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Advantages and disadvantages of Advantages and disadvantages of

surface irrigationsurface irrigation

Surface IrrigationSurface Irrigation:: Refers to water is Refers to water is distributed over the field by overland distributed over the field by overland flow.flow.

A flow is introduced at one edge of the A flow is introduced at one edge of the field and covers the field gradually.field and covers the field gradually.

The rate of coverage (advance) is The rate of coverage (advance) is dependent on almost entirely the dependent on almost entirely the differences between the discharge differences between the discharge onto the field and the accumulating onto the field and the accumulating infiltration into the soil.infiltration into the soil.

Secondary factors include:Secondary factors include:

1.1. field slopefield slope, ,

2.2. surface roughnesssurface roughness,,

3.3. geometry geometry oror shape of the flow shape of the flow cross-sectioncross-section. .

The practice of surface irrigation The practice of surface irrigation is thousands of years old.is thousands of years old.

It collectively represents It collectively represents 9595٪٪ of of common irrigation activity today. common irrigation activity today.

The first water suppliesThe first water supplies were were developed from developed from streamstream or or riverriver flows onto the adjacent flood flows onto the adjacent flood plain;plain;

Through simple check-dams and Through simple check-dams and a canal to distribute water to a canal to distribute water to various locations;various locations;

Farmers could then allocate a Farmers could then allocate a portion of the flow to their fields.portion of the flow to their fields.

Low-lying soilsLow-lying soils were typically were typically high in clay and silt contenthigh in clay and silt content and tended to be most and tended to be most fertile. fertile.

Land slopeLand slope was normally was normally small because of the small because of the structure of the flood plain structure of the flood plain itself.itself.

With the With the advent of modern advent of modern equipmentequipment for moving earth and for moving earth and pumping water, pumping water, surface surface irrigation systems were irrigation systems were extended to upland areas and extended to upland areas and lands quite separate from the lands quite separate from the flood plain of local rivers and flood plain of local rivers and streams.streams.

These lands tend to have more These lands tend to have more variable soils and topographies, variable soils and topographies, are usually better drained, and are usually better drained, and may be naturally less fertile.may be naturally less fertile.

Thus, these lands usually require Thus, these lands usually require greater attention to design and greater attention to design and operation.operation.

Advantages Advantages

Surface irrigation offers a Surface irrigation offers a number of important number of important advantages at both the farm advantages at both the farm and project level.and project level.

local irrigators generally have local irrigators generally have at least minimal understanding at least minimal understanding of how to operate and maintain of how to operate and maintain the system.the system.

Surface systems are often Surface systems are often more acceptable to more acceptable to agriculturalists agriculturalists who who appreciate the effects of appreciate the effects of water shortage on crop yieldswater shortage on crop yields since it appears easier to since it appears easier to apply the depths required to apply the depths required to refill the root zone.refill the root zone.

These systems can be developed These systems can be developed at the farm level with minimal at the farm level with minimal capital investmentcapital investment

The control and regulation The control and regulation structures are:structures are:

1.1. DDurableurable

2.2. SSimple,imple,

3.3. Easily constructedEasily constructed with with inexpensive and readily-inexpensive and readily-available materials like:available materials like:

woodwood,,– concreteconcrete– brickbrick – mortarmortar, etc. , etc.

The essential structural elements The essential structural elements are located at the edges of the are located at the edges of the fields which facilitates operation fields which facilitates operation and maintenance activities.and maintenance activities.

The major capital expense of the The major capital expense of the

surface system is generally surface system is generally associated withassociated with land grading, but if land grading, but if the topography is not too the topography is not too undulating, these costs are not undulating, these costs are not great.great.

Recent developments in surface Recent developments in surface irrigation technology have largely irrigation technology have largely overcome the irrigation efficiency overcome the irrigation efficiency advantage of sprinkler and trickle advantage of sprinkler and trickle systems.systems.

An array of automating devices An array of automating devices roughly equates labour roughly equates labour requirements.requirements.

The major trade-off between The major trade-off between surface and pressurized methods surface and pressurized methods lies in the relative costs of land lies in the relative costs of land levelling for effective gravity levelling for effective gravity distribution and energy for distribution and energy for pressurization.pressurization.

Energy requirements for surface Energy requirements for surface irrigation systems come from irrigation systems come from gravity. gravity.

They are less affected by climatic They are less affected by climatic and water quality and water quality characteristics.characteristics.

Even moderate winds can Even moderate winds can seriously reduce the seriously reduce the effectiveness of sprinkler effectiveness of sprinkler systems.systems.

Sediments and other debris Sediments and other debris reduce the effectiveness of reduce the effectiveness of trickle systems but may actually trickle systems but may actually aid the performance of the aid the performance of the surface systems. surface systems.

Salinity is less under surface Salinity is less under surface irrigation than either of these irrigation than either of these pressurized systems.pressurized systems.

Surface systems are better able Surface systems are better able to utilize water supplies that are to utilize water supplies that are available available less frequently,less frequently, more more uncertain,uncertain, and more variable in and more variable in raterate and and duration.duration.

