Methods of Irrigation

Types of Irrigation Systems

Irrigation systems are designed to supply water to crops and vegetation so that plant growth is sustained with the minimum required water. Apart from agriculture, irrigation methods are also used for landscape maintenance, re-vegetation of disturbed soils, dust suppression, sewage removal and certain mining operations. Farming that relies only on direct rainfall is referred to as rain-fed or dry-land farming.

Techniques of Irrigation

In India, irrigated area is approximately 36 percent of the net sown area. Different parts of the country use different irrigation techniques. Methods differ primarily in how water taken from the source is delivered and distributed over the field. The general objective is to supply the field as uniformly as possible so each plant receives neither excessive nor deficient water. Common sources and systems used in India include wells, tanks, canals, perennial canals and multi-purpose river valley projects.

Surface Irrigation

Surface irrigation is the family of methods in which water flows over and across the soil surface by gravity. The hydraulics are influenced by many field and water-delivery variables and, therefore, achieving efficient surface irrigation depends on careful design and operation.

• Important factors affecting surface irrigation hydraulics: surface slope of the field, roughness of the field surface, depth of water to be applied, length of run and time required, size and shape of the water-course, discharge of the water-course, and field resistance to erosion.
• When a surface irrigation method is well chosen and properly managed, it should: store the required water in the root-zone depth; reduce runoff losses; minimise soil erosion; provide a uniform application of water; require relatively less manual labour; use minimum area for channels, furrows or strips; and permit use of machinery for land preparation, cultivation and harvesting.

Basin Irrigation

Basin irrigation is used where a field is level in all directions and surrounded by low bunds or dykes so that water can be applied in an undirected manner. If the inflow to that level field is undirected and uncontrolled, it is termed a basin even if the field has beds, ridges or furrows for particular crops. Basin irrigation is common for orchards and many field crops where temporary storage of water on the surface is acceptable.

Furrow Irrigation

Furrow irrigation supplies water in small trenches (furrows) dug between crop rows. Water flows along the furrow, usually by gravity, and infiltrates laterally and vertically into the root zone. Flow to individual furrows can be controlled, making this method suitable for row crops, tree crops and crops sensitive to inundation (for example tomatoes, potatoes and many vegetables). It is a long-established, economical and low-technology method.

• Five common furrow types (Surajbhan, 1978): sloppy furrow, levelled furrow, contour furrow, serial furrow and corrugated furrow.
• Advantages of furrow irrigation: large areas can be irrigated at once; labour savings once furrows are filled; relatively low cost; plants receive appropriate quantities of water without direct inundation.
• Drawbacks: ensuring uniform distribution along long furrows can be difficult; runoff losses may increase on sloping land; field levelling is often required to reduce bypassing and inequality; retention ponds or catchments may be used to capture runoff for reuse.

Free Flooding and Uncontrolled Flooding

Uncontrolled flooding describes situations where croplands are flooded without management aimed at efficient water use. This occurs where crop value is low or land is used for grazing or recreation. These fields are commonly small holdings and are not managed to the standards of larger irrigation systems.

Free flooding is an ancient and simple method used on smooth, flat topography. Water is applied to a plot from field ditches without checks or guided flow. The land is divided into plots of suitable size according to soil porosity. Irrigation proceeds from the higher end towards the lower end and is stopped when the desired depth is reached. Free flooding is economical and suited to irregular surfaces or where water is abundant, but it offers little control over flow and therefore cannot ensure high efficiency. Rapid flows may fail to meet soil moisture needs; slow flows may result in excessive infiltration or deep percolation losses.

Border Strip Method

Border strip irrigation divides a field into strips separated by low embankments (borders or levees). Typical strip widths are 10 to 15 metres and lengths vary from 90 m to 400 m. Water is diverted from a field channel into each strip and flows across the strip surface towards the lower end. Strip surfaces should be essentially level across their width with a general longitudinal slope from inlet to outlet. A longitudinal slope of 2 to 4 m per 1000 m is commonly recommended; where slopes are steeper, measures to control erosion must be provided.

Classification Based on Availability of Water

1. Gravity Irrigation

Gravity or flow irrigation supplies water to land by gravity from a higher level source such as a diversion weir, barrage or reservoir and is generally less expensive than lift systems. Gravity systems are commonly classified as perennial, non-perennial and inundation systems.

• Perennial irrigation: Water supply is assured throughout the cropping period. This can be achieved directly or indirectly.
• Direct (perennial) irrigation: Water is diverted directly from a river into a canal by a diversion weir or barrage without large storage. It is used where the river provides adequate flow during crop periods.
• Indirect (storage) irrigation: Water is stored in a reservoir behind a dam during the monsoon and released through canals during the crop period. Storage irrigation increases irrigation potential but is more expensive due to dam construction.
• Non-perennial (restricted) irrigation: Canal supply is available only part of the year, typically supplied from small storages; canals may be closed in the monsoon when rainfall provides sufficient water.
• Inundation irrigation: Water is taken from the flooded river without permanent headworks, using temporary heads or cuts in the river bank. Discharge is controlled by a regulator located downstream of the head. Inundation canals usually flow seasonally and carry silt that is agriculturally beneficial.
• Design notes for inundation canals: the bed level is kept low enough to draw half the design supply at low river levels, and channel width is chosen to limit excessive flood passage. Full supply level is set with reference to steady river levels during the main irrigation season. Design uses standard hydraulic formulae such as Manning's or Chezy's. Typical water surface slope values range from 0.20 to 0.25 m/km for inundation canals.
• Advantages of inundation canals: economical because no permanent headworks; silt carried is beneficial to soils.
• Disadvantages of inundation canals: high maintenance requirements; seasonal operation only.

