrigation; (2) border irrigation; and (3) furrow irrigation.
Fig 4-1: Time-space trajectory of water during a surface irrigation showing its advance, wetting,
depletion and recession phases.
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A surface irrigation event is composed of four phases as illustrated graphically in Figure 4-1. When
water is applied to the field, it 'advances' across the surface until the water extends over the 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. The time difference between the recession and advance curves is the infiltration opportunity
time (IOT). During this period the soil has the opportunity to absorb water from the soil surface. The
time difference between the recession and advance curves is the infiltration opportunity time (IOT).
During this period the soil has the opportunity to absorb water from the soil surface. The depth of
infiltration depends upon the intake characteristics of the soil. Under the assumption of uniform soil
conditions, for uniform application of irrigation water, the IOT should be the same throughout the
length of the field. This means that the advance and recession curves must be parallel. Similarly, the
average depth (or volume) of water infiltrated into the soil depends upon the infiltration opportunity
time and the intake characteristics of the soil. This sum of the infiltrated volume (depth multiplied by
area) and the runoff volume (if any), when compared with the volume of water required in the root
zone is an indication of the application efficiency.
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.
4.2.1. Border irrigation
The border method of irrigation make use of parallel ridges to guide a sheet of flowing water as it
moves down the slope. The land is divided into a number of long parallel strips called borders that are
separated by low ridges. The ends of the strips are usually not closed. The boarder strips has little or
no cross slope but has a uniform gentle slope in the direction of irrigation. The essential feature of
border irrigation is to provide uniform depth. Each stripe is irrigated independently by turning in a
stream of water at the upper end. The water spreads and flows down the strip in a sheet confined by
the border ridges. The irrigation stream must be large enough to spread over the entire width between
the border ridges without overtopping them. The flow rate must be such that the desired volume of
water is applied to the strip in a time equal to or slightly less than, that needed for the soil to absorb the
net amount required. When the desired volume of water has been delivered to the strip, the inflow is
turned off, or when the advanced water front either reaches the lower end, or a few minutes before or
after that the stream is turned off. The water temporarily stored in the border moves down the strip
and infiltrates, thus completing the irrigation. Outflow from the strip may be avoided by closing the
lower end and pounding the water on the lower reaches of the strip until infiltrated.
The border method of irrigation is adapted to most soils where depth and topography permit the
required land leveling at a reasonable cost and without permanent reduction in soil productivity. It is,
however more suitable to soils having moderately low to moderately high infiltration rates. Usually it
is not used in course sandy soils that have very high infiltration rates because excessive deep
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percolation unless the strip length is very short. It is also not well suited to soils having a very low
infiltration rate, since to provide adequate infiltration opportunity time/ intake time, with out excessive
surface runoff at the lower end, the irrigation stream may be too small to completely cover the border
strips.
The border method is suitable to irrigate all close-growing crops like wheat, barley, fodder crops and
legumes. It is, however, not suitable for crops like rice which requires standing water during most
seasons of its growing season. Border irrigation is best suited to slopes of less than 0.5 percent. The
erosion hazard created by rainfall runoff must be considered in determining the permissible border
slopes. Field application efficiency is good to excellent if the border strips are designed and installed
properly and good water management practices followed.
The border method has a number of advantages. They include:
a) Border ridges can be constructed economically with simple farm implements like a bullockdrawn A-farm ridges or bund former or tractor-drawn disk ridger.
b) Labor requirement in irrigation is greatly reduced as compared to the conventional check basin
methods of irrigation.
c) Uniform distribution and high water application efficiencies are possible if the system is
properly designed.
d) Operation of the system is simple and easy
Fig- Border irrigation
Straight and contour borders
Borders may be laid along the general slope of the field (straight or down the slope of borders) or may
be laid across the general slope of the field (contour borders). When fields can be leveled to desired
land slopes economically and without affecting its productivity, graded borders are easier to construct
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and operate. When the land slope exceeds safe limits, fields are undulated and leveling is not feasible,
borders may be laid across the slope and are called contour borders. The design criterion of a contour
border is the same as a straight border. Each contour border is level crosswise and has uniform
longitudinal gradient as in a straight border. The width and length of a contour border are identical to
that of a straight border for a particular set of conditions. In laying contour borders, the vertical
distance between adjacent benches should, as far as possible, be limited to 30cm, but shouldn’t exceed
60cm. the height of bund (ridge) should be sufficient to contain both the normal irrigation stream and
storm runoff.