They are a highly flexible, They are a highly flexible, relatively easily-managed relatively easily-managed method of irrigation.method of irrigation.

Disadvantages Disadvantages

Confronts every designer and irrigator.

The soil must have properties that are highly varied both spatially and temporally.

They become almost undefinable except immediately preceding the 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 water logging and soil salinity if adequate drainage is not provided.

The need to use the field surface as a conveyance and distribution facility 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.

Important disadvantage of surface irrigation methods is the difficulty in:

Applying ligth irrigations. 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 to the 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.

Surface irrigation Surface irrigation systemssystems

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.

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.

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 It may or may not directly wet the entire surface, but all of the the entire surface, but all of the flow paths have been completed.flow paths have been completed.

Then the irrigation water either Then the irrigation water either runs off the field or begins to runs off the field or begins to pond on its surfacepond 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 It either drains from the surface runoffrunoff or or infiltratesinfiltrates into the soil. into the soil.

The drainage period is The drainage period is segregated into the depletion segregated into the depletion phase phase (vertical recession)(vertical recession) and and the recession phase the recession phase (horizontal (horizontal recession).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.

Fig. Time-space trajectory of water during a surface irrigation showing its advance, wetting, depletion and recession phases.

The time and space references shown in Figure 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 The advance and recession curves are therefore:curves are therefore:

Trajectories of the leading and Trajectories of the leading and receding edges of the surface receding edges of the surface flowsflows

The period defined between the The period defined between the two curves at any distance is: two curves at any distance is: the time water is on the surface the time water is on the surface and therefore also the time and therefore also the time water is infiltrating into the soil.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 only involve a depletion phase as the water infiltrates vertically over the entire field.

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.

Thus, surface irrigation may appear in several configurations and operate under several regimes.

For practical purposes, there may not be a depletion phase and recession can be ignored.

Evolution of the practiceEvolution of the practice

Although surface irrigation is Although surface irrigation is thousands of years old, thousands of years old, the most the most significant advances have been significant advances have been made within the last decade.made within the last decade.

In the developed and In the developed and industrialized countries,industrialized countries, – land holdings have become as much land holdings have become as much

as 10-20 times as largeas 10-20 times as large,,– and the number of farm families and the number of farm families

has dropped sharply. 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 l/s and rely on either small mechanized equipment or animal powered farming implements.

The most interesting evolution in The most interesting evolution in surface irrigation surface irrigation is the is the development and application of development and application of microcomputers and microcomputers and programmable calculators programmable calculators to the to the design and operation of surface design and operation of surface irrigation systems. 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.

Researchers made major Researchers made major contributions with efficient and contributions with efficient and accurate numerical solutions to accurate numerical solutions to these equations.these equations.

Today in the graduate and Today in the graduate and undergraduate study of surface undergraduate study of surface irrigation engineering, irrigation engineering, microcomputer and programmable microcomputer and programmable calculator utilization is, or should calculator utilization is, or should be, common practice.be, common practice.

Microcomputers and programmable Microcomputers and programmable calculators provide several calculators provide several features for today's irrigation features for today's irrigation engineers and technicians.engineers and technicians.

They allow a much more They allow a much more comprehensive treatment of the comprehensive treatment of the vital hydraulic processes occurring vital hydraulic processes occurring both on the surface and beneath it.both on the surface and beneath it.

One can find optimal designs and One can find optimal designs and management practices for a management practices for a multitude of conditions because multitude of conditions because designs historically requiring days designs historically requiring days of effort are now made in seconds.of effort are now made in seconds.

The effectiveness of existing The effectiveness of existing practices can be predicted, even to practices can be predicted, even to the extent that control systems the extent that control systems operating, sensing and adjusting operating, sensing and adjusting on a real-time basis are possible.on a real-time basis are possible.

Surface irrigation Surface irrigation methodsmethods

Basin irrigation Border irrigationBorder irrigation Furrow irrigationFurrow irrigation Uncontrolled floodingUncontrolled 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.

Surface methods are classified by Surface methods are classified by the;the;

1.1. slopeslope

2.2. the sizethe size

3.3. shape of the field, shape of the field,

4.4. the end conditions, and the end conditions, and

5.5. how water flows into and over the how water flows into and over the field. field.

Each surface system has unique Each surface system has unique advantages and disadvantages advantages and disadvantages depending on such factors as depending on such factors as were listed earlier like:were listed earlier like:

(1) initial cost; (1) initial cost;

(2) size and shape of fields;(2) size and shape of fields;

(3) soil characteristics; (3) soil characteristics;

(4) nature and availability of the (4) nature and availability of the water supply; water supply;

(5) climate;(5) climate;

(6) cropping patterns; (6) cropping patterns;

(7) social preferences and (7) social preferences and structures;structures;

(8) historical experiences; and (8) historical experiences; and

(9) influences external to the (9) influences external to the surface irrigation system. surface irrigation system.

