Irrigation

Course: B.Sc. Agricultural

Subject : Fundamental of Soil and Water conservation Engineering

Unit: 4

The major sources of water availableeither for agriculture or for humanconsumption is obtained from theprecipitation in the form of rainfall orsnowfall.

Run-off from precipitation drainsthrough streams and rivers or collectsin surface depressions forming tanks orponds

Sources of water:

Water of streams stored in reservoirsor is diverted directly through canalsystem for irrigation.

Run-off water stored in tanks orponds is also regulated for irrigationthrough suitable conveyance systemof rainfall is stored as a Groundwater.

It is only after the soil has absorbedwater to field capacity that waterstarts percolating down to watertable and adds to ground waterreservoir.

Functions of water:

Ecological importance:

The distribution of vegetation over thesurface of earth is controlled by theavailability of water than any other singlefactor. In heavy rainfall area, flushvegetation (forest) is observed.

Physiological importance:

The ecologically importance of water isresult of physiological importance.

It is a constituent of protoplasm:

Water is as important quantitatively asqualitatively Constituting 80 to 90 Percent offresh weight of most herbaceous plant partsand 50 percent of the fresh weight of woodyplant.

It is a very good solvent:

Water acts as a solvent in which gases,minerals (plant Nutrients) and other solutesare dissolved. The dissolved plant nutrientsare absorbed by Plant through soil solution. Itacts as a carrier of food nutrients.

It is a reagent:

Water acts as a reagent in many important processes,such as photosynthesis and hydrolysis of starch andsugar.

It maintains turgidity of plant:

Maintenance of turgidity is essential for cellEnlargement and growth. Turgidity is also importantin opening of the stomata, movement of leaves,flower, petals etc.

It controls the temperature of plant and soil.

It is a major part of plant body

Classification of soil water

Hygroscopic water (Water of adhesion)

Capillary water (water of cohesion)

Gravitational water

Water Vapour

Hygroscopic water:

It is that part of soil water which is very tightly heldon the surface of soil particles in very thin film byadsorption forces such as adhesion and cohesion.

It is mostly in vapor form and forces with which it isheld on surface of soil particles is estimated about10,000 atmosphere towards the inner side and about31 atmosphere at the outer side of hygroscopic waterfilm.

This water is considered to be unavailable water formost of field crops.

Capillary water:

The water which is held in the soil between fieldcapacity and hygroscopic coefficient in micro poresand tension between 0.1 to 31 atmosphere.

This water moves from the area of higher tensionto area of lower tension.

When soil particles absorb water even after thehygroscopic coefficient is reached, additional wateris also held around the particles in the form thinfilm.

The plant food nutrients are dissolved in it andtherefore, it is most useful water for plants.

Water in excess of the field capacity is termed asGravitational water.

This water starts moving as free water throughthe macropores and it is called gravitationalwater.

It is that part of soil water, which moves freely inresponse to gravity and drains out of the soil.

This water has little use for plants.

Gravitational water is loosely held at less then0.1 to 0.3 atmosphere tension.

Gravitational water:

In this category, the water ispresent in gaseous form in the soilatmosphere but it is not directlyused by plants and is therefore, notimportant unlike the first threekinds.

Vapor form:

Absorption and Movement of Water in Soil

The movement of water from thesoil surface into and through thesoil is called water intake. It is theexpression of several factorsincluding infiltration andpercolation.

1. Infiltration:

Infiltration is the term applied to the process ofwater entry into the soil generally (but notnecessarily) through the soil surface and verticallydownward.

This process is of great practical importance since itsrate determines the amount of run-off over the soilsurface.

In other words, infiltration refers to the entry ofwater in to the soil surface. Infiltration is a surfacecharacteristic of a soil.

2. Infiltration rate:

It is the maximum rate of which water entersfrom the surface to the soil when flow is non-diversent.

Initially the infiltration rate is more butafterwards it decreases because the soil getswet.

3. Percolation : Downward movement ofwater through saturated pores.

Compactness of soil surface: A compactsoil surface permits less infiltration whereasmore infiltration occurs from loose soil surface.