Border irrigation design
Border design involves balancing the water advance and recession curves to achieve an equal
opportunity time for intake at any point along a border strip. On sites suitable for border irrigation,
advance and recession curves will be reasonably well balanced if the following two conditions are
met: (a) the volume of water delivered to the border strip is adequate to cover it to an average depth
equal to the gross application; and (b) the intake opportunity time at the upper end of the border strip is
equal to the time necessary for the soil to absorb the net application desired. Border design requires
knowledge of the cumulative intake characteristics of the soils to be irrigated. Hydraulic calculations
in border design are based on the Manning equation, which includes a coefficient (n) that expresses the
flow resistance of boundary conditions. The coefficient varies with crops, stages of crop growth, and
degree of roughness of the soil surface.
4.2.2. Basin Irrigation
The field to be irrigated by the basin method is divided into level rectangular areas bounded by dikes
or ridges. Water is turned in at one or more points until the desired gross volume has been applied to
the area. The flow rate must be large enough to cover the entire basin in approximately 60 to 75
percent of the time required for the soil to absorb the desired amount of water. Water is ponded until
infiltrated.
Most crops can be irrigated with basin irrigation. It is widely used for close-growing crops such as
alfalfa and other legumes, grasses, small grains, mint, and rice. It is used for row crops that can
withstand some inundation, such as sugar beets, corn, grain sorghum, and cotton, and for other row
crops if they are planted on beds so they will be above the water level. It also is well suited to the
irrigation of tree crops, grapes, and berries.
This irrigation method is best suited to soils of moderate to low intake rate (50 mm/h or less). It is an
excellent way of applying water to soils that have a moderately high intake rate, but basin areas may
need to be very small. Basin irrigation is best suited to smooth, gentle uniform land slopes. Undulating
or steep slopes can be prepared for basin irrigation, provided the soils are deep enough to permit
needed land leveling.
High application efficiency can be obtained easily with little labour. Basin irrigation can be used
efficiently by inexperienced workers, and can easily be automated. When basins are leveled with lasercontrolled scrapers, basins can be as large as 16 ha (Erie and Dedricki, 1979). Many different kinds of
crops can be grown in sequence without major changes in design, layout, or operating procedures.
There is no irrigation runoff, there is little deep percolation if no excess is applied, and maximum use
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can be made of rainfall. Leaching is easy and can be done without changing either the layout or
operation method.
Accurate initial land leveling is essential and level surfaces must be maintained. Adequate basin ridge
height may be difficult to maintain on sandy soils or fine-textured soils that crust or crack when dry.
Prolonged ponding and crop scalding can occur if the system is poorly managed. In some areas special
provisions must be made for surface drainage. Drop structures, lined ditches, or pipelines may be
required to control water on steep slopes that require benching. Relatively large inflow rates are
needed for basins and special structures may be needed to prevent erosion.
Design Basin Irrigation
Water should be applied at a rate that will advance over the basin in fraction of the infiltration time to
achieve high efficiency. The volume of water applied must equal the average gross irrigation
application. The intake opportunity time at all appoint in the basin must be greater than or equal to the
time required for the net irrigation to enter the soil. The longest intake opportunity time at any point on
the basin area must be sufficiently short to avoid scalding and excessive deep percolation. The depth
of water flow must be contained by the basin ridges.
