Irrigation and Drainage: All You Need to Know About

The pressure for survival and the need for additional food supplies to meet the demands of increasing populations are necessitating a rapid expansion of irrigation throughout the world. Irrigation is not only important for the arid regions but is becoming equally important in the humid regions.

Although irrigation and drainage is very important and is becoming a basic part of well-developed agriculture throughout the world, it also has its negative sides. Irrigation for example can convert agriculturally productive land into waterlogged land and create problem of salinity. The successful cultivation of crops largely depends upon adequate drainage of the land in which they are grown.

This unit treats all aspect of irrigation and drainage i.e. methods of irrigation and drainage, the advantages and disadvantages of each are discussed. The suitability of each method in terms of crops to be grown and topography of the land is also treated.

Table of Contents

1.            Definition of irrigation

2.            Crops response to water at different stages of growth

3.            Irrigation methods

4.            Surface irrigation

5.            Types of surface irrigation systems

6.            Complete flooding system of irrigations

 7.           Partial flooding

8.            Overhead irrigation

9.            Subsurface irrigation

10.          Efficient water management

11.          Drainage

12.          Drainage system

What is irrigation?

Irrigation is generally defined as the artificial application of water to the soil for the purpose of supplying the moisture essential for plant growth. Irrigation water is supplied to supplement the water available from rainfall.

Important terms and definitions

i.    Water requirement

The water requirement (WR) of a crop may be defined as the quantity of water, regardless of its source, required by a crop in a given period for its normal growth and development under the field condition at a specific place.

Water requirement includes the losses due to evapotranspiration (ET) or consumptive use (CU) plus the losses during the application of irrigation water and the amount required for special operations such as land preparation, transplanting, leaching, etc. it may thus be formulated as follows: WR=ET or CU+ application loses + special needs.

ii.    Irrigation requirement

The field irrigation requirement (IR) of a crop, therefore, refers to the water requirement of the crop, excluding the effective rainfall and the contribution from soil profile, and may be formulated as: IR=WR-(ER+S).

A farm irrigation requirement depends on the irrigation needs of the individual crops, their area and losses in the farm water distribution systems, mainly by seepage.

iii.    Net irrigation requirement

This is the amount of irrigation water required to bring the soil moisture level in the effective root zone to field capacity. Thus it is the difference between field capacity and the soil moisture content in the root zone before starting irrigation.

iv.     Gross Irrigation requirement

This is the total amount of water applied through irrigation. In other words, it is the net irrigation requirement plus losses in water application and other losses.

Gross irrigation requirement can be determined for a field, farm, and outlet command area, or an irrigation project, depending on the need, by considering the appropriate losses at a various stages of the crop growth.

Gross irrigation requirement in field= net irrigation requirement divided by irrigation efficiency. For example, if the net amount of irrigation is 10cm and the irrigation efficiency is 70%, the gross amount of water to be applied to the field is 10 cm divided by 0.70=14.29 cm.

v.      Irrigation efficiency

Irrigation efficiency is a measure of amount of water delivered by irrigation that actually ends up as available water to the plant. To illustrate the point, Let us assume a rooting depth of 1.5m. Ideally, the soil should be wetted evenly down to a depth of at least 4m. In practice this is rarely attained.

Irrigation efficiency is a measure of how close the water delivery system comes to achieving this ideal situation. It gives a measure of the amount of water effectively delivered to a farm and varies from soil to soil. The ratio between water requirement and irrigation is a measure of irrigation efficiency. It indicates how efficiently the available water supply is being used.

Factors influencing irrigation efficiency

─The principle factors influencing irrigation efficiency are the design of the irrigation system.

─ The degree of land preparation.

─ And the skills and care of the irrigation.

Water is lost from the distribution system in several ways, by evaporation, unwanted wetting of banks of delivery ditches, deep percolation, seepage and run-off. In sprinkler system, for example, the tendency of high evaporation reduces irrigation efficiency. Any factor which causes loss of water will also reduce irrigation efficiency.

Ways of minimizing the loss of irrigation water

─        These losses can be minimized by adequate planning of the irrigation system.

─        Proper design of the irrigation method.

─        Proper land preparation.

─        Efficient operation of the system.