Basin irrigationBasin irrigation

Basin irrigation is the most Basin irrigation is the most common form of surface common form of surface irrigation, particularly in regions irrigation, particularly in regions with layouts of small fields. with layouts of small fields.

If a field is level in all directions, If a field is level in all directions, is provided by a dyke to prevent is provided by a dyke to prevent runoff, and provides an runoff, and provides an undirected flow of water onto the undirected flow of water onto the fieldfield, it is herein called a basin., it is herein called a basin.

A basin is typically square in A basin is typically square in shape but exists in all sorts of shape but exists in all sorts of irregular and rectangular irregular and rectangular configurations.configurations.

It may be furrowed or It may be furrowed or corrugated, have raised beds for corrugated, have raised beds for the benefit of certain crops, the benefit of certain crops, but but as long as the inflow is as long as the inflow is undirected and uncontrolled into undirected and uncontrolled into these field modifications, it these field modifications, it remains a basinremains a basin..

water is added to the basin water is added to the basin through a gap in the perimeter through a gap in the perimeter dyke or adjacent ditch. dyke or adjacent ditch.

There are few crops and soils not There are few crops and soils not amenable to basin irrigation, but amenable to basin irrigation, but it is generally favored by it is generally favored by moderate to slow intake soils, moderate to slow intake soils, deep-rooted and closely spaced deep-rooted and closely spaced crops.crops.

Crops which are sensitive to Crops which are sensitive to flooding and soils which form a flooding and soils which form a hard crust following an irrigation hard crust following an irrigation can be basin irrigated by adding can be basin irrigated by adding furrowing or using raised bed furrowing or using raised bed planting. planting.

Reclamation of salt-affected soils Reclamation of salt-affected soils is easily accomplished with basin is easily accomplished with basin irrigation and provision for irrigation and provision for drainage of surface runoff is drainage of surface runoff is unnecessary.unnecessary.

It is always possible to encounter It is always possible to encounter a heavy rainfall or mistake the a heavy rainfall or mistake the cut-off timecut-off time thereby having too thereby having too much water in the basin.much water in the basin.

Consequently, some means of Consequently, some means of emergency surface drainage is emergency surface drainage is good design practice.good design practice.

Basins can be served with less Basins can be served with less

command area and field command area and field watercourses than can border and watercourses than can border and furrow systems because their level furrow systems because their level nature allows water applications nature allows water applications from anywhere along the basin from anywhere along the basin perimeter. perimeter.

Automation is easily applied. Automation is easily applied.

Basin irrigation has a number of Basin irrigation has a number of limitations, which are associated with limitations, which are associated with soil crusting and crops that cannot soil crusting and crops that cannot accommodate inundation.accommodate inundation.

Precision land levelling is very Precision land levelling is very

important to achieving high important to achieving high uniformities and efficiencies. uniformities and efficiencies.

Many basins are so small that Many basins are so small that precision equipment cannot work precision equipment cannot work effectively. effectively.

The perimeter dykes need to be The perimeter dykes need to be well maintained to eliminate well maintained to eliminate breaching and waste, and must be breaching and waste, and must be higher for basins than other higher for basins than other surface irrigation methods. surface irrigation methods.

To reach maximum levels of To reach maximum levels of efficiency, efficiency, the flow per unit width the flow per unit width must be as high as possible must be as high as possible without causing erosion of the without causing erosion of the soil. soil.

When an irrigation project has When an irrigation project has been designed for either small been designed for either small basins or furrows and borders, basins or furrows and borders, the capacity of control and outlet the capacity of control and outlet structures may not be large structures may not be large enough to improve basins.enough to improve basins.

Border irrigationBorder irrigation

Border irrigation can be viewed as Border irrigation can be viewed as an extension of basin irrigation to:an extension of basin irrigation to:– sloping, sloping, – long rectangular or contoured field long rectangular or contoured field

shapes, with free draining conditions shapes, with free draining conditions at the lower end.at the lower end.

Water is applied to individual Water is applied to individual borders from small hand-dug borders from small hand-dug checks from the field head ditch.checks from the field head ditch.

When the water is shut off, it When the water is shut off, it recedes from the upper end to the recedes from the upper end to the lower end.lower end.

Sloping borders are suitable for Sloping borders are suitable for nearly any crop except those nearly any crop except those that require prolonged ponding.that require prolonged ponding.

Soils can be efficiently irrigated Soils can be efficiently irrigated which have moderately low to which have moderately low to moderately high intake rates moderately high intake rates but, but, as with basins, should not as with basins, should not form dense crusts unless form dense crusts unless provisions are made to furrow or provisions are made to furrow or construct raised borders for the construct raised borders for the crops.crops.

The stream size per unit width The stream size per unit width must be large, particularly must be large, particularly following a major tillage operation, following a major tillage operation, although not so large for basins although not so large for basins owing to the effects of slope.owing to the effects of slope.

The precision of the field The precision of the field topography is also critical, but the topography is also critical, but the extended lengths permit better extended lengths permit better levelling through the use of farm levelling through the use of farm machinery. machinery.

