Mechanics of Tillage and Traction

LESSON 9. Concept And Definition In Minor Irrigation

9.1 Cultivable Command Area (CCA):

The area which can be irrigated from a scheme and is fit for cultivation.

9.2 Cultivable area:

It consists of net area sown, current fallow, fallow lands, other lands, current fallow, culturable waste and land under miscellaneous tree crops.

9.3 Gross Irrigated Area:

The area irrigated under various crops during a year, counting the area irrigated under more than one crop during the same year as many times as the number of crops grown and irrigated.

 9.4 Net Irrigated area:

It is the area irrigated through any source in a year for a particular crop.

 9.5 Irrigation Potential Created (IPC):

The total gross area proposed to be irrigated under different crops during a year by a scheme. The area proposed to be irrigated under more than one crop during the same year is counted as many times as the number of crops grown and irrigated. If original Irrigation Potential of the scheme is not known then the maximum area irrigated during the past five year or so may be taken as the IPC.

 9.6 Irrigation Potential Utilised (IPU):

The gross area actually irrigated during reference year out of the gross proposed area to be irrigated by the scheme during the year.

 9.7 Minor Irrigation (M.I.) Scheme:

A scheme having CCA up to 2,000 hectares individually is classified as minor irrigation scheme.

9.8 Medium Irrigation Scheme:

A scheme having CCA more than 2,000 hectares and up to 10,000 hectares individually is a medium irrigation scheme.

 9.9 Major Irrigation Scheme:

A scheme having CCA more than 10,000 hectares is major irrigation scheme.

 9.10 Sprinkler Irrigation System:

Sprinkler Irrigation is a method of applying irrigation water which is similar to rainfall. Water is distributed through a system of pipes usually by pumping. It is then sprayed into the air of entire soil surface through spray heads so that it breaks up into small water drops which fall to the ground.

 9.11 Drip irrigation system:

Drip irrigation system delivers water to the crop using a network of mainlines, sub‐mains and lateral lines with emission points spaced along their lengths. Each dripper/emitter, orifice supplies a measured, precisely controlled uniform application of water, nutrients, and other required growth substances directly into the root zone of the plant.

 9.12 Centrifugal Pump:

The most common type of pump. Typically the pump is close‐coupled to an electric motor, that is, the pump is mounted right on the end of the motorʹs drive shaft and the pump case is bolted straight into the motor so that it looks like a single unit. The water typically enters the pump through a suction inlet centered on one side of the pump, and exits at the top. Almost all portable pumps are end‐suction centrifugals. If the pump is not one of the next two types, then chances are it is an end‐suction centrifugal. This type of pump needs to be installed on a pad above the high water level if pumping from a lake or river.

 9.13 Submersible Pump:

Submersible pumps are installed completely underwater, including the motor. The pump consists of an electric motor and pump combined in a single unit. Typically the pump will be shaped like a long cylinder so that it can fit down inside of a well casing. Although most submersibles are designed to be installed in a well, many can also be laid on their side on the bottom of a lake or stream. Another common installation method for lakes and rivers is to mount the submersible pump underwater to the side of a pier pile (post). Submersibles do not need to be primed since they are already under water. They also tend to be more efficient because they only push the water, they do not need to suck water into them. Most submersible pumps must be installed in a special sleeve if they are not installed in a well, and sometimes they need a sleeve even when installed in a well. The sleeve forces water coming into the pump to flow over the surface of the pump motor to keep the motor cool. Without the sleeve the pump will burn up. Because the power cord runs down to the pump through the water it is very important that it be protected from accidental damage

9.14 Turbines and Jet Pumps:

A turbine pump is basically a centrifugal pump mounted underwater and attached by a shaft to a motor mounted above the water. The shaft usually extends down the center of a large pipe. The water is pumped up this pipe and exits directly under the motor. Turbine pumps are very efficient and are used primarily for larger pump applications. They are typically the type of pumps used on municipal water system wells. When you see a huge motor mounted on its end over a well, that is most likely a turbine pump. I use turbine pumps for large parks and golf courses where we are pumping from lakes. The turbine pump is mounted in a large concrete vault with a pipe connecting it to the lake. The water flows by gravity into the vault where it enters the pump. The pump motors are suspended over the vault on a frame. A jet pump is similar to a turbine pump but it works by redirecting water back down to the intake to help lift the water.

9.15 Water body:

All natural or artificial units with some or no masonary work used for storing water for irrigation or other purposes are covered under water body. These are usually of various types known by different names like tank, reservoirs, ponds and bundhies etc. The distinction among various types of water body, however, is not very explicit.

9.15.1 Pond:

A small body of water usually earthen though masonry dykes are also included and shallow made through excavations which represent a restricted environment. Ponds usually describe small bodies of water generally no one would require a boat to cross.