Impact of rain drop: The force (speed) withwhich the rain drop falls on the ground is saidto be impact of rain drop. Ordinary size variesfrom 0.5 to 4 mm in diameter. The speed ofraindrop is 30 ft per second and force is 14times its own weight.

Factors affecting the rate of infiltration:

Soil cover: Soil surface with vegetative cover hasmore infiltration rate than bare soil because sealingof capillary is not observed.

Soil Wetness: If soil is wet, infiltration is less. Indry soil, infiltration is more.

Soil temperature: Warm soil absorbs morewater than cold soils.

Soil texture: In coarse textured soils, infiltrationrate is more as compared to heavy soils. In coarsetextured soil, the numbers of macro-pores aremore.

Depth of soil: Shallow soils permit less water toenter into soil than too deep soils.

Types of Pumps

Basically four principles involved in pumping

water:

Atmospheric pressure.

Positive displacement

Centrifugal force

Movements of columns of water caused by the difference in specific gravity.

Pump Characteristic:

Capacity

Static suction head

Static discharged Head

Total Suction Head

Total Discharge head

Friction head

Pressure head

Velocity Head

Net positive suction Head [NPSH]

Maximum practical Suction lift of pump

Water horse power [WHP]

Shaft horse power [WHP]

Brake Horse power

Efficiency

A. Centrifugal Pumps

Definition:

It may be defined as one in which an impeller rotatinginside a close fitting case draws in the liquid at centerand by virtue of centrifugal force throws out the liquidthrough an opening at the side of casing.

Centrifugal pumps are most widely used in irrigationpractice. They are simple in construction, easy tooperate, low initial cost and produce a constant steadydischarge. This type of pump is well adapted to usualpumping services such as irrigation, water supply andsewage services.

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A centrifugal pump is rotary machinesconsisting of two basic parts:

1. Rotary element or impeller

2. Stationary element or casing

Principles of Operation:

B. Vertical Turbine Pumps:

Vertical turbine pump [deep well turbine pump] isvertical axis centrifugal or mixed flow type pumpcomprising of stages which accommodate rotatingimpellers and stationary bowls possessing guidevanes.

These pumps are used where the pumping water levelis below the limits of Volute centrifugal pump. Theyhave higher initial cost and are more difficult to installand repair. The pressure head developed depends onthe diameter of impeller and the speed at which it isrotated. The pressure head developed by singleimpeller is not great. Additional head is obtained byadding more bowl assemblies or stage.

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C. Submersible Pumps:

An vertical turbine pump close coupled to asmall diameter submersible electric motor istermed as “submersible pump”.

The motor is fixed directly below the intakeof the pump.

The pump element and the motor operateunder submerged condition.

It can be used in very deep tube well where along shaft would not be practical.

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Irrigation water measurement

Weir :General Requirement for the Setting andOperation of Weirs

Properly constructed and installs weirs proved mostaccurate devices for measuring flow.

However improper setting and operation may resultin large errors in discharge measurements withweirs.

1. The weirs should be set at lower end of a longpool sufficiently wide and deep to five smoothflow having velocity less than 15cm/sec.

2. Baffles may be put in weir pond to reducevelocity.

3. The weir wall must be vertical [not leaning toupstream or down stream].

4. The center line of the weir should be parallel tothe direction of flow.

Precautions necessary in the use of weirs

5. The crest of weir should be level so thatwater passing over it will be of same depth atall point along the crest.

6. Notch should be of regular shape and itsedge must be rigid and straight.

7. The weir crest should be above the bottomof the approach channel.

8. The crest of weir is placed high enough sothat water will fall freely below weir.

1. Weirs are not always suitable for measuringflow.

2. They are not accretes unless proper conditionsare maintained

3. They require a considerable loss of head whichis mostly not available in channels on flat grads.

4. Weirs are not suitable for water carrying silt

5. Weirs are not easily combined whit turnoutstructures.

Limitation in the use of weirs:

Parshall flume

The Parshall flume is a fixed hydraulic structureoriginally developed to measure surface water andirrigation flows.

The Parshall flume is now frequently used to measureindustrial discharges, municipal sewer flows, andinfluent / effluent at wastewater treatment plants. Ofall the flumes, the Parshall flume is the mostrecognized and commonly used.