Furrow irrigationFurrow irrigation Furrow irrigation avoids flooding the Furrow irrigation avoids flooding the

entire field surface by channelling entire field surface by channelling the flow along the primary direction the flow along the primary direction of the field using 'furrows,' 'creases,' of the field using 'furrows,' 'creases,' or 'corrugations'.or 'corrugations'.

Water infiltrates through the wetted Water infiltrates through the wetted perimeter and spreads vertically and perimeter and spreads vertically and horizontally to refill the soil horizontally to refill the soil reservoir. reservoir.

Furrows are often employed in basins Furrows are often employed in basins and borders to reduce the effects of and borders to reduce the effects of topographical variation and crusting.topographical variation and crusting.

The distinctive feature of furrow The distinctive feature of furrow

irrigation is that the flow into each irrigation is that the flow into each furrow is independently set and furrow is independently set and controlled as opposed to furrowed controlled as opposed to furrowed borders and basins where the flow is borders and basins where the flow is set and controlled on a border by set and controlled on a border by border or basin by basin basis.border or basin by basin basis.

Furrows provide better on-farm water Furrows provide better on-farm water management flexibility under many management flexibility under many surface irrigation conditions. surface irrigation conditions.

The discharge per unit width of the field The discharge per unit width of the field is substantially reduced and is substantially reduced and topographical variations can be more topographical variations can be more severe. severe.

A smaller wetted area reduces A smaller wetted area reduces evaporation losses. evaporation losses.

Furrows provide the irrigator more Furrows provide the irrigator more opportunity to manage irrigations opportunity to manage irrigations toward higher efficiencies as field toward higher efficiencies as field conditions change for each irrigation conditions change for each irrigation throughout a season. throughout a season.

Disadvantages of furrow Disadvantages of furrow irrigation irrigation

1)Accumulation of salinity between 1)Accumulation of salinity between furrows;furrows;

2)Increased level of tailwater losses;2)Increased level of tailwater losses;

3)The difficulty of moving farm 3)The difficulty of moving farm equipment across the furrows;equipment across the furrows;

4) the added expense and time to make 4) the added expense and time to make extra tillage practice (furrow extra tillage practice (furrow construction);construction);

5) an increase in the erosive potential of 5) an increase in the erosive potential of the flow;the flow;

6) a higher commitment of labour to 6) a higher commitment of labour to operate efficiently;operate efficiently;

7) generally furrow systems are more 7) generally furrow systems are more difficult to automate, particularly with difficult to automate, particularly with regard to regulating an equal discharge regard to regulating an equal discharge in each furrow. in each furrow.

(a) graded furrow irrigation system

(b) contour furrows

There are many cases where There are many cases where croplands are irrigated without croplands are irrigated without regard to efficiency or uniformity. regard to efficiency or uniformity.

These are generally situations where These are generally situations where the value of the crop is very small or the value of the crop is very small or the field is used for grazing or the field is used for grazing or recreation purposes.recreation purposes.

Small land holdings are generally not Small land holdings are generally not subject to the array of surface subject to the array of surface irrigation practices of the large irrigation practices of the large commercial farming systems. commercial farming systems.

Also in this category are the surface Also in this category are the surface irrigation systems like check-basins irrigation systems like check-basins which irrigate individual trees in an which irrigate individual trees in an orchard, for exampleorchard, for example. .

While these systems represent While these systems represent significant percentages in some areas, significant percentages in some areas, they will not be discussed in detail in they will not be discussed in detail in this paper. this paper.

The evaluation methods can be The evaluation methods can be applied if desired, but the design applied if desired, but the design techniques are not generally techniques are not generally applicable nor need they be since the applicable nor need they be since the irrigation practices tend to be irrigation practices tend to be minimally managed. minimally managed.

Requirements for optimal Requirements for optimal performanceperformance

There is substantial field evidence There is substantial field evidence that surface irrigation systems can that surface irrigation systems can apply water to croplands uniformly apply water to croplands uniformly and efficiently, but it is the general and efficiently, but it is the general observation that most such systems observation that most such systems operate well below their potential. operate well below their potential.

A very large number of causes of A very large number of causes of poor surface irrigation performance poor surface irrigation performance have been outlined in the technical have been outlined in the technical literature. literature.

They range from inadequate design They range from inadequate design and management at the farm level and management at the farm level to to inadequate operation of the inadequate operation of the upstream water supply facilities.upstream water supply facilities.

However, in looking for a root cause, However, in looking for a root cause, one most often retreats to the fact one most often retreats to the fact that infiltration changes a great deal that infiltration changes a great deal from irrigation to irrigation, from soil from irrigation to irrigation, from soil to soil,to soil, and is neither predictable nor and is neither predictable nor effectively manageable. effectively manageable.

The infiltration rates are an unknown The infiltration rates are an unknown variable in irrigation practice. variable in irrigation practice.