Read Also: Sowing Guide for Different Kind of Crops

vi.    Irrigation frequency

This refers to the number of days between any two subsequent irrigation during periods without rainfall. It depends on the consumptive use rate of a crop and the amount of available moisture in the crop root zone.

It is a function of crop, soil and climate. Sandy soils are irrigated more often than fine textured soils. Moisture use rate increases as the crop grows and the day becomes longer and hotter.

In general, irrigation should start when about 50 % of the available moisture has been used from the zone in which most of the roots are concentrated. A record of the growth stages of the crop with reference to the critical periods of growth is also kept with a view to determining the frequency of the irrigation.

vii.  Irrigation scheduling

The number and timing of irrigation vary widely for different crops. Earlier concept for scheduling were based on the soil water regime in which water content at field capacity (the upper limit of the regime)was considered as 100% available for crop growth, and that at the permanent wilting point as 0% available.

About 50% available water was accepted as the lower limit of the regime and it was taken as a criterion for scheduling irrigation. Later on it was realized that climatic parameters play a predominant role in governing the water needs of crops.

This leads to the concept of evapo-transpiration, which has been used as a criterion for timing irrigations. The latest approach for scheduling irrigation is the plant water status itself. This may be considered as an ideal criterion as the plant is a good integrator of soil, water and climatic factors.

2. Crops Response to Moisture Level at Different Stages of Growth

It has been found that the water requirement of a crop vary with the different stages of its growth. When water is in abundance, irrigation can be given whenever needed, but when the water supply is limited, it is necessary to take into account the critical stages of crop growth with respect to moisture.

The term critical stage is commonly used to define the stage of growth when plants are most sensitive to water shortage. Each crop has certain critical stages at which, if they is a shortage of moisture, yield is reduced drastically. Therefore, when there is a shortage of water, it is better to take care of the critical stages first to obtain increased water use efficiency.

1.  Critical stages of crop growth in relation to moisture availability

Crop	Critical periods
Wheat	Crown root initiation, heading, flowering, and grain formation
Rice	Tillering, heading and flowering
Maize	Early  vegetative stage, flowering, and milk ripe stage, i.e. tassel ling to hard –dough stages
Sorghum	Seedling, booting  to heading stage
Groundnuts	Flowering and pod development
Cotton	Start of flowering and during boll development

3.  Irrigation methods

Irrigation methods vary in different parts of the world and on different farms in the same area because of differences in soil, topography, water supply, crops and customs. There are four methods of irrigation:

─                Surface irrigation (flooding, check basin method, border strip method, furrow method and corrugated method).

─             Overhead irrigation (sprinkler method).

─             Sub-surface irrigation.

─             Drip irrigation.

1.    Surface irrigation

In the surface irrigation method, water is applied directly to the soil surface from a channel located at the upper reach of the field. Highly efficient irrigation can be achieved in surface methods by an appropriate combination of the size of the irrigation stream, the size, shape and slope of irrigation bed, in infiltration rate of the soil and plant population.

Surface irrigation could be made more efficient by observing the following

─             The water distribution system should be properly constructed to provide adequate control of water to the field.

─             The land should be well prepared to permit the uniform distribution of water over the fields.

─             Fine texture soils with low infiltration rate require smaller streams to avoid excessive losses due to run-off at the downstream end and deep percolation at a lower reaches.

─             Coarse –texture soils with high infiltration rate require larger streams to spread over the entire strip rapidly and avoid excessive losses due to percolation at the upper ridges.

Advantages of surface irrigation

1              Adaptability: surface irrigation can be used on nearly all types of soil and crops. The system can be designed to accommodate a wide range of stream sizes and still maintain high water application efficiency.

2.            Flexibility: surface irrigation systems permit ample latitude to meet emergencies. The capacity of surface system is efficient to permit an entire farm to be irrigated in a small time period.

3.            Economy: it is usually inexpensive to operate because of low power requirement. Water is usually applied directly to the farmland by gravity flow from the irrigation projects canals and laterals. Where water is pump from wells, rivers, storage reservoirs or other sources of supply, only enough power to raise the water slightly above the land surface to be irrigated is needed.