9.15.2 Tanks:

A shallow water unit usually larger than a pond created by constructing earthen or masonry barricades which receives water either from tube wells or rains.

9.15.3 Reservoirs:

A large man made impoundment of varying magnitude created by erecting, bunds, dams barrages or other hydraulic structures across streams or rivers serving one or more purposes such as irrigation, power generation, flood control or other water resource development projects.

9.16 GROUND WATER SCHEMES

9.16.1 Dug well: It covers ordinary open wells of varying dimension dug or sunk from the ground surface into water bearing stratum to extract water for irrigation purposes. These are broadly masonary wells, kuchcha wells and dug‐cum‐bore wells. Most of such schemes are of private nature belonging to individual cultivator. The parameter of the well ranges between 2 to 6 m and the depth between 8 and 15 meter. Their CCA generally varies from 1 to 2 hectares.

9.16.2 Shallow tube well: It consists of a bore hole built into ground with the purpose of tapping ground water from porous zones. In sedimentary formations depth of a shallow tube well does not exceed 60-70 meters. These tube wells are either cavity tube wells or strainer tube wells. These are usually drilled by percussion method using hand boring sets and sometimes percussion rigs. Success and popularity of the scheme depends on how cheap they are. A coir structure formed by binding coir strings over an iron frame is being used as strainer. In shallow water table areas, bamboo frames are also used. Sometimes steel pipe casing are replaced by pipes constructed by rapping bituminised gunny bags over the bamboo frame. These are called bore wells, in which bore hole is stable without a lining in the bottom portion and a tube is inserted only in the upper zone. The tube wells are generally operated for 6 to 8 hours during irrigation season and give yield of 100-300 cubic meter per day, which is roughly 2 to 3 times that of a dug well. Their CCA may go upto 15 hectares.

9.16.3 Deep tube wells: It usually extends to the depth of 70 meter and more and is designed to give a discharge of 100 to 200 cubic metre per hour. The deep tube wells are drilled by rotary percussion or rotary cum percussion rigs. These tube wells operate round the clock during the irrigation season, depending upon the availability of power. Their annual out put is roughly 15 times that of an average shallow tube well and are usually constructed as public scheme which are owned and operated by government departments or corporations. Their CCA may go upto 50 hectares.

9.17 SURFACE WATER SCHEMES

9.17.1 Surface flow irrigation scheme:

These schemes use rainwater for irrigation purposes either by storing it or by diverting it from a stream, nala or river. Some times, permanent diversions are constructed for utilising the flowing water of a stream or river. Temporary diversions are also constructed in many areas which are usually washed away during the rainy season. The small storage tanks are called ponds or bundhis which are mostly community owned. The command areas of such schemes are 20 hectares or less. The large storage tanks whose command varies from 20 to 2000 hectares are generally constructed by government departments or local bodies. These are the biggest items of surface minor irrigation works.

(i) Storage schemes (Tanks and other storages)

Storage schemes include tanks and reservoirs which impound water of streams and rivers for irrigation purposes. After wells, tanks occupy a very important place under the minor irrigation programme. They provide nearly two third of the total irrigation from minor sources in the states of Andhra Pradesh, Karnataka, Kerala, Maharashtra, Orissa and Tamilnadu. Tracts with undulating topography and rocky substrata are eminently suitable for tank irrigation. Besides, there exists scope for further construction of tanks in many areas. A large number of existing tanks in southern States have gone into disuse due to long neglect of repairs. Renovation of these tanks so as to restore the lost irrigation potential is being accorded priority under the minor irrigation programme.

The essential features of these schemes are

a. a bund or a dam which is generally of earth, but is also sometimes partly or fully masonary,

b. anicut and feeder channels to divert water from adjoining catchments,

c. a waste weir to dispose of surplus flood water,

d. sluice or sluices to let out water for irrigation, and

e. conveyance and distribution system.

 The size of the storage is determined by the runoff expected on the basis of dependable monsoon rainfall in the catchment and by the fact whether the rainfall and cropping pattern would permit more than one filling of the tank.

(ii) Diversion schemes :

These schemes aim at providing gravity flow irrigation by mere diversion of stream water supply without creating any storage. As compared to storage schemes they are economical but their feasibility is dependent on the presence of flow in the stream at the time of actual irrigation requirements. Essentially such schemes consist of

a. an obstruction (weir) or bund constructed across the stream for raising and diverting water; the weir being called anicut in the South, bandhara in Maharashtra and Gujarat, and Bandh in the Assam region, and

b. an artificial channel, known as kul in the hilly areas, pyne in Chhota Nagpur and Bihar and dong in the Assam region.