In theory, maximum depth of flow and maximum deep percolation both occur where water is
introduced into a basin, usually considered as a “strip” of unit width for computational purposes. For
any given site conditions, the depth of flow varies directly and the amount of deep percolation varies
inversely with the inflow rate per unit width of basin strip. Thus, if a limit is set on flow depth, deep
percolation may be reduced only by shortening the length of the basin strip. If limits are established
for both depth of flow and deep percolation, then the design limit for length is determined. Flow at the
head end of basin strips must not exceed some practical depth related to the construction and
maintenance of basin ridges.
The average deep percolation (the difference between the net and gross irrigation application) should
be minimized. On some sites excess deep percolation causes acute drainage problems. To avoid this
condition, the design efficiency usually should not be less than about 80 percent. This efficiency can
be obtained if the time required to cover the basin is not more than 60 percent of the time required for
the net application to enter the soil. A design efficiency of less than 70 percent should be considered
only for soils having excellent internal drainage. On sites where irrigation water supplies are limited or
costly, where subsurface drainage problems are acute, or where crops can be damaged by prolonged
surface flooding, design efficiencies in excess of 90 percent are often practical. These efficiencies are
easily obtained when laser-controlled scrapers are used (Erie and Dedricki, 1979).
Basin strips usually are designed to be level; however, they may be constructed with a slight grade in
the direction of water flow. A slight grade will minimize adverse effects of variations in the finished
land surface, such as low areas or reverse grades, which result in a slower rate of advance, reduced
efficiency, excessive deep percolation or prolonged flooding that may damage crops. The total fall in
the length of the basin strips should not be greater than one-half the net depth of application used as a
basis for design. No adjustment is made in the design to compensate for such slight grades.
Drainage facilities may be needed to remove excess water from basins resulting from an accidental
over irrigation or heavy rainfall. Large furrows formed when constructing basin ridges facilitate
removal of excess rainfall or irrigation water. They also speed the water coverage rate over the basin
and reduce flow depths and deep percolation adjacent to the point or points of intake soils, and in high
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rainfall areas, on moderate intake soils. Basin ridges, or levees, should be provided for basins on low
intake soils, and in high rainfall areas, on moderate intake soils. Basin ridges, or levees, should be
constructed so that the top width is at least as great as the ridge height. The settled height should be at
least equal to the greater of (a) the design gross depth of application, or (b) the maximum depth of
flow plus a freeboard of 25 percent of the maximum depth of flow.
4.2.3. Furrow Irrigation
Small, evenly spaced, shallow channels are installed down or across the slope of the field to be
irrigated. Water is turned in at the high end and conveyed in the small channels to the vicinity of plants
growing in, or on beds between, the channels. Water is applied until the desired application and lateral
penetration is obtained.
Furrow irrigation is primarily used with clean tilled crops planted in rows. Most furrow in row crops
are either parabolic in cross section or have flat bottoms and about 2 to 1 side slopes. Most crops can
be irrigated by the furrow method except those grown in ponded water, such as rice. The furrow
method is particularly suitable for irrigation crops subject to injury if water covers the crown or stem
of the plants, as crops may be planted on the beds between furrows. This irrigation method is best
suited to medium to moderately fine textured soils of relatively high available water holding capacity
and conductivities which allow significant water movement in both the horizontal and vertical
directions. The method is suited to fine textured very slowly permeable soils on levels sites, which
permit water impoundment.
On sloping sites excessive surface runoff occurs because these soils require very small streams for
long periods of time to obtain the desired intake. The movement of irrigation water applied by furrows
on coarse textured sands and loamy sands is mainly downward with very little lateral penetration.
Efficient furrow irrigation on these soils requires very short furrows, small application times, relatively
close row spacing and small depths of water application. Furrow grades should be limited so that soil
loss from rainfall runoff or irrigation flow is within allowable limits. Furrow grades should generally
be 1.0 percent or less, but can be as much as 3.0 percent in arid areas where erosion from rainfall is not
a hazard. In humid areas furrow grades should generally not exceed 0.3 percent; however, grades up to
0.5 percent may be permissible if the lengths are sufficiently short. A minimum grade of 0.03 to 0.05
percent in humid and sub-humid areas is necessary to assure adequate surface drainage. Maximum
furrow grades for erosive soils can be estimated by the equation:
Smax = 67/(P30)1.3
(3.40)
Where P30 is the 30- min rainfall in mm on a 2-year frequency and Smax is the maximum allowable
furrow grade in percent.