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Development of the Parshall flume began in 1915 byDr. Ralph L. Parshall of the U.S. Soil ConservationService.

Using the sub-critical Venturi flume as his startingpoint, Dr. Parshall introduced several radicalmodifications with his Improved Venturi Flume – themost greatest of which was a drop in elevation throughthe throat of the flume. The drop producedsupercritical flow through the throat of theflume. With supercritical flow, only one headmeasurement is necessary to determinee the flow rate,greatly simplifying the use of the flume.

Development of the Parshall Flume

Submerged orifices

An orifice used as a measuring device is a well-defined, sharp-edged opening in a wall or bulkheadthrough which flow occurs.

7 kPa pressure

sensor

200 kPa

pressure sensor

Pressure

reducing

valve

Shut off

valve

Flow

control

valve

Orifice

Volumetric

Detector

For irrigation use, orifices are commonly circularor rectangular in shape and are generally placedin vertical surfaces, perpendicular to the directionof channel flow.

A submerged orifice and the same orificedischarging freely have nearly the samecoefficient of discharge.

Submerging an orifice provides the capability tomeasure relatively large flows with a small dropin water surface, conserving delivery headcompared with weirs.

However, the submerged orifice requires headmeasurements upstream and downstream.

Drip Irrigation

It is termed as trickle irrigation also and canbe define as slow and frequent application ofwater to the plant root zone under lowpressure.

The drip irrigation is an advanced method ofirrigation suitable for water scarcity areas. Ituses the water very economically and efficient.It applies the correct quantity of water to theplant at the root zone through a network oftubing’s.

The design of drip system is simple. Theserequirements are pipes, accessories and otherequipment.

This method can be very well adopted for widespaced crops like Coconut, Mango, Sapota, Banana,etc

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Drip Irrigation System - Components and their Function

A drip irrigation system consists essentially of mainline, submains, lateral, drippers, filters and other small fittings andaccessories like valves, pressure regulators, pressure gauge,fertilizer application components etc.

1. Filter: It is the heart of drip irrigation. A filter unit cleans the

suspended impurities in the irrigation water so as to preventblockage of holes and passage of drip nozzles. The type offiltration needed depends on water quality and emitter type. Atwo-stage filter unit is usually needed.

a) Gravel Filter (Sand Filter):

These filters are effective against inorganicsuspended solids, biological substances and otherorganic materials.

Gravel filter consist of small basalt gravel or sand(usually 1-2 mm dia) placed in cylindrical tank, madeof metal.

Water enters form the top and flows through thegravel while leaving the dirt in the filter.

The clean water is discharge at the bottom.

b) Screen Filter:

These are installed with or withoutgravel filter, depending upon quality ofwater.

The screens are usually cylindrical shapeand are made of non-corrosive metal orplastic material.

c) Disc Filter:

The filtration elements are groovedplastic disc, which are piled togetheraround a telescopic core, acceding to thedesired degree of filtration.

Both sides of the discs are grooved andthe grooves cross each other when piledup and frightened together.

The main line conveys the water fromfiltration system to the sub main.

They are normally made of rigid PVC pipesin order to minimize corrosion andclogging.

Their diameter is based on the system flowcapacity. The velocity of flow in mainsshould not be greater than 1.5 m/s

2. Main Line:

The Submain conveys the water mainlineto the laterals. They are also buried inground below 2 to 2.5 ft and made of rigidPVC.

The diameter of Submain is usually smallerthan main line.

There may be number of Submain fromone mainline depending upon the plot sizeand crop type.

3. Submain:

Laterals are small diameter flexible pipes ortubing made of low density polyethylene(LDP) or liner low density polyethylene(LLDPE) and of 12 mm, 16mm, and 20 mmsize.

Their colour is black to avoid the algae growthand effect of ultra- violet radiation.

They can withstand the maximum pressure of2.5 to 4 kg/cm2. They are connected toSubmain at predetermined distance.

4. Laterals:

It is the main component of Drip irrigation systemfor discharging water from lateral to the soil. Theyare made of plastic, such as polythene orpolypropylene.

