What is Irrigation?
Irrigation is the process in which water is stored during periods of excess rainfall and thus that stored water is put 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.
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 per cent of moisture holding capacity, and when is the time to irrigate. In other words what should be the frequency of irrigation.
Definition 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.
Crop Yield and Productivity
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.
The following table gives the data for productivity
|Crop||water applied. |
Increase in yield can be achieved by the following methods
- Land shaping or land levelling
- Suitable crop rotation and crop planning
- Using high yielding varieties of seeds.
- Using chemicals and fertilizers like N.P.K, green manures.
- Modern methods of irrigation such as sprinkler, drip, furrow, etc.
- The lining of canals, distributaries and watercourses by an economical lining material.
- Drainage of irrigated land by surface and subsurface irrigation.
Advantages and distadvantages of irrigation
- Increase in food production: Increase in crop yield leads to an increase in food production, thus developing people as well as society
- Protection against drought: The provision of adequate watering facilities in any region ensures protection against failure of crops from famines and droughts.
- 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.
- Mixed Cropping: Means growing two or more crops together in the same field. This practice is followed so that if weather conditions are not favourable 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.
- 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.
- 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 s human beings.
- Employment: During the construction of irrigation works, employment is provided
- An abundant supply of water tempts the cultivators to use more water than required.
- Excess water supplied to the field would percolate into the soil. Hence, due to constant percolation, groundwater table would be raised and will lead to waterlogging.
- The groundwater can get polluted due to 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 PROJECTS
|major projects||>10000 ha||>5 crores|
|medium projects||2000-10000 ha||0.25-5 crores|
|small projects||<2000 ha||0.25-0.5 crores|
TYPES OF IRRIGATION
1 Surface irrigation
Surface irrigation is defined as the group of water application techniques where water is applied and distributed over the soil surface either by gravity or by pumping. More than 75% of irrigated lands in India is supplied water by surface irrigation methods. 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.
If the water is lifted up by some mechanical or manual means, such as by pumps, etc. and then supplied for irrigation, then it is called lift 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 practised 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. Rather it flows underground and nourishes the plant roots by capillarity.
It may be divided into the following two types
(i) Natural sub-irrigation (ii) Artificial sub-irrigation.
(1) Natural sub-irrigation
Leakage water from channels goes underground and during passage through the sub-soil, it may irrigate crops, sown on lower lands, by capillarity. Sometimes, leakage causes the water table to rise up, which helps in the irrigation of crops by capillarity. When underground irrigation is achieved, simply by natural processes without any additional extra efforts, it is called natural sub-irrigation.
(ii) Artificial sub-irrigation
When a system of open-jointed drains is artificially laid below the soil, so as to supply water to the crops by capillarity, then it is known as artificial sub-irrigation. It is a very costly process and hence, adopted on a very small scale. It may be recommended only in some special cases with favourable soil conditions and for cash crops of very high return.
ADVANTAGES OF LIFT IRRIGATION OVER FLOW IRRIGATION
- Lift irrigation can be installed directly on the farm (mostly it is installed centrally) at any time with a small investment. Thus the length of the field channels (water courses) is less and can be lined with an economic lining material locally available.
- Water can be applied to the required depth when it is time to irrigate.
- There are no pollution and evaporation losses (due to pumping from nearby tub wells).
- The subsoil water table can be controlled and prevents waterlogging, soil salinity and alkalinity.
- High irrigation efficiency and improved methods of irrigation like the perforated pipe, sprinkler and drip can be practised.
- Cash crops can be grown, which payback for the high initial investment.
- Irrigation is possible even during a year of drought.
- Isolated patches of land can be easily irrigated where the canal water cannot reach.
- High yields can be obtained with good water management and fertilizer application.
IRRIGATION & METHODS OF IRRIGATION
- Free flooding.
- Check flooding.
- Furrow method.
- Drip irrigation method.
- Border flooding.
- Basin flooding.
- Sprinkler irrigation method.
1) Free flooding or Ordinary flooding
In this method, ditches are excavated in the field, and they may be either on the contour or 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 labour requirements are usually high. Water application efficiency is also low. Wild flooding is most suitable for close-growing crops, pastures, etc, particularly where the land is steep. This method may be used on rolling land (topography irregular) where borders checks, basins and furrows are not feasible.
2) Border flooding
In this method, the land is divided into a number of strips, separated by low levees called borders. 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 metres in width, and 100 to 400 metres in length to prevent water from concentrating on either side of the border. The land should be levelled 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. When the advancing water reaches the lower end of the strip, the supply of water to the strip is turned off.
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 is similar to ordinary flooding except that the water is controlled by surrounding the check area with low and flat levees. 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 applied to the land to be irrigated 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
6) Sprinkler irrigation method
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 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 levelling is not necessary. This method is used mainly by the cultivators of tea, coffee and vegetables in our country.
Advantages of Sprinkler Method
- It can be efficiently used for a wide range of topographic conditions, soils and crops.
- With the use of sprinklers, erosion of soil can be controlled (as surface runoff is eliminated).
- Uniform application of water is possible with sprinklers.
- In this method, better control on water usage can be enforced and also light supply of water is possible which is required for seedlings and plants which are very young.
- Labour cost is reduced as no land preparations are required.
- Does not require borders field channels etc, and hence more land is available for cropping.
- The time and amount of application of fertilizer can be better controlled in sprinkler irrigation.
- About 80% of water application efficiency is possible (water application efficiency is high).
- The sprinkler irrigation method is especially adaptable to more humid regions.
- Can be used when there is water scarcity because evaporation and percolation losses are less.
Disadvantages of Sprinkler Method
- Under windy conditions and high temperatures, the water distribution and water application efficiencies are low saline water may cause leaf burns in many crops.
- The system is costly to install, operate and maintain.
- Continuous supply of power is generally required for operating the system.
- Corners remain under-irrigated and therefore uniformity of application is to some extent affected.
7) Drip irrigation method
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 applied 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. Centrifugal pump is best suited for this method. Drip irrigation is best suited for row crops and orchards such as tomatoes, grapes, corn, citrus melons, fruits, cauliflower, cabbage and turnips.
Advantages of drip irrigation
- Water requirement is very less (since water is delivered directly to the plant).
- Evaporation loss and wind loss are almost negligible.
- There is the least wetting of soil surface due to the direct water supply to the plants.
- Suitable for all types of soils, especially for coarse-textured soil.
- The highest rate of vegetative growth.
- Land levelling is not required.
- Roots stay within the moist zoneReduction of deleterious effects of salts.
- Low labour requirements, as most drip systems are permanent setups.
- Field operations are easier to manage.
Disadvantages of drip irrigation
- Does not offer frost protection (as sprinklers do).
- Plastic drip-lines and sub-mains may be attacked by rodents and small animals.
- Requires regular flushing (to clear off the dirt collected near the ends of the drip lines) supervision.
- High skill is required in the design, installation, operation and maintenance.
Note: Sprinkler and drip systems fall under a category known as pressurized irrigation system