Grades on less erosive soils may be increased by approximately one fourth. Moderate to high
application efficiency can be obtained if good water management practices are followed and the land
is properly prepared. The initial capital investment is relatively low on lands not requiring extensive
land forming as the furrows are constructed by common farm implements. Water does not contact
plant stems and scalding is thus avoided. Erosion hazards on steep slopes limit use in climate areas
where precipitation intensities and volume result in surface runoff, which, when concentrated in
furrow channels may cause excessive soil erosion or crop damage from flooding. Surface runoff
occurs except where the field is level and water is impounded until intake is completed. Labour
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requirements may be high as irrigation streams must be carefully regulated to achieve uniform water
distribution. Salts from either the soil or water supply may concentrate in the ridges and depress crop
yields.
Design of furrow irrigation
A furrow system may be designed only after gathering soils, crops, topography, size and shape of
irrigable areas, farm equipment available, farmer operational practices, and farmer personal
preferences for the proposed area. The designer must know the intake characteristics and water storage
capacities of the various soils, which along with the crop to be grown, will determine the design depth
of application and whether furrow will be used. For acceptable uniformity and adequacy of
application, the minimum time for water at any point is the time for intake of the net desired
application. The maximum time is limited by excessive deep percolation. The time water is available
for intake at any point, the opportunity time, is the time interval between water advance and recession.
i) Design Assumptions
Development of design relationships requires assumptions for intake vs time, advance and recession
rates, flow retardance, and intake as related to the furrow wetted perimeter. Rate of advance is
assumed to be a function of water inflow rate, soil intake characteristics, furrow shape, grade, length
and roughness.
ii) Design Limitations
Flow rates into furrows must not exceed the channel capacity as limited by cross-sectional shape and
size, slope and hydraulic roughness. The inflow must advance at a rate which will achieve a
reasonably uniform opportunity time throughout the length. Maximum flows are also limited to nonerosive velocities. Erosive soils may erode excessively when the flow velocity exceeds approximately
0.15 m/s while less erosive soils may safely withstand velocities of 0.18 m/s. Velocity and depth of
flow for a given cross-section and grade depend on the roughness or retardance of the furrows.
Manning roughness coefficients of 0.04 for furrows are commonly used in estimating flow velocity.
Recession time, the time for water to disappear at any point after inflow ends is primarily affected by
flow rate and by furrow length, shape and slope for a specific soil. Recession time is relatively short
and can be ignored when slopes exceed approximately 0.05 percent. Recession time is a very
significant portion of the opportunity time on low gradient (<0.05 percent) or level furrows. Excess
opportunity time results in deep percolation, which should not exceed 20 to 25 percent of the design
application depth.
4.3. Pressurized Irrigation Systems
4.3.1. Introduction
Sprinkle and trickle irrigation together represent the broad class of “ pressurized” irrigation methods,
in which water is carried through a pipe system to a point near where it will be consumed . This is in
contrast to surface irrigation methods, in which water must travel over the soil surface for rather long
distances before it reaches the point where it is expected to infiltrate and be consumed. Thus, surface
irrigation methods depend on critical uncertainties associated with water infiltration into the soil while
being conveyed, as well as at the receiving site.