2.   Types of surface irrigation systems

Surface irrigation may be grouped into two broad classifications:

i.             Complete flooding of the soil surface which includes flooding from the field ditches, flooding strips between border dikes, and flooding in basins or checks. In this method the entire land surface in the area being irrigated is covered with water.

ii.            Partial flooding or furrow method where the entire irrigated area is only partially flooded. Closely spaced furrows (small ditches) contain and distribute the water which moves both laterally and downward from the furrow to moisten the plant root zone.

Flooding method of irrigation is most suitable for:

─             Land having such regular surfaces than the other surface irrigation methods is impractical.

─             Areas where irrigation water is abundant and inexpensive.

─             Crops such as rice which requires standing water during most parts of their growing season.

3.  Complete flooding systems of irrigation

i.             check basin method

This is the simplest and most common method of irrigation. It consists of applying irrigation water to the level areas enclosed by ridges. Fairly level field are well graded and then divided by ridges into rectangular or square basins of 3 ₓ 2 m to 30ₓ30 m, so that each has a nearly level surface.

The size of the basin depends on the soil type and head of stream available. The water is retained in the basin and then slowly percolates into the soil.

When irrigation is orchards, square basin may be used as for other crops, but when the plants are widely spaced, the ring method of basin irrigation may be used. The rings are circular basins formed around each tree.

Advantage of check basin method

─             An advantage of the ring method is that the entire area is not flooded, thus obtaining high water use efficiency.

Suitability of check basin method

─             check basin irrigation is suited to smooth, gentle and uniform land slopes and for soils with moderate to slow infiltration rates.

─             the method is especially well suited to irrigating grain and fodder crops  in heavy soils where water is absorbed very slowly and is required to stand for a relatively long time to ensure adequate irrigation.

ii. Border strip method

The well leveled and graded land is divided into a number of long parallel strips called borders that are separated by low ridges. Each border strip should be level and should have a uniform gentle slope in the direction of water flow.

Each border is irrigated by allowing the water to flow from the upper end of the border in a thin sheet. The water moves towards the lower end with a non-corrosive velocity and covers the entire width of the border.

When the advancing water reaches the lower end, the stream is turned to the second trip. The water temporarily stored in the border moves down the strip and infiltrates the soil, thus completing the irrigation.

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Suitability of border strip method

─             This method of irrigation is more suitable for soil with moderately low to moderately high

Infiltration rate

─             This method is suitable for irrigating close-growing crops such as wheat, barley and fodder crops.

─             It’s not suitable for rice which requires standing water during the greater part of the growing season.

4.    Partial Flooding Methods

i.      Furrow method

With furrow irrigation, small channels or furrows are used to convey the water over the soil surface in small individual parallel streams. Infiltration occurs through the sides and bottom of the furrow containing water. From the point of infiltration, the water moves both laterally and vertically downwards to moisten the plant root zone.

The degree of flooding of the land surface depends on the shape, size and spacing of the furrows, the land slope and hydraulic roughness of the furrow. Furrows are made between the crops rows and the crops are grown on the ridges. With furrow irrigation it is difficult to prevent some erosion on steep slopes; the furrows should be laid out on the contour, i.e. across the slopes.

Suitability of furrow method

─  Nearly all row crops such s maize, sorghum, groundnut, cotton, tobacco, potatoes and sugarcane are irrigated by the furrow method.

─  Furrow method is suitable for most soils except  sandy soils that have a very high infiltration rate and provides the poor lateral distribution of water between the furrows.

ii.  Corrugation method

This is a partial flooding method, as the water does not cover the entire field surface. The stream of water is guided to flow through small furrow called corrugations evenly spaced across the field. The water spread laterally, saturating the area between the corrugations.

The main difference between this and regular furrow irrigation is that more but smaller furrows are used and the crop rows are not necessarily related to the irrigation furrows.  The corrugations are made after sowing but before germination have taken place.

The corrugations are U- shaped or V-shaped channels (furrows) of about 6-10 cm deep, spaced 50-150 cm apart, running down the slope from field ditches, or preferably from portable   gated pipes made of aluminum or hosepipes, in either case with outlet tubes. The length of the corrugation varies from 40 to 120 m and the slope is usually 2-6%.

The entire soil surface is wetted slowly by the capillary movement of the water which flows in the corrugation.

Advantages of corrugation method

─  This method of wetting the soil minimizes the crusting effect on the surface soil, which may be a problem when the entire surface is flooded.