Where high levels of uniformity and Where high levels of uniformity and efficiency are being achieved, efficiency are being achieved, irrigators utilize one or more of the irrigators utilize one or more of the following practices:following practices:

1) precise and careful field 1) precise and careful field preparation;preparation;

2) irrigation scheduling;2) irrigation scheduling;

3) regulation of inflow discharges; and3) regulation of inflow discharges; and

4) tailwater runoff restrictions, 4) tailwater runoff restrictions, reduction, or reuse. reduction, or reuse.

Land preparation is largely a land Land preparation is largely a land grading problem. grading problem.

The attention here then is focused on The attention here then is focused on inflow regulation and tailwater inflow regulation and tailwater control. control.

Inlet discharge controlInlet discharge control Surface irrigation systems have two Surface irrigation systems have two

principal sources of inefficiency:principal sources of inefficiency:– deep percolation and deep percolation and – surface runoff or tailwater The remedies surface runoff or tailwater The remedies

are competitive.are competitive. To minimize deep percolation the To minimize deep percolation the

advance phase should be completed advance phase should be completed as quickly as possible so that the as quickly as possible so that the intake opportunity time over the field intake opportunity time over the field will be uniform will be uniform and then cut the and then cut the inflow off when enough water has inflow off when enough water has been added to refill the root zone.been added to refill the root zone.

This can be accomplished with a high, This can be accomplished with a high, but non-erosive, discharge onto the but non-erosive, discharge onto the field.field.

However, this practice increases the However, this practice increases the tailwater problem because the flow at tailwater problem because the flow at the downstream end must be the downstream end must be maintained until a sufficient depth maintained until a sufficient depth has infiltrated. has infiltrated.

The higher inflow reaches the end of The higher inflow reaches the end of the field sooner but it increases both the field sooner but it increases both the duration and the magnitude of the duration and the magnitude of the runoff. the runoff.

There are three options available to There are three options available to solve this problem, at least partially: solve this problem, at least partially:

1) dyke the downstream end to prevent 1) dyke the downstream end to prevent runoff as in basin irrigation;runoff as in basin irrigation;

2) reduce the inflow discharge to a rate 2) reduce the inflow discharge to a rate more closely approximating the more closely approximating the cumulative infiltration along the field cumulative infiltration along the field following the advance phase, a following the advance phase, a practice termed 'cutback'; orpractice termed 'cutback'; or

3) select a discharge which minimizes 3) select a discharge which minimizes the sum of deep percolation and the sum of deep percolation and tailwater losses, i.e., optimize the tailwater losses, i.e., optimize the field inflow regime.field inflow regime.

In this configuration, the head ditch is In this configuration, the head ditch is divided into a series of level bays divided into a series of level bays which are differentiated by a small which are differentiated by a small change in elevation.change in elevation.

Water levels are regulated in two Water levels are regulated in two bays simultaneously so that the lower bays simultaneously so that the lower bay has sufficient head to produce an bay has sufficient head to produce an advance phase flow in the furrows advance phase flow in the furrows while in the upper bay the head is while in the upper bay the head is only sufficient to produce the cutback only sufficient to produce the cutback flow.flow.

Thus, the system operates by moving Thus, the system operates by moving the check-dam from bay to bay along the check-dam from bay to bay along the upper end of the field. the upper end of the field.

Two very recent additions to Two very recent additions to the efforts to control surface the efforts to control surface irrigation systems more irrigation systems more effectively are:effectively are:

1.1. 'Surge Flow' system developed 'Surge Flow' system developed at USA.at USA.

2.2. 'Cablegation' system developed 'Cablegation' system developed at the USA. at the USA.

Surge Flow system

Wastewater reuseWastewater reuse

The tailwater deep percolation trade-The tailwater deep percolation trade-off can also be solved by collecting off can also be solved by collecting and recycling the runoff to improve and recycling the runoff to improve surface irrigation performance.surface irrigation performance.

Reuse systems have not been widely Reuse systems have not been widely employed historically because water employed historically because water and energy have been inexpensive. and energy have been inexpensive.

It is often more economical to It is often more economical to regulate the inflow rather than regulate the inflow rather than to collect and pump the runoff to collect and pump the runoff back to the head of the field or back to the head of the field or to another field.to another field.

Tailwater reuse systems are Tailwater reuse systems are more cost-effective when the more cost-effective when the water can be added to the flow water can be added to the flow serving lower fields and thereby serving lower fields and thereby saving the cost of pumping.saving the cost of pumping.

Surface irrigation Surface irrigation structuresstructures

Surface irrigation systems are Surface irrigation systems are supported by a number of on- supported by a number of on- and off-farm structures which and off-farm structures which control and manage the flow and control and manage the flow and its energy. its energy.

In order to facilitate efficient In order to facilitate efficient surface irrigation, these surface irrigation, these structures should be:structures should be:– easily and cheaply constructedeasily and cheaply constructed– easy to manage and maintain. easy to manage and maintain.

Each should be standardized for Each should be standardized for mass production and fabrication in mass production and fabrication in the field by farmers and technicians.the field by farmers and technicians.