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4.3.2. Sprinkle Irrigation
With sprinkle irrigation, water is jetted through the air to spread it from the pipe network across the
soil surface. This adds a degree of uncertainty to sprinkle irrigation, as wind and other atmospheric
conditions affect the application efficiency. The usual goal of sprinkling is uniform watering of an
entire field. Sprinkling as an important method of agricultural irrigation had its beginning in the early
part of this century. By 1950, better sprinklers, aluminium pipe, and more efficient pumping plants
further reduced the cost and increased the usefulness of sprinkle irrigation. Today sprinkling is a major
means of irrigation on all types of soils, topographies and crops. Sprinkle irrigation systems can be
broadly divided into set and continuous-move systems. In set systems, the sprinklers remain at a fixed
position while irrigation, whereas, in continuous-move systems, the sprinklers operate while moving in
either a circular or a straight path. Set systems include systems moved between irrigations, such as
hand-move and gun sprinklers (referred to as periodic-move systems). Set systems also include such
systems as solid-set sprinklers (referred to as fixed systems). The principal continuous-move systems
are centre-pivot and linear moving laterals and travelling sprinklers.
Advantages
Adaptability: Sprinkle irrigation is an adaptable means of supplying all types of crops with frequent
and uniform application of irrigation over a wide range of topographic and soil conditions. Sprinkle
irrigation can be partly or fully automated to minimize labour costs, and systems can be designed to
minimize water requirements.
Some of the more important objective that can be attained by sprinkling are: Effective use of small,
continuous streams of water, such as from springs and small boreholes; Proper irrigation of shallow
soils that cannot be graded without detrimental results; Irrigation of steep and rolling topography
without producing runoff or erosion; and Effective, light, frequent watering whenever needed, such as
for germination of a crop like lettuce, which may latter be surface irrigated.
Labour Savings: Following are some features of the sprinkle method relative to labour and
management requirements. Periodic-move sprinkle systems require labour for only one or two
relatively short periods each day to move the sprinkler laterals in each field. Labour requirements can
be further reduced by utilizing mechanically moved, instead of hand-moved, laterals. Furthermore,
unskilled labour can be used, because irrigation decisions are made by the manager, rather than by the
irrigators. Most mechanized and automated sprinkle systems require very little labour and are simple
to manage; and Fixed sprinkle systems can eliminate field labour during the irrigation season.
Special uses: Some of the more important special uses of sprinkle irrigation include: Modifying
weather extremes by increasing humidity, cooling crops, and alleviating freeze damage to buds and
leaves by use of special systems design; and using light, intermittent irrigation to supplement erratic or
deficient rainfall.
Water Savings: High application efficiency can be achieved by properly designed and operated
sprinkle irrigation systems. Properly engineered systems are easy to manage or automate to achieve
overall seasonal irrigation efficiencies of 75 % or greater. It is because much of the finesse needed to
operate them can be designed into the systems hardware, thus reducing the management and labour
inputs and training needed.
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Disadvantages
The disadvantages of sprinkle systems are mainly in the areas of high costs, water quality and delivery
problems, and environmental constraints.
High Costs: Both initial and pumping costs for sprinkle irrigation systems are higher than for surface
irrigation systems on uniform soils and slopes. However, surface irrigation may be potentially more
efficient.
Water Quality and Delivery: The sprinkle method is restricted by the following water-related
conditions. Large flows intermittently delivered are not economical to use without a reservoir, and
even minor fluctuations in rate cause difficulties. Saline water may cause problems because salt is
absorbed by the leaves of some crops and high concentrations of bicarbonates in irrigation water may
spot and affect the quality of fruit when used with overhead sprinklers. Certain waters are corrosive to
the metal pipes typically used in many sprinkle irrigation systems.
Environmental and Design Constraints: Some important constraints that limit the applicability of
the sprinkle method are: Sprinkling is not well-adapted to soils having an intake rate of less than about
3 mm/h. Windy and excessively dry conditions cause low sprinkle irrigation efficiencies. Field shapes
other than rectangular are not convenient to handle, especially for mechanized sprinkle systems.
Types of Sprinkle Systems
Sprinkler systems may be divided into two basic groups: set systems that operate with the sprinklers
set in a fixed position, and continuous-move systems that operate while the sprinkler in moving
through the field. Set systems may be further divided according to whether or not sprinklers must be
moved through a series of positions during the course of irrigating a field. Those systems that must be
moved are called periodic-move systems, and those not requiring any movement are call fixed
systems.
