Crops Response to Moisture Level at Different Stages of Growth

It has been found that the water requirements 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 there 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.

Table: Critical Stages of Crop Growth in Relation to Moisture Availability

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 irrigation);

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 the irrigation bed, the 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 fields.

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

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

Coarse-textured soils with high infiltration rates 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

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.

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.

Economy: It is usually inexpensive to operate because of low power requirements. Water is usually applied directly to the farmland by gravity flow from the irrigation projects canals and laterals.

Where water is pumped 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.

Types of Surface Irrigation Systems

Surface irrigation systems may be grouped into two broad classifications:

Complete flooding of the soil surface which includes flooding from 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.

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 that the other surface irrigation methods are impractical.

Areas where irrigation water is abundant and inexpensive.

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

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 fields are well graded and then divided by ridges into rectangular or square basins of 3 x 2 m to 30 x 30 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 basins and then slowly percolates into the soil.

When irrigating orchards, square basins 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 strip.

The water temporarily stored in the border moves down the strip and infiltrates the soil, thus completing the irrigation.

Suitability of Border strip method

This method of irrigation is more suitable for soils with moderately low to moderately high infiltration rates.

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.

Partial Flooding Methods

1. 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 downward 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 as maize, sorghum, groundnuts, cotton, tobacco, potatoes and sugar cane are irrigated by the furrow method.

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

2. 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 furrows 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 aluminium or hosepipes, in either case with outlet tubes. The length of the corrugations varies from 40 to 120 m and 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 it 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.

It’s most suitable for fine – to moderately coarse-textured 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.

2. Overhead Irrigation (Sprinkler irrigation)

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

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

Factors to consider when selecting a sprinkler

The sprinkler should have a capacity to meet the water requirements 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

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.

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

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

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

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

Advantages of sprinkler system

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

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

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.

Water use efficiency is also greater with sprinkler irrigation.

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

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.

This method does not interfere with the movement of farm machinery

Disadvantage of sprinkler system

The capital investment for equipment is relatively high.

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

It is not well suited to very windy areas.

3. Sub-surface 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 roots through capillary action. Water may be introduced either through correctly spaced open ditches in 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 suitably spaced to cover the field adequately and drainage ditches for the disposal of excess water.

Read Also : Proper Irrigation and Drainage Guide

Types of subsurface irrigation

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.

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

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.

It permits the application of fertilizer through the system.

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

The system has a greater advantage over other sub-surface irrigation systems because it is easily laid 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 reasonably 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

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

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

Evaporative loss of water from land surface is minimal.

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

The amount of water for irrigation can be controlled and even distribution is possible. Normal farm operations can be carried out without interference or major alteration of the lay-out.

Disadvantages of subsurface irrigation

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

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

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

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

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.

1. 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 : Agronomic Measures to Control Soil Erosion

2. Efficient methods of conveyance and distribution of water

For efficient water use, irrigation channels should be stable, have negligible scour and negligible deposition 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 rubber, plastic and asphalt 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.

3. Judicious methods of water application

Whatever the method of irrigation, the essential requirement in water use 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.

4. 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 amounts of rainwater within the root zone, and improving the soil structure for increased water availability.

5. 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.

6. 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.

7. 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.

Drainage and Its Importance

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 include:

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.

Drainage Systems

There are two main types of drainage systems: Surface and subsurface drainage.

a. Surface drainage

Surface drainage involves smoothing the soil surface and creating enough slope 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.

b. Sub-surface drainage

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.

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 include;

– Low maintenance costs.

– Unobstructed passage of farm implement over them.

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

– They also indirectly help in providing water for irrigation.

– Assists in protecting the soil from erosion.

– 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.

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.

In summary, the importance of irrigation and drainage in sustainable crop production cannot be over emphasized. Too much or too little moisture is a determinant factor in the growth, development and crop yield.

Various agronomic practices carried out in the process of crop production are aimed at creating the most suitable moisture regimes that could guaranty optimum growth and productivity of crop plants.

Irrigation supplies the needed moisture at the right quantity and time while drainage removes excess moisture from the field thereby creating suitable moisture regime that encourages microbial activities in the soil as well as nutrient availability and absorption by the plants.

Various method of irrigation and drainage, their advantages and disadvantages and suitability to given crops and locality were highlighted.

In this article, you have studied the various methods of irrigation i.e. surface and sub-surface irrigation methods, the suitability, the advantages and disadvantages of each method were treated.

Also the various methods of draining excess water from the land, the advantages and the disadvantages of each method were discussed. Irrigation and drainage are integral part of any sustainable crop production programme.

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