─ The movable pipes make the method more efficient.

─ The advantage of this method is that is makes it possible to irrigate on relatively steep slopes without causing erosion.

Suitability of corrugation method

─  This method is suited to close-growing crops and for pasture growing on steep slopes.

─  Its most suitable for fine-to moderately coarse texture soils.

─ It is not recommended for saline soils or when the irrigation water has a high salt content.

Disadvantages

─             The method is very conducive to increasing salinity.

─             This method has a high requirement as each field must be corrugated at least once every year.

─             Field operation is difficult due to rough surface.

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5.   Overhead irrigation

In this method, the irrigation water is supplied to the crop above the ground surface in the form of spray. A sprinkler irrigation system consists of a pump to develop the desire operating pressure and main lines, laterals and risers to convey the water.

Sprinkler head and nozzles discharge the water in the form of spray. For sprinkler irrigation, the water must be clean and free of sand, debris and large amount of dissolved salt and a stable supply of water must be available.

Factors to consider when selecting a sprinkler

─             The sprinkler should have a capacity to meet the water requirement of the crop.

─             Should apply water at a rate that does not exceed the minimum intake rate of the soil.

─             The sprinkler should be able to apply water with some minimum economic uniformity.

─             Should minimize the total annual cost of irrigation.

─             Produce a crop that economically justifies the use of the system.

Types of sprinkler system

1.            Rotating sprinkler heads are spaced equally along the lateral lines. The lateral lines remain in one place until required amount of water has been applied and are moved the same distance for each successive setting.

2.            Perforated pipes: water is pumped through very small, closely spaced orifices in the pipe. These perforated pipes form the lateral lines and provide fairly uniform distribution along both sides of the pipe.

Suitability of sprinkler system

1.            The sprinkler irrigation is both technically and economically very suitable for terrain that is too uneven for surface irrigation, as well as for sandy soils.

2.            This method can be used for nearly all crops except rice and jute.

3.            It is not suitable for heavy clay soils where the infiltration rate is very low.

Advantages of sprinkler system

i.              Soluble fertilizers, herbicides and fungicides can be applied to the irrigation water economically.

ii.             It is used to protect crops against frost and high temperature that reduce the quality and quantity of the produce.

iii.            Water application can be more uniform and carried out with greater precision with sprinkler system than with surface irrigation, except during times of high wind.

iv.           Water use efficiency is also greater with sprinkler irrigation.

v.            Sprinkler during the hot hours of the day may improve the micro climate, prevent transient wilting, and increase stomata opening and thereby improving the photosynthetic effectiveness.

vi.           The elimination of the field ditches required for surface irrigation increases the net area available for crop production and reduces water losses to seepage and percolation.

Vii           This method does not interfere with the movement of farm machinery.

Disadvantages of sprinkler system

i.              the capital investment of equipment is relatively high.

ii.             Water loss due to evaporation and the interception of water by the foliage is greater   with sprinklers than with surface irrigation method.

iii.            It is not well suited to very windy areas.

6.    Subsurface irrigation    

In subsurface irrigation, water is applied below the ground surface by maintaining an artificial water table at a predetermined depth, depending upon the soil texture and rooting depth of the plant roots. Water reaches the plant root through capillary action.

Water may be introduced either through correctly spaced open ditches in the field or underground pipelines such as tile drains or mole drains. The depth of open ditches or trenches varies from 30- 100 cm and they are spaced about 15-30 m apart.

The water application system consists of field supply channels, ditches or trenches suitable spaced to cover the field adequately and drainage ditches for the disposal of excessive water.

Types of surface irrigation

1.     Open Ditches system:  It is most widely used sub-surface system. Feeder ditches are excavated on the contour and spaced close enough to ensure control of water table.

They are connected to a supply ditch that runs down the predominant field slope and has control structure as needed to maintain the desired water level in the feeder ditches. The lower ends are connected by an outlet tile which is used to carry excess irrigation water and storm water to a satisfactory outlet.

2.      Perforated tubes (Drip irrigation): The perforated tube is buried 4”-8” under the ground depending on the type of crop to be grown. It is generally used for row crops especially cotton.

Water is pumped through these tubes under a low pressure and it oozes out through the numerous tiny holes to supply the roots. At this slow rate of application, water percolates immediately downwards and sideways into the soil.