4.3.3. Trickle Irrigation
With trickle irrigation the distribution of the water after it leaves the pipe network depends only on
localized lateral movement above or on the soil surface or in the soil profile. Thus, water is conveyed
through the pipe system almost directly to each plant, and only the soil immediately surrounding each
plant is wetted. This leads to the potential high application efficiency associated with trickle irrigation.
The first experiments leading to the development of trickle irrigation were introduced in Germany in
1860, where short clay pipes with open joints were used to combine subsurface irrigation with
drainage. In the 1920s, perforated pipe was introduced, and subsequent experiments centered on
development of perforated pipe made of various materials and on control of flow through the
perforations. The early use of trickle irrigation was confined to greenhouses. The technique as we
know it today was not practical for field crops until the introduction of low-cost plastic tubing in the
early 1940s. Another significant step in the evolution of trickle irrigation took place in Isreal in the
latter 1950s, when long-path emitters were greatly introduced.
The trickle irrigation systems can be classified into the following four categories which require
different layout or hydraulic design procedure: Drip irrigation, where water is slowly applied through
small emitter openings to the soil surface; Spray irrigation, where water is sprayed over the soil
surface near individual trees; Bubbler irrigation, where a small stream or fountain of water is applied
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to flood small basins or the soil surface adjacent to individual trees; and Subsurface irrigation, where
water is applied through emitters below the soil surface. (Subsurface irrigation is not the same as subirrigation, which is done by controlling the water table).
For trickle irrigation, water is delivered by a pipe distribution network under low pressure in a
predetermined manner. Emitters are affixed to the hose, which lies on the soil surface alongside the
row of young trees. The emitters dissipate the pressure in the pipe distribution network by discharging
water through narrow nozzles or long flow paths. The discharge rate is only a few liters per to each
tree.
Upon leaving an emitter, water flows through the soil profile by capillary and gravity. Therefore, the
area that can be watered from each emission point is limited. Choosing a duration and frequency of
application and emission point spacing that meet both the evapotranspiration demands of the crop and
the infiltration and water holding characteristics of the soil is important.
For wide-spaced permanent crops, such as trees and vines, emitter are manufactured individually as
units that are attached by a barb to a flexible supply line called an emitter lateral, lateral hose, or
simply lateral. Some emitters have several outlets that supply water though small diameter” spaghetti”
tubing to two or more emission points. These are used in orchards to wet a larger area with a minimum
increase in costs.
For seasonal row crops, such as tomatoes, sugarcane and strawberries, the lateral with emitter outlets
is manufactured as a single disposable unit. These disposable laterals may have either porous walls
from which water oozes or single or double-chambered tubing with perforations spaced every 0.15 to
1.0 m. For all types of trickle systems, the laterals are connected to supply pipe lines called manifolds.
Advantages
Trickle irrigation is a convenient and efficient means of supplying water directly to the soil along
individual crop rows or surrounding individual plants, such as trees and vines. A trickle irrigation
system offers special agronomical, agro-technical and economical advantages for efficiently use of
water and labour. Furthermore, it provides an effective means for efficiently utilizing small continuous
streams of water.
Water and Cost Savings: The high interest in trickle irrigation is because of its potential to reduce
water requirements and operating costs. Trickle systems can irrigate some kinds of crops with
significantly less water than is required by the other irrigation methods. For example, young orchards
irrigated by a trickle system may require only one-half as much as orchards irrigated by sprinkler or
surface irrigation. Trickle irrigation can reduce the cost of labour, because the water needs only to be
regulated, not tended. The regulation is usually accomplished by automatic timing devices, but the
emitters and system controls should be inspected frequently.
Easier Field Operations: Trickle irrigation does not stimulate weed growth, because much of the soil
surface is never wetted by irrigation water. This reduces costs of labour and chemicals needed to
control weeds. Also, because a trickle system wets less soil during an irrigation, uninterrupted orchard
or field operations are possible. With row crops on beds, for example, the furrows in which farm
workers walk remain relatively dry and provide firm footing. Injecting fertilizers into the irrigation
water can eliminate the labour needed for ground application. Greater control over fertilizer placement
and timing through trickle irrigation may improve its efficiency.
