What is Irrigation?
Irrigation is a simple process. In this water storage takes place during periods of excess rainfall. Thus that water in storage is made available to irrigate plants during periods of less or no rainfall.
Plants require water and air for their survival. Different types of plants require different quantities of water at different times until they grow completely. Water is normally made available to these plants by nature through the flood water of rivers. The supply of water by nature does not match the requirement of the crops.
The following are the main concerns on irrigation.
• How to apply?…. That is what should be the method: Border flooding method, furrow method, sprinkler method, drip method, etc.
• How much to apply?….. How much moisture the soil can hold in its pores which is the water holding capacity of the soil.
• When to apply?… That is when has the moisture level depleted to 50 to 60 percent of moisture-holding capacity, and when is the time to irrigate. In other words what should be the frequency of irrigation.
Irrigation may be defined as the science of the artificial application of water to the land, in accordance with crop requirements throughout the crop period for the full-fledged nourishment of the crops.
The crop yield from irrigation is expressed as quintal/ha or tonnes/ha. The productivity of the crop is expressed as crop yield per mm of water applied.
Advantages and disadvantages of Irrigation
1. Increase in food production: Increase in crop yield leads to an increase in food production, thus developing people as well as society.
2. Protection against drought: The provision of adequate watering facilities in any region ensures protection against the failure of crops from famines and droughts.
3. Revenue generation: When the regular supply of water is assured, the farmers can grow certain superior or high priced crops in place of inferior or low priced crops. As a result, revenue is generated.
4. Mixed Cropping: Means growing two or more crops together in the same field. This practice is followed so that if weather conditions are not favorable for one crop, it may be suitable for other crops. But if enough water facilities are made available, the need for mixed cropping is eliminated.
1. Power generation: Major river valley projects are usually planned to provide hydroelectricity power together with irrigation. However, a relatively small quantity of hydroelectric power may be also generated at a small cost on projects which are primarily planned for irrigation.
2. Transportation: Most of the canals are provided with unsurfaced roads primarily for the purpose of inspection and maintenance. These roadways provide a good pathway for local people. The network of canals can be used as the most economical means of transportation of goods as well as human beings.
3. Employment: During the construction of irrigation works, employment is provided
1. An abundant supply of water tempts the cultivators to use more water than required.
2. Excess water provided seeps in the soil. Hence, due to constant percolation, groundwater table would be raised and will lead to waterlogging.
3. The groundwater can get polluted due to the seepage of the nitrates into the groundwater (applied to the soil as fertilizers).
NOTE: If water is used judiciously with proper scientific consideration then there won’t be ill effects of irrigation.
Types of Irrigation
Surface irrigation is the process of supplying water to the soil by means of gravity or pumps. Out of the total land under irrigation, 76 percent of land receives water by surface irrigation. This method is best suited to soils with low to moderate infiltration capacities and to lands with
relatively uniform terrain (slopes less than 3%)
It can be further classified into:
1) Flow irrigation 2) Lift irrigation.
When the water is available at a higher level, and it is supplied to a lower level, by the mere action of gravity, then it is called flow irrigation.
In lift irrigation, water is lifted to a certain level by means of pumps. Then this water is provided in the fields for irrigation. Use of wells and tube wells for supplying irrigation fall under this category.
Flow irrigation can be further subdivided into
(a) Perennial irrigation (b) Flood irrigation.
(a) Perennial Irrigation
In the perennial system of irrigation, constant and continuous water supply is assured to the crops in accordance with the requirements of the crop, throughout the crop period. In this system, water is supplied through the canal distribution system taking-off from above a well or a reservoir.
When water is supplied by diverting the river runoff into the main canal by constructing a diversion or a barrage across the river, then it is called as direct irrigation.
But if a dam is constructed across a river to store water during monsoons, so as to supply water in the oil-taking channel during periods of low flow, then it is termed as storage irrigation.
b) Flood Irrigation
In this method, the soil is kept submerged and thoroughly flooded with water, so as to cause thorough saturation of the land. It is usually practiced in delta regions where the river water level during the flood is sufficiently high to supply water to the land by flow, or partly by flow and partly by lift.
This system of water supply is also called uncontrolled or inundation irrigation.
In this type, water does not actually wet the soil surface. Here the supply of takes place beneath the soil by capillary action.
Following are its types-
(i) Natural sub-irrigation (ii) Artificial sub-irrigation.
(i) Natural sub-irrigation
Leakage water via channels travels underground. When this water passes through the sub-soil, it irrigates crops sown on lower lands by capillarity action.
Sometimes, leakage causes the water table to rise up, which helps in the irrigation of crops by capillarity. Here underground water supply is maintained to the field without any extra efforts in a natural way.
(ii) Artificial sub-irrigation
Application of water by capillarity action of drains laid underground is known as artificial sub-irrigation. It is a very costly process and hence, adopted on a very small scale.
This method is applicable only where the soil conditions are favorable and for cash crops that give a substantial amount of returns.
Advantages of Lift Irrigation over Flow Irrigation
1. Installation of lift irrigation is possible anywhere on the field as per the requirement.
2. Proper levels of water can be maintained.
3. There are no pollution and evaporation losses (due to pumping from nearby tub wells).
4. The subsoil water table can be controlled and prevents water logging, soil salinity and alkalinity.
5. High irrigation efficiency and improved methods of irrigation like the perforated pipe, sprinkler, and drip can be practiced.
6. Cash crops can be grown, which payback for the high initial investment.
7. Irrigation is possible even during a year of drought.
8. Parts of land that are difficult to irrigate can be irrigated well.
9. Better yield is possible by applying proper irrigation and water management processes.
Types of Flood Irrigation
1. Free flooding.
2. Check flooding.
3. Furrow method.
4. Border flooding.
5. Basin flooding.
1) Free flooding or Ordinary flooding
In this particular method, ditches are dug in the field. These ditches and are on the contour or they are up and down the slope. Water from these ditches flows across the field.
