The design, the equipment and the technique of replenishing the soil-water deficit by applying irrigation water is referred to as “irrigation system”. The system adopted for irrigation must ensure a uniform distribution of water in the root-zone of a crop and high efficiency of water application i.e. the ratio of water stored in the root-zone to that delivered to the field, should be the maximum. There should be the minimum or no wastage of waste either through surface run-off or through deep percolation below the root-zone of a crop. The method used should be inexpensive and economically justifiable.
Several systems of irrigation are in vogue to suit different types of crops, topography, soil types, water resources, climatic conditions and costs. These systems are: surface-irrigation system, sprinkler irrigation system, subsoil-irrigation system, and drip-irrigation system.
Surface-irrigation system. In the surface-irrigation system, water is directly applied to the surface of the soil and is spread by gravity flow incidental to the slope of the land. There are several methods in this system, the commonest being flooding from a ditch, check basin, ring and basin, border strip and furrow. For irrigation with the surface system, fields are laid out every time before the crops are sown, since these layouts are destroyed during preparatory tillage. In order to achieve a higher water-application efficiency in the surface system of irrigation, the fields must be leveled as well.
The check-basin method of irrigation is widely practiced in India, as it is well suited to all irrigable soils and to a variety of crop. Checks, rectangular or square, with sizes, varying from about10 to 100 square meters or even more, are used. The basins are leveled in both directions. Slopes up to two to three per cent can be irrigated by using this method with a good control on irrigation water and high water-application efficiency. On steeper slopes, this method can be used after proper terracing. The limitations of this method are that it has too many ridges which not only occupy the land but also hinder interculture and harvesting with mechanical means.
The ring and basin method is commonly used for widely spaced orchard crop to irrigate individual trees or groups of trees. The important consideration in the case of the ring and basin method is to wet the soil only around the tree and not the entire land, thus economizing on the water.
In the case of the border-strip irrigation method, the field is divided into long narrow strips, with small paralleled ridges on the sides. The strips are 3 to 10 meters wide, and across the width the strip is leveled perfectly. The length of a strip ranges from 50 to 300 meters or more, depending upon the slope, the rate of water-intake of the soil and the stream size. This method is also suited well to all irrigable soils and to closely spaced rowed crop and even to pasture crop. Slopes up to seven per cent can be irrigated when the pasture crop are grown. On steep slopes, this method can be used by proper terracing or trenching along the contours. For laying out border strips, the land needs to be graded uniformly to achieve a high water-application efficiency. Larger flows are required for irrigating border strips than those in the case of other layouts.
In furrow irrigation, water is applied to the field in furrow between ridges and the top of the ridge is not directly wetted. Crop, such as sugarcane, cotton, tobacco, potato, some vegetables and other widely spaced row crops, are irrigated by using the furrow method. The furrows can be along the slope when the level of the land is sloping gently up to about three per cent. When the slope exceeds three per cent and is up to about fifteen per cent, the furrows are laid out in graded contours. Water distribution can be controlled well to achieve uniform application and the consequent high efficiency. The length of the furrows varies with the soil type, the slope and the quantity of water to be applied, and may vary from 10 meters to 1,000 meters for different situations. Under limited water resources, alternate furrow irrigation is adopted to save water. Small furrows known as corrugations are sometimes prepared in the case of border-strip method to increase efficiency and the uniformity of overflooding from the ditch method. Corrugations can be made by using an implement called ‘corrugator’. This implement is suited to close-growing crops grown on a medium type of soil with the rather uneven topography.
Sprinkler irrigation system. Sprinkler irrigation is a system in the case of which water pressure is applied to the surface of any crops or soil in the form of a thin spray from above. This method is advantageous, as water can be applied at any controlled rate and uniform distribution and high efficiency can be ensured. This method can be adopted in the case of all crops and is very popular in the case of cash and some orchard crops. The sprinkler system is specially suited to shallow sandy soils of uneven topography, where leveling is not practicable, and in areas where labor and water are scarce. On some soils with salinity problems, the sprinkler system is advocating for leeching of salt more effectively, emergence and to secure quicker and better growth of plants. The sprinkler system is also used for cooling the crops during high temperatures and for frost control during freezing temperatures. The pesticides, weedicides and fertilizers have also been applied successfully by adopting the sprinkler irrigation system.
A typical sprinkler system consists of a pump to lift and convey water under pressure, pipes or tubing for conveyance of water the sprinkler heads or nozzles, the risers which connect the sprinkler heads with the pipeline. Based on the equipment with which spraying is done, the sprinkler system has been classified as rotating-head type and perforated-pipe type. The sprinkler system is also classified on the basis of the portability of the equipment as (i) portable, (ii) semi-portable, and stationary or permanent. The stationary type is more expensive than the portable one. In the portable and the semi-portable systems, again there are manual (hand-moved) systems or power-moved systems. These are self propelled sprinkler systems which move laterally or radially around central-pivot feeding-line. These portable systems can be designed to cover any area ranging from 3 to 4 hectares to 50 60 hectares or more. There are boom type sprinkler systems which employ one boom sprinkler head on each lateral. These systems irrigate an area of 75 to 100 meters radius, depending on the nozzle size and the pressure. The boom-type sprinklers are commonly used for tall crops and orchards.