 In case of small schemes which have prominent scope in the hilly tracts and foot hill plains, the water is usually diverted by constructing temporary bunds across the streams, made up of earth, stones or even bamboos. The discharge handled being of small order, the bund on the head of the channel is not provided with any gated structure for controlling and regulating the flow. Construction of work, is, therefore, simple and cheap and can be handled to a large extent by the people themselves. However, these constructions being temporary, require frequent renovation. The bunds are liable to be washed away by every major flood. The channels also get silted up and scoured frequently. It is essential that whenever such schemes aim at diverting higher discharges, say more than 5 to 10 cusecs, or tackle streams having high intensity of flood discharge, proper regulation structures equipped with suitable types of gates are provided. Weir has to be provided with scouring sluices in order to regulate the flow of silt in the offtaking channels. The construction of masonary weir is comparatively simpler and cheaper where rocky foundation is available beneath the streambed. The design of the weir on permeable and erodable foundation is more complicated and requires specialised engineering knowledge. The irrigation capacity of the diversion schemes is dependent on the actual flow in the stream at the time the irrigation is required. The cold weather and the hot weather flow, therefore, need to be ascertained carefully before deciding the feasibility and economics of these schemes. This is particularly important in the case of non-snow fed flashy streams that spurt to lift suddenly in the rainy season after which the discharge in them dwindles down to appreciable quantity. Some diversion schemes are also constructed as kharif or monsoon channels supplying water only during the monsoon season. Such schemes are useful for providing supplemental irrigation for paddy and preliminary watering for sowing of rabi.

In most of the hilly tracts, small irrigation channels called ‘kuls’ are the only means of irrigation. These channels carry water diverted from streams by constructing temporary or pucca bunds across the streams. These channels are often constructed in hazardous hilly terrain under very difficult conditions. To avoid seepage of water and for the sake of stability, these channels are lined in most of the reaches.

(iii) Water conservation cum ground water recharging Schemes:

Under this head are included schemes which serve primarily one or more of the following purposes:

a. submerging agricultural land during monsoon for sowing post monsoon crops,

b. improving moisture regime of the adjoining fields down stream for raising of post monsoon crops without irrigation and replenishing the ground water.

An additional advantage of these schemes is that they help to conserve the soil. When constructed in the head water region serving catchment area of tanks down below, they serve the important purpose of retarding the silting rate of these tanks. The system of water conservation through field embankments is peculiar to central Indian tracts and is commonly in vogue in the northern Madhya Pradesh, Bundhelkhand region of Uttar Pradesh and eastern Rajasthan.

In the Bundhelkhand region, these works are popularly known as ‘bundhies’, which consist of earthen embankments thrown across gently sloping ground. During the rainy season, water is stored upstream and the land gets submerged. If the land slope is gradual, often large areas get submerged even by low embankments. Ordinarily, no direct irrigation is carried out and benefit is mostly due to submergence. In nearly all these areas, the soil is generally black which is retentive of moisture. After remaining submerged under water during the rainy season, the soil retains sufficient moisture to grow good rabi crops. The remaining water is let out and the submerged land released for cultivation. The other advantage of submerging land in this manner is that the first flood brings a lot of silt which acts as rich manure. By preventing free flow of water across steep gradient, the soil of the land is also conserved.

Ahars in Bihar, which store water for irrigation of paddy fields also function somewhat in a similar manner. Water is let out in October for irrigating the rice fields and the drained out fields in the bed of the ahars are cultivated with rabi crops. The head water tanks popularly in vogue in Orissa have a similar role to perform. These consist of bunds put up across slope at the head of gullies with the objective of impounding and diverting the cumulative run off into the wider valley area down stream of the bunds by percolation, seepage and surface flow. Surface channels are provided in the flanks to carry floodwater received in excess of the storage capacity of the bunds during the monsoon season.

Percolation tanks primarily constructed for the purpose of recharging ground water are in vogue in Maharashtra, Tamilnadu, Kerala and Rajasthan. Check dams or rapats are in vogue in Rajasthan. They consist of bunds constructed across the streams for the purpose of retarding the surface flow and also the subsurface flow to some extent by making the bed slope of the stream flattened. This results in increased percolation of water in the sub‐soil with consequent increase of the ground water supply.

9.17.2 Surface Lift Irrigation Scheme:

In regions where the topography does not permit direct flow irrigation from rivers and streams, water has to be lifted into the irrigation channels. These works are similar to diversion schemes, but in addition pumps are installed and pump houses constructed. These schemes, being costly in operation, are feasible only in areas where

a. gravity flow irrigation is not possible

b. there is keen demand for irrigation and cultivators are enthusiastic,

c. water is available in the streams for at least about 200 days in a year, and

d. cheap electric power is available.

Installation of diesel operated pump sets for lifting water makes the operation and maintenance cost of these schemes exorbitantly high. However, for lifting small order of discharge by individual cultivators, portable diesel engine pump sets are feasible as they provide greater flexibility and mobility for installation at different points of the water source or sources. In some areas Solar Pumps are also used for lifting water. The CCA of such schemes may go upto 20 hectares.