The structural elements of a surface The structural elements of a surface system perform several important system perform several important functions which include:functions which include:

1) turning the flow to a field on and off; 1) turning the flow to a field on and off;

2) conveying and distributing the flow 2) conveying and distributing the flow among fields; among fields;

3) water measurement, sediment and 3) water measurement, sediment and debris removal, water level debris removal, water level stabilization;stabilization;

4) distribution of water onto the field. 4) distribution of water onto the field.

Diversion structuresDiversion structures

Most surface irrigation systems Most surface irrigation systems derive their water supplies from derive their water supplies from canal systems operated by public or canal systems operated by public or semi-public irrigation departments, semi-public irrigation departments, districts, or companies.districts, or companies.

Some irrigation water is supplied in Some irrigation water is supplied in piped delivery systems and some piped delivery systems and some directly pumped from groundwater. directly pumped from groundwater.

Diversion structures perform several tasks Diversion structures perform several tasks including including

1) on-off water control which allows the 1) on-off water control which allows the supply agency to allocate its supply and supply agency to allocate its supply and protects the fields below the diversion from protects the fields below the diversion from untimely flooding; untimely flooding;

2) regulation and stabilization of the 2) regulation and stabilization of the discharge to the requirements of field discharge to the requirements of field channels and watercourse distribution channels and watercourse distribution systems; systems;

3) measurement of flow at the turnout in 3) measurement of flow at the turnout in order to establish and protect water order to establish and protect water entitlements; and entitlements; and

4) protection of downstream structures by 4) protection of downstream structures by controlling sediments and debris as well as controlling sediments and debris as well as dissipating excess kinetic energy in the dissipating excess kinetic energy in the flow. flow.

Typical turnout from a canal or lateral

Conveyance structures Conveyance structures

Conveying water to the field Conveying water to the field requires requires similar structures to similar structures to those found in major canal those found in major canal networks. networks. The conveyance can The conveyance can be:be:

Earthen ditch or lateral,Earthen ditch or lateral, Buried pipe,Buried pipe, Lined ditch.Lined ditch.

Lined sections can be elevated, Lined sections can be elevated, or constructed at surface level.or constructed at surface level.

Elevated concrete channel

Gated pipe for furrows

Pipe materials are:Pipe materials are:– Plastic,Plastic,– Steel,Steel,– Concrete,Concrete,– Clay,Clay,– Asbestos cement,Asbestos cement,– Or simple as a wooden or Or simple as a wooden or

bamboo construction. bamboo construction.

Pipe and Lining Pipe and Lining MaterialsMaterials

Lining materialsLining materials

slip-form cast-in-place,slip-form cast-in-place,prefabricated concrete, prefabricated concrete, shotcrete or gunite, shotcrete or gunite, asphalt, asphalt, surface and buried plastic or surface and buried plastic or rubber membranes,rubber membranes,compacted earth.compacted earth.

The management of water in The management of water in the field channels involves;the field channels involves;1.1. Flow measurement,Flow measurement,

2.2. Sediment and debris removal,Sediment and debris removal,

3.3. Divisions,Divisions,

4.4. Checks,Checks,

5.5. Drop-energy dissipators,Drop-energy dissipators,

6.6. Water level regulators.Water level regulators.

Management of Management of WaterWater

Associated with these are Associated with these are various flow measuring devices various flow measuring devices like:like:Weirs,Weirs,Flumes,Flumes,OrificesOrifices. .

The designs of these structures The designs of these structures have have been standardizedbeen standardized since they are since they are small in size and capacity. small in size and capacity.

Designs for flow measurement and Designs for flow measurement and drop-energy dissipators structures drop-energy dissipators structures need:need:more attentionmore attentionconstruction must be more precise since construction must be more precise since

their hydraulic responses are quite their hydraulic responses are quite sensitive to their dimensionssensitive to their dimensions..

Design of Management Structures

On-farm water management structures:

A simple Drop Structure

A Typical Check-Divider

Field distribution systemsField distribution systems

After the water reaches the field, it is After the water reaches the field, it is distributed by a variety of means: distributed by a variety of means: Simple constructed Simple constructed Elaborately constructedElaborately constructed. .

Most fields have a head ditch or Most fields have a head ditch or pipeline running along the upper side pipeline running along the upper side of the field from which the flow is of the field from which the flow is distributed onto the field. distributed onto the field.

In a field irrigated from a head ditch, In a field irrigated from a head ditch, the spreading of water over the field the spreading of water over the field depends somewhat on the method of depends somewhat on the method of surface irrigation. surface irrigation.

For borders and basins, open or For borders and basins, open or piped cutlets are generally used. piped cutlets are generally used.

Furrow systems use outlets which Furrow systems use outlets which can be directed to each furrow. can be directed to each furrow.

Head ditch outlets for borders and basins

Field distribution and spreading can also be Field distribution and spreading can also be through:through: portable pipelines running along the surfaces or portable pipelines running along the surfaces or permanent pipelines running underground.permanent pipelines running underground.

Basins and borders usually receive water Basins and borders usually receive water through buried pipes serving one or more through buried pipes serving one or more gated risers within each basin or border.gated risers within each basin or border.