In India, the portable systems, which are hand operated, are available commercially. They involve an investment 15-20 thousand rupees to install a system for irrigating five to ten hectares in one crop season.
Subsoil-irrigation system. In the subsoil-irrigation system, water is applied into a series of field ditches deep down to the impervious layer. It then moves laterally and then vertically through capillaries and saturates the root-zone. A continuous supply of moisture in the crop conditions created by the ponding of irrigation water on the impervious layer. In artificial sub-irrigation, which is also known as sub-surface irrigation perforated or porous pipes are laid underground in the vicinity of the root-zone and water under pressure is distributed through these pipes. This system is adaptable on soils having an impervious layer at a suitable depth in the sub-stratum. The initial investment and maintenance are prohibitive. The system is, however, very efficient, since the water losses through evaporation from the wet surface can be minimized. This system is practiced at present in Gujarat and Kashmir for growing cash crop on the sandy-loam soils.
Drip-irrigation system. Drip-irrigation, also termed ‘trickle irrigation’, involves the slow application of water, drop by drop, as the name signifies, to the root-zone of a crop. The method was initiated in Israel and is now being tried in other countries. In this method, water is used very economically, since losses due to deep percolation and surface evaporation are reduced to the minimum. This method is, therefore, very much suited to arid regions and is being followed for irrigating orchard crops at present. The successful growing of orchards even on saline soils has been made possible by the drip system irrigation. The system can also be used for applying fertilizers in solution.
The equipment consists of a pumping-unit to create a pressure of about 2.5 kg per square cm, pipe-lines which may be of PVC tubing with drip type of nozzles or emitters, and a filter unit to remove the suspended impurities in the water. The amount of water dripping from the nozzles can be regulated, as desired, by varying the pressure at the nozzles, and the size of the orifice of the nozzles. Work is in progress in India to design, and adapt drip irrigation to conditions in this country. The initial high cost of the equipment and its maintenance are the major limitations of this system. it may, however, work out to be cheaper than the sprinkler system, especially for orchards and other widely spaced crops.
Water-lifting devices. There are several devices for lifting water from low-level water reservoirs, such as well, lakes and rivers.
Earthen pots, swinging buckets, wooden scoops, the Archemedian screw and picota are some of the ancient devices worked with manual labor and are well known in this country. They are suitable for low lifts. The discharge varies from 6,000 to 12,000 liters per hour. The mhote and the Persian-wheel have also been used in India for centuries. They are operated with animal power and can lift water from depths of 20 meters or so, with a discharge varying from 4,000 to 12,000 liters per hour. Horizontal and turbine pumps worked with petrol, diesel or electric motors, are the commonest present-day devices for raising water for irrigation. Wind velocities are not high enough throughout the year in India to raise water with wind-mills.
The power of a pump (hp), required to lift water, depends on the quantity of water, the lift involved, the efficiency of the pump and the frictional losses in the pipe-line. It is given by the relationship,
Hp = (62.4 x Q x H)/(550 x E)
Where, hp is the brake horse-power of the pump;
62.4 is the weight in lb of the cubic foot of water;
q is the discharge in cubic feet per second;
H is the total head in feet;
550 is the foot-pounds per second;
and E is the efficiency of the pump.
The total head (H) is the difference in the elevation between the water levels and the area to be irrigated plus the fractional losses in the pipe-line. It represents the suction height plus the delivery height plus the resistance in the pipe due to friction expressed in feet. Where belt-drive or gears are used, the efficiency (E) is the product of the pump efficiency and the belt-drive or gear efficiency .
The installation of pumps should receive careful attention. The pump should be located as near the water source as possible. It should be free from floods and inundation hazards. The foundation for the base plate should be strong and rigid to absorb vibrations. The pump and the driving-shaft should be perfectly aligned and should be checked periodically. The suction and delivery pipes should be supported adequately to avoid strain on the pipe casing. The piping should be adjusted with the minimum of bends. The pump should be protected from excessive pressure, and for the prevention of the back-flow of water by installing in the discharge line a non-return valve and the gate-regulating valve. A foot valve should be attached to the suction pump to facilitate priming.
Some of the important relations and equivalents in pump operations are as follows :
1 cubic foot of water = 62.4 lb
1 cubic foot per second = one cusec = 448.83 gallons per minute
= 0.99 acre-inch per hour = 100 tons
1 acre-inch of water = 101 tons = 102.8 cubic metres
= 22,660 gallons
1 cubic metre = 35.32 cft = 220 gallons = 1,000 litres
1 horse-power = 550 foot-pounds per second = 33,000 foot-pounds per minute = 746 watts
Irrigation practices comprise three interlinked components, viz. the quantity of water at each irrigation, the interval between two irrigations and the total number of irrigations in the life-span of the crop.
The total number of irrigations received by a crop is governed by the availability of water, the needs of the crop, and the profitability of that crop.