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Use of Saline Water: Frequent irrigation maintains most of the soil in a well-aerated condition and at
soil moisture content that does not fluctuate between wet and dry extremes. Less drying between
irrigation keeps the salts in the soil more dilute, making it possible to use more saline water than can
be used with other irrigation methods.
Use on Rocky Soils and Steep Slopes: Trickle irrigation systems can be designed to operate
efficiently on almost any topography. Because the water is applied close to each tree, rocky areas can
be irrigated effectively by a trickle system even when the spacing between trees is irregular and tree
sizes vary. Furthermore, problem soils with intermixed textures and profiles and shallow soils that
cannot be graded can be efficiently irrigated by a trickle system.
Disadvantages
The main disadvantages inherent in trickle irrigation systems are their comparatively high initial cost,
their susceptibility to clogging, their tendency to build up local salinity, and where improperly
designed or maintained, their spotty distribution of water.
High Costs: The initial and pumping costs are relatively very high.
Clogging: Because emitter outlets are very small, they can easily become clogged by particles of
mineral or organic matter. Clogging reduces emission rates and uniformity of water distribution, which
causes damage to plants. To guard against clogging, particles of mineral or organic matter present in
the irrigation water must be removed before the water enters the pipe network. However, particles may
form within the pipes as water stands in the lines or evaporates from emitter orifices between
irrigations. Iron oxide, calcium carbonate, algae, and microbial slimes are problems in many trickle
systems chemical treatment of the water is necessary to prevent or correct most of these causes of
clogging.
Distribution Uniformity: Most trickle irrigation emitters operate as pressures ranging from 2 to 14 m.
of head. If a field slopes steeply, the individual emitter discharges may differ by as much as 50% from
the volume intended. Furthermore, the lines will drain through lower emitters after the water is shut
off; hence some plants receive too much water and others too little.
Soil Conditions: Some soils may not have sufficient infiltration capacity to absorb water at the usual
emitter discharge rate. Under these conditions, ponding and runoff can be expected. Sandy soils are
usually well adapted to trickle irrigation, especially those that have horizontal stratification.
Stratification aids trickle irrigation, because it promotes lateral water movement, which wets a greater
volume of soil.
Salt Accumulation: Salts often concentrate at the soil surface and become a potential hazard. This is
because light rains can leach them downward into the root zone. Therefore, when rain falls after a
period of salt accumulation, irrigation should continue on schedule unless about 50 mm of rain have
fallen. This is necessary to ensure leaching of salts below the root zone.
Hazards: Some of the more prevalent hazards associated with trickle irrigation are: If uncontrolled
events interrupt an irrigation, crop damage may occur rather quickly. The ability of roots to forage of
nutrients and water is limited to the relatively, small volume of soil wetted, which should be at least
33% of the total potential root zone. Rodents sometimes chew polyethylene laterals. Rodents control
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or use of polyvinylchloride (PVC) laterals may be necessary to prevent this damage. Should a main
supply line break or the infiltration system malfunction, contaminants may enter the system. This
could result in plugging up a larger number of emitters that would need to be cleaned or replaced;
therefore, safety screens should be provided and maintained at the lateral inlets.
Summery
This section discusses some of the important factors which should be taken into account when
determining which surface irrigation method is most suitable: basin, furrow or border irrigation.
Again, it is not possible to give specific guidelines leading to a single best solution; each option has its
advantages and disadvantages.
Factors to be taken into account include:
- natural circumstances (slope, soil type)
- type of crop
- required depth of irrigation application
- level of technology
- previous experience with irrigation
- required labour inputs.
NATURAL CIRCUMSTANCES
Flat lands, with a slope of 0.1% or less, are best suited for basin irrigation: little land leveling will be
required. If the slope is more than 1%, terraces can be constructed. However, the amount of land
leveling can be considerable.