A typical riser outlet, known as an alfalfa A typical riser outlet, known as an alfalfa valve, is shown in the Figure. valve, is shown in the Figure.

The most common piped method The most common piped method of furrow irrigation uses: of furrow irrigation uses: • Plastic or Plastic or • Aluminum gated pipeAluminum gated pipe

The gated pipe may be connected The gated pipe may be connected to the main water supply via:to the main water supply via:A piped distribution network with a A piped distribution network with a

riser assembly directly to a canal riser assembly directly to a canal turnout, orturnout, or

An open channel to a piped An open channel to a piped transition.transition.

An alfalfa valve riser

Field MeasurementsField Measurements

The evaluation of surface irrigation at The evaluation of surface irrigation at the field level the field level is an important aspect is an important aspect of both management and design.of both management and design.

Field measurements are necessary to Field measurements are necessary to characterize the irrigation system in characterize the irrigation system in terms of:terms of:its most important parameters,its most important parameters,to identify problems in its function,to identify problems in its function,to develop alternative means for to develop alternative means for

improving the system. improving the system.

System characterization System characterization necessitates a series of basic necessitates a series of basic field measurements field measurements before,before, during,during, and and afterafter the irrigation. the irrigation.

The objectives of the evaluation The objectives of the evaluation will dictate whether the field will dictate whether the field measurements are:measurements are:comprehensive or are comprehensive or are simplified for special purposes. simplified for special purposes.

Five classes of field Five classes of field measurements are: measurements are:

1) Field topography and 1) Field topography and configurationconfiguration; ;

2) Water requirements2) Water requirements;;

3) Infiltration3) Infiltration; ;

4) Flow measurement; and 4) Flow measurement; and

5) Irrigation phases. 5) Irrigation phases.

Classes of Field Classes of Field MeasurementsMeasurements

Field Topography and Field Topography and Configuration Configuration

All field evaluations should include a All field evaluations should include a relatively simple assessment of the relatively simple assessment of the field topography and layout. field topography and layout.

These measurements are well enough These measurements are well enough known that only their brief mention is known that only their brief mention is required.required.

There is first of all the field's primary There is first of all the field's primary elevations.elevations.

This information requires that a This information requires that a surveying instrument be used to surveying instrument be used to measure elevations of the principal measure elevations of the principal

The following measurements need The following measurements need notnot be comprehensive be comprehensive nornor as as formalized as one would expect formalized as one would expect for for a land leveling projecta land leveling project. . Field boundaries (including dykes if Field boundaries (including dykes if

present),present),The elevation of the water supply The elevation of the water supply

inlet (an invert and likely maximum inlet (an invert and likely maximum water surface elevation),water surface elevation),

The elevations of the surface and The elevations of the surface and subsurface drainage system if subsurface drainage system if possible.possible.

The field topography and geometry should be The field topography and geometry should be measured. measured. This requires:This requires:placing a simple reference grid on the field, placing a simple reference grid on the field,

usually by staking, and then usually by staking, and then surveying the elevations of the field surface surveying the elevations of the field surface

at the grid points to establish slope and at the grid points to establish slope and slope variations.slope variations.

Usually one to three lines of stakes placed 20-30 Usually one to three lines of stakes placed 20-30 meters apart or such that 5-10 points are measured meters apart or such that 5-10 points are measured along the expected flow line will be sufficient. along the expected flow line will be sufficient. For For example:example:

A border or basin would require at most A border or basin would require at most three stake lines,three stake lines,

A furrow system as little as one, depending A furrow system as little as one, depending on the uniformity of the topography. on the uniformity of the topography.

Topography and Topography and GeometryGeometry

The The surveysurvey should establish the should establish the distance of each grid point from the distance of each grid point from the field inlet as well as the field field inlet as well as the field dimensions dimensions (length of the field in the (length of the field in the primary direction of water movement as primary direction of water movement as well as field width).well as field width).

There are important items of There are important items of information that should be available information that should be available from the survey:from the survey: The field slope and its uniformity in the The field slope and its uniformity in the

direction of flow and normal to it;direction of flow and normal to it; The slope and area of the field; andThe slope and area of the field; and A reference system in the field establishing A reference system in the field establishing

distance and elevation changes.distance and elevation changes.

The SurveyThe Survey

It is also worthwhile at this stage of It is also worthwhile at this stage of the evaluation the evaluation to record the location to record the location and extent of major soil typesand extent of major soil types (this (this may require may require samplingsampling and and some some laboratory analyseslaboratory analyses). ).

The cropping pattern should be The cropping pattern should be determined and, if a crop is on the determined and, if a crop is on the field at the time of the evaluation, field at the time of the evaluation, any obvious differences in growth and any obvious differences in growth and vigor should be noted.vigor should be noted.

Similarly, the cultivation practices Similarly, the cultivation practices should be recorded. should be recorded.

General Notes

Determining Water RequirementsDetermining Water Requirements

The irrigation system may not be The irrigation system may not be designed to supply the total amount designed to supply the total amount of moisture required for crop growth.of moisture required for crop growth.