After the water leaves the ditches, no attempt is made to control the flow by means of levees, etc. It is sometimes called wild flooding (as the movement of water is not restricted).
The initial cost of land preparation is low but labor requirements are usually high. Water application efficiency is also very low.
Wild flooding is best for pastures, close-growing crops, etc, mostly where the land is steep. This type of process is applicable where the land is irregular and construction of borders checks, basins and furrows are not feasible.
2) Border flooding
Border flooding involves the dividing of the land into strips. Borders are long, uniformly graded strips of lands, separated by earth bunds. These bunds are to guide the flow of water down the field
The land areas confined in each strip is of the order of 10 to 20 meters in width, and 100 to 400 meters in length to prevent water from concentrating on either side of the border. The land should be leveled perpendicular to the flow. Water is allowed to flow from the supply ditch into each strip.
Waterflow slowly towards the lower-end and it infiltrates into the soil as it advances. As the flowing water comes near the bottom end of the strip, water supply to the strip is stopped.
Supply ditch may either be in the form of an earthen channel or a lined channel or an underground concrete pipe having risers at intervals. The size of the supply ditch depends upon the infiltration rate of the soil and the width of the channel.
3) Check flooding
Check flooding and ordinary flooding are the same. In check flooding, each compartment is filled with water at a fairly high rate and allowed to stand until the water infiltrates. Close-growing crops such as Jowar or paddy are preferred for this method of irrigation
Deep homogenous loam or clay soils with medium infiltration rates are preferred for this method. This method is suitable for both more permeable and less permeable soils.
Water can be quickly spread in case of highly permeable soils thus reducing percolation losses. Water can also be held for a longer time in case of less permeable soils for assuring adequate penetration.
4) Basin flooding
This method is a special type of check flooding adopted specially for orchard trees. One or more trees are generally placed in the basin & the surface is flooded as in check method.
The shape of the basin can be square, rectangular, circular or it may be irregular. Flatter the land surface, easier it is to construct the basins.
5) Furrow method
In this method, water is supplied to the land by a series of furrows.
Furrows are small parallel channels made to carry water for irrigating the crops. The water flowing in the furrows infiltrates into the soil and spreads laterally to irrigate the land between the furrows.
The crops are usually grown on the ridges between the furrows. In the furrow method, only a part of the land varying from one-half to one-fifth is wetted which results in reduced evaporation losses.
Note: Corrugations are in general similar to furrows but furrows are the channels of relatively larger cross-section compared to corrugations
In this method, water is applied to the land in the form of a spray, somewhat as in ordinary rain through a network of pipes and pumps.
It is also sometimes known as overhead irrigation (the overhead system is required).
The sprinkler method can be used for all the crops except rice and jute and for almost all the soils except very heavy soils with a very low infiltration rate. Best suited for very light soils as deep percolation losses are avoided.
This system is flexible to suit undulating topography and hence land leveling is not necessary. This method is used mainly by the cultivators of tea, coffee, and vegetables in our country.
Advantages of Sprinkler Method
1. It can be efficiently used for a wide range of topographic conditions, soils, and crops.
2. With the use of sprinklers, erosion of soil can be controlled (as surface runoff is eliminated).
3. Uniform application of water is possible with sprinklers.
4. In this method, better control of water usage can be enforced and also the light supply of water is possible which is required for seedlings and plants which are very young.
5. Labour cost reduction is possible as no land preparations are required.
6. It does not require borders field channels etc, and hence more land is available for cropping.
7. The time and amount of application of fertilizer can be better controlled in sprinkler irrigation.
8. The sprinkler irrigation method is especially adaptable to more humid regions.
9. It can be used when there is water scarcity because evaporation and percolation losses are less.
Disadvantages of Sprinkler Method
1. Under windy conditions and high temperatures, the water distribution and water application efficiencies are low saline water may cause leaf burns in many crops.
2. The system is costly to install, operate and maintain.
3. A continuous supply of power is necessary for operating the system.
4. Corners remain under-irrigated and therefore uniformity of application is to some extent affected.
Drip irrigation method
Drip irrigation is one of the latest methods of irrigation which is becoming increasingly popular in areas with acute scarcity of water and salt problems.
In this method, water and fertilizer are slowly and directly made available to the root zone of the plants in order to minimize the losses due to evaporation and percolation.
It is also known as trickle irrigation. This is achieved with the help of specially designed emitters and drippers. The centrifugal pump is best for this method. Drip irrigation is best for row crops and orchards such as tomatoes, grapes, corn, citrus melons, fruits, cauliflower, cabbage, and turnips.
Advantages of Drip Irrigation
1. Low consumption of water.
2. Evaporation loss and wind loss are almost negligible.
3. There is the least wetting of soil surface due to the direct water supply to the plants.
4. Suitable for all types of soils, especially for coarse-textured soil.
5. The highest rate of vegetative growth.
6. Land leveling is not necessary.
7. Roots stay within the moist zone reduction of deleterious effects of salts.
8. Low labor requirements, as most drip systems are permanent setups.
9. Field operations are easier to manage.
Disadvantages of Drip Irrigation
1. It does not offer frost protection (as sprinklers do).
2. Plastic drip-lines and sub-mains are prone to attack by rodents and small animals.
3. Requires regular flushing (to clear off the dirt collected near the ends of the drip lines) supervision.
4. High skill is necessary for the design, installation, operation, and maintenance.
Note: Sprinkler and drip systems fall under a category known as a pressurized irrigation system.