2. Tank Irrigation

Tank irrigation uses small reservoirs (tanks) formed by earthen embankments on local streams and is especially important in the peninsular region (for example in Karnataka, Maharashtra and Tamil Nadu). Tanks store runoff for irrigation and are a form of storage irrigation.

• Site selection and catchment protection (vegetation and soil conservation) are important to reduce sedimentation and preserve storage capacity.
• Essential components: tank embankment, surplus or escape weir, outlet sluice and sometimes a designed breaching section to safely pass very large inflows without uncontrolled overtopping.

3. Lift Irrigation

Lift irrigation raises water by mechanical means (pumps) from a river, canal or well to lands that cannot be served by gravity. Lift systems are used where gravity supply is impossible or groundwater is inadequate in quantity or unsuitable in quality.

• Gross lift area: the gross irrigable area that can be irrigated only by pumping because its elevation is too high for gravity supply.
• Pumping installations commonly use electric or diesel pumps; electrical pumping is generally preferred, with diesel sets held as standby.
• Comparison (brief):
• Lift irrigation: relatively costly; less silt in water; operation depends on machinery; higher water rates; more complex.
• Gravity irrigation: cheapest means of irrigation; silt in water may be beneficial; no lifting equipment required; lower water rates; simple and economical.

4. Well Irrigation and Subsurface Irrigation

• Groundwater: often a more dependable source than surface water and is normally free from weed seeds and many plant pests. The first cost of well or tube-well installation is high. The ideal aquifer is a coarse gravel formation free from fine sand, although such formations are not always available. An aquifer is a saturated geological formation that yields sufficient water to wells or springs (commonly unconsolidated sand, gravel or fractured rock).
• Subsurface irrigation: water does not wet the soil surface; the water table or subsurface flows supply moisture to roots by capillarity. Two types:
• Natural sub-irrigation: leakage from channels or percolation raises the water table and irrigates lower lands naturally by capillary rise.
• Artificial sub-irrigation: drainage or open-jointed underground distribution systems are deliberately constructed to supply water to root zones by capillarity. This is generally expensive and used only in special cases with favourable soils and for high-value crops.

5. Sprinkler Irrigation Systems

Sprinkler irrigation applies water under pressure through a network of mains, submains and laterals to sprinklers or nozzles that spray or jet water over the field surface. Rotating sprinkler heads or fixed nozzles distribute water in patterns intended to approximate rainfall.

• System components: pump and headworks, mains and submains, lateral pipes and sprinkler heads or perforated pipes with nozzles. Perforated pipes operate at low heads; rotating heads may operate at low or high heads depending on design.
• Advantages: seepage losses from earthen channels are eliminated; only the required water is applied; land levelling is not essential; no agricultural area is lost to ditches (increasing cropped area by up to about 16%); application rates can be kept below infiltration capacity to avoid runoff; fertilisers can be applied through the system; salts are leached and waterlogging can be controlled; less labour intensive; system efficiencies up to about 80% are possible with good design and operation.
• Limitations: wind distorts sprinkler patterns causing non-uniform application; high evaporation losses in hot and windy climates; unsuitable for crops like paddy requiring large, frequent depths; high initial cost and technical skill required; pumps and machinery need clean (sand-free) water to prevent impeller/nozzle damage; requires significant electrical power; less effective on heavy soils with poor intake; continuous water supply is needed for commercial operation.

6. Drip (Trickle) Irrigation

Drip or trickle irrigation is a micro-irrigation method designed to apply water slowly and directly to the root zone of each plant. It is especially suited to areas with acute water scarcity or saline water problems because it minimises evaporation and deep percolation losses.

• System elements: a head (pump), mains, submains, laterals and drip emitters or small drop nozzles located at plant positions. Water oozes from emitters at a low, controlled rate directly into the root zone.
• Typical head pressure: around 2.5 atmospheres is commonly used to ensure consistent flow through the network and emitters (exact pressure depends on system design and emitter type).
• Fertigation and filtration: water is commonly passed through a fertiliser tank so that nutrients are delivered with irrigation (fertigation). A filter is essential to remove suspended particles and prevent emitter clogging.
• Drip irrigation advantages (summary): very high water-use efficiency; reduced evaporation and percolation losses; targeted application reduces weed growth and foliage wetting; suitable for orchards, vineyards, vegetable crops and high-value plantations; allows precise fertiliser application.

Each irrigation method has site-specific suitability that depends on topography, soil texture and infiltration, crop type, availability and reliability of water, capital and operating cost, energy availability, and management capacity. Proper choice, design and operation are essential to match a method to local requirements and to achieve economical and sustainable irrigation.