Advantages of drip irrigation

i.              There is considerable saving in water by adopting this method since the water can be applied almost precisely to the root zone and there is no need to wet the entire area between the crops.

ii.             It permits the application of fertilizer through the system.

iii.            It minimizes such conventional losses as deep percolation, run-off and soil water evaporation.

iv.           The system has a greater advantage over other sub-surface irrigation system because it is easily layed down and can be removed at any time after the crop has been harvested.

Suitability of subsurface irrigation method

─             This method can be used for most soils with a low water-holding capacity and a high infiltration rate.

─             Subsurface irrigation is suited to soils having reasonable uniform texture and permeable enough for water to move rapidly both horizontally and vertically within and for some distance below the crop’s root zone.

─             this method is suited to irrigating vegetables, most field crops, small grains, pasture grass, most forage crops and flowers.

Advantages of subsurface irrigation

i.              Effective on soils having low water holding capacity and high intake rates where other methods are impracticable due to labour, equipment and water costs.

ii.             Dispersion of weed seeds is reduced, thus reducing weed control costs.

iii.            Evaporative loss of water from land surface is minimal.

iv.           Special tillage and frequent land preparation for conveying surface water is eliminated, thus less damage to soil structure.

V             The amount of water for irrigation can be controlled and even distribution is possible.

vi.           Normal farm operations can be carried out without interference or major alteration of the lay-out.

Disadvantages of subsurface irrigation

i.              Subsurface irrigation tends to cause salt accumulation in the root zone.

ii.             Requires a more complex combination of physical conditions not readily found in nature.

iii.            Drainage and leaching practices must be more intensive to assure adequate salinity control.

iv.           It is expensive and should be used only for high-value crops.

4.  Water management in irrigation scheme

The important aspects of a comprehensive irrigation development programme are:

─             Integrated development of water resources.

─             Efficient method of conveyance and distribution of water.

─             Judicious methods of water application.

─             Proper soil management practices.

─             Cropping pattern for high water –use efficiency.

─             Proper timing of irrigation based on the development stages of the plant.

─             Removal of excess water.

i.      Integrated development of water resources

 Watershed management and harvesting are important aspects of water resources development Programme. Loss of water by seepage and evaporation from farm tanks can be minimized by lining and covering reservoirs with plastic, artificial rubber or chemical.

Read Also: Systems of Crop Production

ii.  Efficient methods of conveyance and distribution of water

For efficient water use, irrigation channels should be stable, have negligible scour and negligible disposition of sediments.  To achieve this, irrigation channels and canals are lined with suitable materials which include concrete, rock masonry, brick, bentonite-earth mixtures, natural clays of low permeability, and various rubbers, plastic and alsphalt compound.

If canals and channels are not lined, or not properly lined, weeds and willows will grow on the canal banks, and moss and other aquatic plants will grow in the canals. These greatly retard water velocity and so decrease canal capacity. Silt and clay sedimentation in canal also restrict water flow.

iii.    Judicious methods of water application              

whatever the method of irrigation, the essential requirement in water uses is the application of right amount of water and its uniform distribution in the field so as to wet the root zone to its storage capacity. Excessive depth of application would result in low efficiency.

iv.   Proper soil management practices

Soil management practices which relates to irrigation are land grading, land preparation and cultivation practices. These aim at obtaining a uniform distribution of irrigation water on the farm, storing large amount of rainwater within the root zone, and improving the soil structure for increased water availability.

v.  Cropping pattern for high water-use efficiency

An efficient cropping pattern must ensure the most efficient use of land, fertilizer, irrigation water and other inputs. In the cropping pattern, the selection of crops and varieties is most important.

A crop or variety should be short-duration, photo-insensitive, have a low water requirement, be fertilizer-responsive and high yielding, all of which may enable the farmer to increase the intensity of cropping and thus raise the production per unit input.

Proper timing of irrigation based on the development stages of the plant. To raise a good crop of rice about 2000mm of water is required. Of this 1500mm is lost by percolation during land preparation.

This huge loss can be prevented and the water used during the growing period of rice. The loss of water through percolation can be minimized by the incorporation of a small quantity of bentonite in the top 25 cm of soil.

vi.  Removal of excess water

A large mass of land is water-logged due to seepage from canals. There should be a proper drainage programme to drain out excess water either into the canal or to a distant place to be used as irrigation water, but with proper salinity-checking devices.