Furrow irrigation can be used on flat land (short, near horizontal furrows), and on mildly sloping land
with a slope of maximum 0.5%. On steeper sloping land, contour furrows can be used up to a
maximum land slope of 3%. A minimum slope of 0.05% is recommended to assist drainage.
Border irrigation can be used on sloping land up to 2% on sandy soil and 5% on clay soil. A minimum
slope of 0.05% is recommended to ensure adequate drainage.
Surface irrigation may be difficult to use on irregular slopes as considerable land leveling may be
required to achieve the required land gradients.
All soil types, except coarse sand with an infiltration rate of more than 30 mm/hour, can be used for
surface irrigation. If the infiltration rate is higher than 30 mm/hour, sprinkler or drip irrigation should
be used.
TYPE Of CROP
Paddy rice is always grown in basins. Many other crops can also be grown in basins: e.g. maize,
sorghum, trees, etc. Those crops that cannot stand a very wet soil for more than 12-24 hours should not
be grown in basins.
Furrow irrigation is best used for irrigating row crops such as maize, vegetables and trees.
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Border irrigation is particularly suitable for close growing crops such as alfalfa, but border irrigation
can also be used for row crops and trees.
REQUIRED DEPTH OF IRRIGATION APPLICATION
When the irrigation schedule has been determined (see Volume 4) it is known how much water (in
mm) has to be given per irrigation application. It must be checked that this amount can indeed be
given, with the irrigation method under consideration.
Field experience has shown that most water can be applied per irrigation application when using basin
irrigation, less with border irrigation and least with furrow irrigation. In practice, in small-scale
irrigation projects, usually 40-70 mm of water are applied in basin irrigation, 30-60 mm in border
irrigation and 20-50 mm in furrow irrigation. (In large-scale irrigation projects, the amounts of water
applied may be much higher.)
This means that if only little water is to be applied per application, e.g. on sandy soils and a shallow
rooting crop, furrow irrigation would be most appropriate. (However, none of the surface irrigation
methods can be used if the sand is very coarse, i.e. if the infiltration rate is more than 30 mm/hour.)
If, on the other hand, a large amount of irrigation water is to be applied per application, e.g. on a clay
soil and with a deep rooting crop, border or basin irrigation would be more appropriate.
The above considerations have been summarized in Table 5. The net irrigation application values used
are only a rough guide. They result from a combination of soil type and rooting depth. For example: if
the soil is sandy and the rooting depth of the crop is medium, it is estimated that the net depth of each
irrigation application will be in the order of 35 mm. The last column indicates which irrigation method
is most suitable. In this case medium furrows or short borders.
The sizes of the furrows, borders and basins have been discussed in the previous chapters. The
approximate rooting depths of the most Important field crops are given in Volume 4.
LEVEL OF TECHNOLOGY
Basin irrigation is the simplest of the surface irrigation methods. Especially if the basins are small,
they can be constructed by hand or animal traction. Furrow irrigation - with the possible exception of
short, level furrows -requires accurate field grading. This is often done by machines. The maintenance
- ploughing and furrowing - is also often done by machines. This requires skill, organization and
frequently the use of foreign currency for fuel, equipment and spare parts.
Short, level furrows - also called furrow basins - can, like basins, be constructed and maintained by
hand.
Borders require the highest level of sophistication. They are constructed and maintained by machines.
The grading needs to be accurate. Machine operation requires a high level of skill, organization and
usually foreign currency.
PREVIOUS EXPERIENCE WITH IRRIGATION
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If there is no tradition in irrigation, the most simple irrigation method to introduce is basin irrigation.
The smaller the basins, the easier their construction, operation and maintenance.
If irrigation is used traditionally, it is usually simpler to improve the traditional irrigation method than
it is to introduce a previously unknown method.
REQUIRED LABOUR INPUTS
The required labour inputs for construction and maintenance depend heavily on the extent to which
machinery is used.
In general it can be stated that to operate the system, basin irrigation requires the least labour and the
least skill. For the operation of furrow and border irrigation systems more labour is required combined
with more skill.
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