Irrigation Systems

Precipitation is the principal source of water, part of whichpercolates underground and a part flows over the surface as runoff.Thus there are two main sources of water for irrigation viz., surfacewater and groundwater. The rivers carrying surface water may be snowfedor perennial capable of feeding the offtaking canals throughout theyear or they may carry seasonal supply and the canals taking off fromthem may carry water for seasonal irrigation only. Furthermore, thesource of water may be at such a level that it can command the area bygravity flow or alternatively the water may have to be lifted to thedesired level to irrigate the fields. The design, equipment andtechnique of replenishing the soil water deficit by applying irrigationwater is referred to as irrigation system.

Gravity Irrigation

Gravityor flow irrigation is the type of irrigation in which water isavailable at a higher level so as to enable supply to the land bygravity flow. In flow irrigation water is supplied to the fieldsthrough the canals offtaking from headworks. Gravity flow irrigation ischeaper compared to lift irrigation. The gravity irrigation is furtherclassified as under:

Perennial Irrigation: In this system assuredsupply of water throughout the crop period to meet irrigationrequirement of the crops is made available to the command area throughstorage

of water done at the dam or diversion of supply made bymeans of headworks at the offtake point of the canal. Perennialirrigation may be either direct or indirect, as follows:

Directirrigation. In direct irrigation system water is directly diverted fromthe river into the canal by construction of diversion weir or barrageacross the river without attempting to store water. This method ispractised where the river has adequate perennial supply to feed thecanal system at all times of crops periods.

Indirect irrigation. Itis also termed as storage irrigation. Here water is stored in areservoir during monsoon period by construction of a dam across theriver for supply into the offtaking canals. Evidently indirectirrigation is adopted where the river is non-perennial or flow in theriver is inadequate during lean period. Storage irrigation has greaterirrigation potential than direct irrigation but is costly due to thecost of construction of dam.

Non-Perennial irrigation. Also calledrestricted irrigation. Canal supply is generally made available innon-monsoon period from the storage in small dams as in Kandi areaswhich is inadequate to feed all the year round, and/or canal water isnot required during monsoon due to adequate rainfall in the commandarea.

Well irrigation is essentially a type of lift irrigation extracting water from the groundwater reservoir. An open well is essentially a vertical hole of 2 to 6 m diameter and of shallow depth. It has small discharge capacity, usually 0.004 cumec. It draws water from one pervious stratum only. The contribution of water is from the bottom and sides in the case of an unlined well, and from bottom only (not sides) in a lined well. It is preferable that the well penetrates to such a depth below water table that there is a water depth of 3 to 4 m during a dry year. From economic standpoint, the depth of open well is limited to 30 m.

Yield of an open well can be increased to a considerable extent by boring 8 to 10 cm diameter hole in the centre of the well penetrating into the lower rich water bearing formation. A suitable arrangement consists of digging open well up to a clayey or kankar layer (mota formation) which shall support the well steining and a bore hole made in the centre of well to tap the lower sandy strata.

The type of well depends on local ground conditions and availability of equipment. Open wells may be either dug, bored, drilled or driven. Large-sized wells which have to be lowered to shallow depths through comparatively soft soils are usually dug manually or by semi-mechanical means. Radial wells are a variation of the dug wells.

In lift irrigation water is lifted from a river or a canal to the bank to irrigate the lands which are not commanded by gravity flow. Lift irrigation is being increasingly practised in India. In every State such areas exist where irrigation can be extended only by lift canals. Lift irrigation also includes tubewell irrigation but the latter is not feasible in areas where scarcity of water exists, climate is dry and groundwater is low, i.e., groundwater is in insufficient quantity and of unsuitable quality. Lift canal then constitutes the only means of extension of irrigation to such perched lands. A lift canal can cater for much larger areas than a tubewell and is suitable where supplies either from a river or a canal are available for lifting to higher elevation.

Lift area is defined as the area the level of which is too high to permit irrigation by gravity flow

from the source, but which can be irrigated by lifting water to the necessary level by means of pumps. Gross 1ft area is the portion of gross irrigable area which can be irrigated only by pumping.

Surface water may be lifted from a reservoir, a river, or from a gravity flow canal. From a river, water may be lifted by installing pumps either on the ground or on floating barges. Floating barge installation envisages economical and expeditious execution of the project for lift irrigation. Moreover, the pumping installation can be made mobile by rowing to the next pumping site after completing irrigation at one installation. In a canal, water may be lifted at the point of the offtake of the lift distributary or inbetween the length of a canal wherefrom high tract to be irrigated starts. Fig. 3.3 shows the longitudinal section of a lift canal.

