TYPES OF IRRIGATION

Irrigation MethodsThere are three principle methods of irrigation viz. surface, sub surface and aerial, overhead or sprinkler irrigation.

A.Surface irrigation : 

4 types of irritation :

(1) Flooding,
(2) Bed or border method (Saras and flat beds),
(3) Basin method (ring and basin) and
(4) Furrow method (rides and furrows, broad ridges or raised beds)

Flooding: It consist of opening a water channel in a plot or field so that water can flow freely in all directions and cover the surface of the land in a continuous sheet. It is the most inefficient method of irrigation as only about 20 percent of the water is actually used by plants. The rest being lost as a runoff, seepage and evaporation. Water distribution is very uneven and crop growth is not uniform. It is suitable for uneven land where the cost of leveling is high and where a cheap and abundant supply of water is available. It is unsuitable for crops that are sensitive to water logging the method suitable where broadcast crops, particularly pastures, alfalfa, peas and small grains are produced.

Bed or border method (Sara and Flat beds or check basin): In this method the field is leveled and divided into small beds surrounded by bunds of 15 to 30 cm high. Small irrigation channels are provided between two adjacent rows of beds. The length of the bed varies from 30 meters for loamy soils to 90 meters for clayey soils. The width is so adjusted as to permit the water to flow evenly and wet the land uniformly. For high value crops, the beds may be still smaller especially where water is costly and not very abundant. This method is adaptable to most soil textures except sandy soils and is suitable for high value crops. It requires leveled land. It is more efficient in the use of water and ensures its uniform application. It is suitable for crops plant in lines or sown by broadcast. Through the initial cost is high requires less labour and low maintenance cost. This may also be called a sort of sara method followed locally in Maharashtra but the saras to be formed in this method are much longer than broader.

Basin irrigation: This method is suitable for orchids and other high value crops where the size of the plot to be irrigated is very small. The basin may be square, rectangular or circular shape. A variation in this method viz. ring and basin is commonly used for irrigating fruit trees. A small bund of 15 to 22 cm high is formed around the stump of the tree at a distance of about 30 to 60 cm to keep soil dry. The height of the outer bund varies depending upon the depth of water proposed to retain. Basin irrigation also requires leveled land and not suitable for all types of soil. It is also efficient in the use of water but its initial cost is high.

There are many variations in its use, but all involve dividing the field into smaller unit areas so that each has a nearly level surface. Bunds or ridges are constructed around the areas forming basins within which the irrigation water can be controlled. Check basin types may be rectangular, contour and ring basin.

Furrow method (rides and furrow, broad ridges, counter furrow etc.): Row crops such as potatoes, cotton, sugarcane, vegetable etc. can be irrigated by furrow method. Water is allowed to flow in furrow opened in crop rows. It is suitable for sloppy lands where the furrows are made along contours. The length of furrow is determined mostly by soil permeability. It varies from 3 to 6 meters. In sandy and clay loams, the length is shorter than in clay and clay loams. Water does not come in contact with the plant stems. There is a great economy in use of water. Some times, even in furrow irrigation the field is divided into beds having alternate rides and furrows. On slopes of 1 to 3 percent, furrow irrigation with straight furrows is quite successful. But on steeper slopes contour furrows, not only check erosion but ensure uniform water penetration.

B. Subsurface method:

Subsurface irrigation or sub-irrigation may be natural or artificial. Natural sub surface irrigation is possible where an impervious layer exists below the root zone. Water is allowed in to series of ditches dug up to the impervious layer, which then moves laterally and wets root zone.

In artificial sub surface irrigation, perforated or porous pipes are laid out underground below the root zone and water is led into the pipes by suitable means. In either case, the idea is to raise the water by capillary movement. The method involves initial high cost, but maintaince is very cheap. There is a risk of soil getting saline or alkaline and neighboring land damaged due to heavy seepage.

It is very efficient in the use of water as evaporation is cut off almost completely. The plant roots do not suffer from logging, there is no loss of agricultural land in laying out irrigation system and implements can be worked out freely. This method is however rarely noticed in our country but followed in other countries like Israel.

C. Drip or trickle irrigation:

It involves slow application of water to the root zone. The drip irrigation system consist of

1) Head
2) Main line and sub line
3) Lateral lines
4) Drip nozzles.

The head consists of a pump to lift water and produce the desired pressure (about 2.5 tmosphere) and to distribute water through nozzles. A fertilizer tank for applying fertilizer solution directly to the field along with the irrigation water and filter which cleans the suspended impurities in irrigation water to prevent the blockage of holes and passage of drip and nozzles

Mains and sub mains are normally of flexible material such as black PVC pipes. Laterals or drip lines are small diameter flexible lines (usually 1 to 1.25 cm diameter black PVC tubes) taking off from the mains or sub mains. Laterals are normally laid parallel to each other. Lateral lines can be up to about 50 meters long and are usually 1.2 cm diameter black plastic tubing. There is usually one lateral line for each crop row. By laying the main line along the center line of the field, it is possible to irrigate either side of the field alternately by shifting the laterals. A pressure drop of 10 percent is permitted between the ends of lateral.

Drip nozzles are also known as emitters or values and are fixed at regular intervals in the laterals. These PVC values allow water to flow at the extremely slow rates, ranging from 2 to 11 liters per hour and they are of different shapes and design.

The spacing between laterals is controlled by the row-to-row spacing of the crop to be irrigated. Drip laterals laid on soil surface are buried underground at the depth of 5 to 10 cm..

D. Sprinkler or overhead irrigation:

This method consists of application of water to soil in the form of spray, somewhat as rain. It is particularly useful for sandy soils because they absorb water too fast. Soils that are too shallow, too steep or rolling can be irrigated efficiently with sprinklers.

This method is suitable for areas having uneven topography and where erosion hazards are great.

In sprinkler irrigation, water is conveyed under pressure through pipes to the area to be irrigated where it is passed out through or sprinklers the system comprises four main parts
i.  Power generator
ii.  Pump
iii. Pipeline and
iv. Sprinkler

The power generator may be electrical or mechanical. A centrifugal pump may be used for suction lift up to 37 to 50 cm. A piston type pump is preferable where water is very deep. The pipe consists of two sections, the main line and the laterals.

The main line may be permanently buried underground or may be laid above ground, if it is to be used on a number of fields. The main pipes are usually made of steel or iron.

The laterals are lightweight aluminum pipes and are usually portable. The sprinkler nozzles may be single or double, revolving or stationery and mounted or riser pipes attached to riser. Each sprinkler head applies water to circular area whose diameter depends up on the size of water, which varies from ¼ to ¾ inch per hour is determined by selecting the proper combination of nozzles.

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