Read Also: The Different Cropping Systems in Africa

5. Drainage

Drainage may be defined as the means by which soil and subsoil-water is controlled in, and removed from, the root zone in relation to the health and vigour of the crop. A soil may need artificial drainage because the water table is high or because of excess surface water.

In both cases, all the pore spaces are filled with water and aeration is poor. The result is reduced root development and possibly an accumulation or concentration of ions such as manganese.

The major sources of excess water that make drainage necessary are:

─             Seepage losses from reservoirs or canals.

─             Deep percolation loss from irrigated lands.

─             Flooding of low lands.

─             Flow of groundwater towards waterlogged lands in the arid region.

Aims of Drainage

The basic aim of field drainage is to assist land to get rid of water from the upper layers of the soil in a manner that will maintain the conditions which provide aeration, warmth and adequate moisture within the root zone of the crop. The adequate drainage of crop-producing lands requires a general lowering of shallow water tables.

Ways of lowering the water table:

─             Eliminating or controlling sources of excess water.

─             Improving natural drainage facilities.

─             Providing man-made artificial drainage systems such as open channel drains, covered clay or concrete pipes pumping ground water.

Benefits of drainage

─ Draining cultivatable land promotes a number of environmental conditions in the soil that are favorable to higher plants and the micro flora and fauna.

─    It improves soil aggregation or granulation and thus encourages aeration, better plant root development, biological activity and nutrient uptake.

─  Providing more available soil moisture and plant food by increasing the depth of the root zone soil.

─  It decreases losses of soil nitrogen due to denitrification.

─  Decreasing soil erosion by increasing water infiltration into soils.

─  Leaching excess salts from the soil.

─ Assuring higher soil temperatures.

1.  Drainage system

There are two main types of drainage systems:

─             Surface.

─             And sub-surface drainage.

1.            Surface drainage

Surface drainage involves smoothing the soil surface and creating enough slopes to ensure water run-off.

Lowland areas often receive water from the surrounding uplands. Impermeable soils may be unable to get rid of excess water by downward movement through the soil profile. Sometimes excess water is applied to a field during irrigation.

In all of these cases, surface drainage is used to dispose of the excess water. Ditches are built of concrete to ensure durability, especially where rapid water movement occurs. Ditches must be cleaned and weeded periodically.

Disadvantages of surface drainage

─             Silt and clay sedimentation and the growth of weeds and willows restrict the flow of water.

─             Surface drains are troublesome to maintain and water distribution interferes with them.

─             Another major disadvantage of surface ditches is that they may interfere with the use of machinery.

2. Sub-surface drainage

i.             Mole drains are cut in the soil at a pre- arranged depth, below the main root zone. Mole drains are usually 10-15 cm in diameter, circular or nearly so in cross section, 50-60 cm deep, and 3-4 m apart. Some cuts are made in the drains.

These cuts assist the passage of water from the surface and through the soil to the drains. Mole drains require not only suitable land but proper grading of the drains and free outlets at the lower ends, leading into surface cuts of sufficient depth which discharge to main drainage canals or a natural water course.

ii.            Tile drains are formed by hollow cylindrical tiles of 10-25 cm internal diameter. The tiles are made of concrete and are laid in deep trenches cut at predetermined intervals to a depth of 75 cm or more. When the soil surrounding the tile is saturated with water, the water seeps into the tile and eventually reaches an outlet where it is discharged.

Advantages of underground drainage

i.              Low maintenance costs.

ii.             Unobstructed passage of farm implement over them.

iii.            Arable land is not sacrificed as is often the case with surface drainage.

iv.           They also indirectly help in providing water for irrigation.

v.            Assists in protecting the soil from erosion.

vi.           The firmer particles of surface soil carried away in large quantities and deposited in the trenches and main drains are cleaned and reformed after some years.

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Maintenance of Drains

─             The maintenance of drainage systems requires the regular removal of soil and vegetation from the drains.

─             To keep closed drains clean, it is essential to destroy the penetrating roots periodically by adding some chemicals to the drain water. To achieve this, all undesirable vegetation in the field should be killed with chemicals.