Tanks on local streams form a significant source of irrigation especially in the peninsular areas in the States of Karnataka, Maharashtra and Tamil Nadu. Tank irrigation belongs to the category of storage irrigation. Tanks are small sized reservoirs formed by small earthen embankments to store runoff for irrigation. The site is selected within a watershed protected by vegetation and containing minimum of cultivated land so as to ensure minimum rate of sedimentation which lowers its storage capacity. Adequate soil conservation measures are essentially adopted to ensure quantity and quality of water inflow into the tank. The essential components of irrigation tanks are tank embankment, surplus or escape weir, and outlet sluice. A suitable breaching section is also sometimes provided to ensure that the tank embankment is not overtopped in the event of excessive discharge from the catchment. The breaching section is a low level embankment of a certain length designed to have a localised breach to escape excessive inflow.

Irrigation tanks are classified, as under, according to the nature of supply of water:

1,. System tanks: The system tanks get assured supply from nearby rivers or canal system and as such they may not have their own catchment.

2. Non-system tanks: Also called ‘isolated’ tanks. The non-system tanks depend on the runoff from their own catchment. They are not connected to any other tank.

3. Grouped tank: The grouped tanks, as the name implies, consist of a series of tanks connected together such that outflow from the upper tank is stored in the lower one for irrigation.

They usually have limited depth of 5 to 10 m and fill up two or three times in the rainy season and redistribute to some extent the available supplies and tide over breaks in the monsoon. The maximum flood discharge from the catchment is usually determined from Ryve’s formula. The length of escape weir is worked out from the formula, Q = CLIP’2, where C is a coefficient of discharge with value 1.66 for broad crested weir and 1.84 for short crested weir. Water losses from tanks are enormous as they usually have more wetted area for the given storage capacity. The water losses due to evaporation and absorption are taken as 1.75 m per year in Southern India and about 1.25 m in Mumbai.

It envisages construction of a series of small dams or bandharas 1.25 to 4.5 m high across a river or stream to raise water level locally to feed offtaking canal and passing downstream excess supply over the top of bandhara which forms overflow crest. The supply downstream can be further arrested by constructing other bandharas in series depending on the availability of command area at each bandhara site. Usually one canal takes off from a bandhara and the supply into the canal is regulated by a head regulator. The length of canal is usually less than 7 km. Once the bandhara system is developed, the. cultivators can maintain it. It is practised in Maharashtra state.

It is an economical type of inigation. The area served by a bandhara may vary from a few hectares to 400 hectares. The area irrigated is termed as Thai which is divided into a number of parts called phad each of size 8 to 40 ha. A phad may contain a number of fields owned by various cultivators. In a particular year same kind of crop is grown in a particular phad. The other phad may have other type of crop. The area near the outlet is irrigated first and then the next until tail-end field is reached.

Site selection: The various considerations which govern selection of a site for a bandhara are: (i) Adequate supply, preferably perennial, (ii) Below the confluence of a tributary, (iii) Good foundation at reasonable depth at site to facilitate construction of bandhara, (iv) River section straight, narrow and well-defined, (v) High river banks at site, (vi) Command nearest to site, (vii) Accessibility of site, (viii) Good command of the offtaking canal, and (ix) Economical.

Inundation irrigation is done by a canal taking off from a river in flood without any diversion work. It depends on the periodical rise in water level of the river and the supply is drawn through open cuts in the river bank or creeks which are called heads. Owing to changes in the river course, the heads have often to be changed. A regulator is, however, provided at the canal about 5 km downstream from the offtake, where the discharge passing below in the canal is controlled and the surplus supply is escaped back into the river. Inundation canals usually flow only during the summer months and bring in large quantity of silt beneficial to crops.

Selection of Offtake Point: The various factors which govern the selection of offtake point are (i) Site which is least likely to silt, (ii) Site on a creek of suitable size is preferred, since it is more constant in discharge than main river and also subject to milder attacks by floods, (iii) Site at the tail end of a creek is preferred as the flow there has less silt and is less liable to changes, (iv) Site on concave curve towards the downstream is preferred for minimum silt entry. A part of concave reach remains always within the range of outside curvature for low as well as high river discharges, otherwise a site in a straight reach free from erosion upstream is suitable, (v) Site with stable and high river banks is suitable, (vi) A back water reach also offers a good site, (vii) Wider river section at the offtake point which embodies two benefits, low velocity in the river and as such low silt charge in the water drawn in the canal, and low variation in water level, and (viii) If other conditions are favourable, a site close to the command area is preferred.

Design of Inundation Canal

The bed level in the channel is kept low enough to draw about half the full supply discharge of the canal in low river, i.e., in the middle of April and again in October, and narrow enough to limit the excessive high floods to the minimum possible. Full supply level is fixed with due consideration to the steady water level in the river during about 1½ months, usually fair irrigation season. Beyond the head regulator, design considerations of unlined canals apply equally to the inundation canals. However, higher values of silt factorf and water surface slope are adopted than those allowed for canals offtaking fràm permanent headworks. Generally the slope ranges between 0.20 and 0.25 rn/km. Manning’s, Chezy or Lacey formulae may be used for the design of inundation canal.