In some cases, precipitation or In some cases, precipitation or upward flow from a water table may upward flow from a water table may contribute substantially towards contribute substantially towards fulfilling crop water requirements. fulfilling crop water requirements.

It is also unrealistic to expect that It is also unrealistic to expect that irrigation can be practiced without irrigation can be practiced without losses due to deep percolation, or losses due to deep percolation, or tailwater runoff. tailwater runoff.

The fraction of the water that is The fraction of the water that is used should be maximized, but used should be maximized, but this fraction cannot be 100 this fraction cannot be 100 percent without other serious percent without other serious problems developing such as a problems developing such as a salt build-up in the crop root salt build-up in the crop root zone. zone.

The dependency on irrigation in The dependency on irrigation in an area requires some analyses an area requires some analyses of the water balance. of the water balance.

Water balance may have three Water balance may have three perspectives:perspectives:The firstThe first is the balance of agricultural is the balance of agricultural

demands within a watersheddemands within a watershed. .

The outcome of such an analysis The outcome of such an analysis establishes the safe yield of water establishes the safe yield of water from various sources and thereby from various sources and thereby indicates:indicates: the area of a project,the area of a project, the priorities among projects,the priorities among projects, the configuration of the large systemic the configuration of the large systemic

components of the projectcomponents of the project..

An evaluation at the field level An evaluation at the field level presumes that this information is presumes that this information is available,available,

Should be generally understood Should be generally understood in as much as the limits of on-in as much as the limits of on-farm irrigation may be dictated farm irrigation may be dictated by the magnitude and by the magnitude and distribution of the total water distribution of the total water supply. supply.

The perspective of water balance at the river basin level

The secondThe second water balance water balance perspective, is the water balance perspective, is the water balance within the farm or command area. within the farm or command area. – An individual field is generally irrigated in An individual field is generally irrigated in

concert with others in the command or concert with others in the command or farm through sharing the water delivered farm through sharing the water delivered through a canal turnout or a well.through a canal turnout or a well.

– Fields also typically share drainage Fields also typically share drainage channels. channels.

– Water balance at the farm or command Water balance at the farm or command area level is established on a field's access area level is established on a field's access to water, its priority, timing and duration. to water, its priority, timing and duration.

– Again, a field evaluation presumes that Again, a field evaluation presumes that these factors have been formulated and these factors have been formulated and can be determined. can be determined.

Figure 17 illustrates the perspective of Figure 17 illustrates the perspective of water balance at the field level. water balance at the field level.

A perspective of the on-farm water balance

The perspective of water balance at the field level

The water balance within the confines The water balance within the confines of a field is a useful concept for of a field is a useful concept for characterizing, evaluating or characterizing, evaluating or monitoring any surface irrigation monitoring any surface irrigation system.system.

In using this aspect of water balance, In using this aspect of water balance, an important consideration is the time an important consideration is the time frame in which the computations are frame in which the computations are made, i.e. whether the balance will use made, i.e. whether the balance will use annual data, seasonal data, or data annual data, seasonal data, or data describing a single irrigation event.describing a single irrigation event.

If a mean annual water balance is If a mean annual water balance is computed, then it becomes reasonable computed, then it becomes reasonable that the change in root zone soil that the change in root zone soil moisture storage could be assumed as moisture storage could be assumed as zero. zero.

In some irrigated areas, In some irrigated areas, precipitation events are so light precipitation events are so light that the net rainfall can be that the net rainfall can be reasonably assumed to equal the reasonably assumed to equal the measured precipitation. measured precipitation.

Under other circumstances, various Under other circumstances, various other terms can be neglected. other terms can be neglected.

The time base and field conditions The time base and field conditions are often selected to eliminate as are often selected to eliminate as many of the parameters as many of the parameters as possible in order to study the possible in order to study the behavior of single parameters. behavior of single parameters.

One of the more important is crop One of the more important is crop evapotranspiration. evapotranspiration.

The upward movement of The upward movement of groundwater to the root zone can groundwater to the root zone can usually be ignored if the water table usually be ignored if the water table is at least a metre below the root is at least a metre below the root zone. zone.

Then if the soil moisture is measured Then if the soil moisture is measured before and after a period when there before and after a period when there is no precipitation or irrigation, the is no precipitation or irrigation, the depletion from the root zone is a depletion from the root zone is a viable estimate of crop water use. viable estimate of crop water use.

There are two particularly important There are two particularly important components in the field water balance components in the field water balance which impact design and evaluation. which impact design and evaluation.

The first is the irrigation requirement The first is the irrigation requirement of the crop, or its evapotranspiration of the crop, or its evapotranspiration and leaching needs.and leaching needs.

This is a design parameter and will be This is a design parameter and will be briefly described here, but a detailed briefly described here, but a detailed treatment is left to the FAO. treatment is left to the FAO.

The second important component The second important component deals with field evaluation and deals with field evaluation and concerns the nature of moisture concerns the nature of moisture content changes in the soil profile. content changes in the soil profile.