The drippers can be classified according to workingprinciple, discharge, type, structure, workingpressure, designation, durability, regulated and nonregulated discharge.

The main principle when planning a dripper is toachieve the minimum discharge with maximum sizeof water passage.

5. Emitters or Drippers:

6. Controls Valves (Ball Valves):

These are used to control the flow through particularpipes. Generally, they are installed on filtration system,mainline, and on all Submain.

They are made up of gunmetal, PVC cast iron and theirsize ranges from ½” to more than 5”.

7. Flush Valve:

It is provided at the end of each sub main to flush outthe water and dirt’s.

8. Air Release Cum Vacuum Breaker Valve:

It is provided at the highest point in the main line torelease the entrapped air during the start of thesystem and to break the vacuum during shut off.

It is also provided on Submain if Submain length ismore.

9. Non Return Valve:

It is used to prevent the damage of pump from flowof water hammer in rising main line.

10. Pressure Gauge:

It is used to indicate the operating pressure of the dripsystem.

12. Fertilizing System:

It is used to add the chemical irrigation water;however, fertigation is not free of hazards. Chemicalsadded to water may be toxic human begins andanimals so, safeguard must be taken to prevent backflow of irrigation water into the water source, whichmight be used for drinks purpose. Only water-solublefertilizers should be used to minimize the clogginghazard.

1. Maximum use of available water.2. No water being available to weeds. 3. Maximum crop yield. 4. High efficiency in the use of fertilizers.5. Less weed growth and restricts population of potential hosts. 6. Low labour and relatively low operation cost. 7. No soil erosion.8. Improved infiltration in soil of low intake. 9. No runoff of fertilizers into ground water. 10.Less evaporation losses of water 11. Improves seed germination. 12. Decreased to tillage operations.

Advantages of Drip Irrigation:

In spite of the fact that drip irrigation has so manypotential benefits , they’re a certain limitation also,there are as follow:

1. Sensitivity to clogging

2. Moisture distribution problem

3. Salinity hazards

4. High cost compared to furrow.

5. High skill is required for design, install andoperation.

Disadvantages of Drip Irrigation:

Sprinkler Irrigation:

It is a overhead irrigation, which spray thewater through nozzles under pressure over thesoil and crop.

In sprinkler method of irrigation, water isspread into the air and allowed to fall on theground surface somewhat resembling rainfall.

The spray is developed by flow of water underpressed through small orifices of nozzles.

The pressure is usually obtained by pumping.

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Sprinkler irrigation can be used for almost all crops (exceptrice and jute).

It is not usually suitable in very fine textured soils (heavyclay soils), where infiltration rates are less than 4 mm perhour.

The method is particularly suitable to sandy soils that havehigh infiltration rates.

Adaptability of Sprinkler Irrigation:

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Components of the Sprinkler System

1. Pumping Set:

The pump usually lifts the water from the sourcesand pushes it through the distribution system andthe sprinkler.

2. Main Line:

Main lines may be permanent or portable.

Permanent lines are used where crop requires fullseason irrigation and steel pipes or PVC pipes areused for permanent main line.

Light weight aluminum pipe with quick couplers areused for most portable mainline.

3. Lateral Lines:

The lateral lines usually are portable quickcoupled aluminum pipe are usually available inlength of 5 m, 6 m or 12 m. Each length has quickcouplings.

4. Sprinkler Heads:

It is the most important components of sprinklerirrigation system its operating characteristicsunder optimum water pressure and climaticconditions mainly the wind velocity willdetermine its suitability and the efficiency of thesystem.

5. Debris Screen:

Debris screens are usually needed whensurface water is used as the source ofirrigation. The function of screens is to keepthe system free of trash that might plung thesprinkler nozzles.

6. Desilting Basins:

Desilting basin may be required to trap sand orsuspended silts where the water comes fromstreams, open ditches or well water having silt.

7. Booster Pumps:

Booster pump should be used when a sprinklerirrigation system is used with an existingpumping system installed in a well and the pumpcapacity is insufficient to force the water throughsprinklers.

8. Take off Valves:

There are generally needed to control thepressure in the lateral lines. The valves shouldalways be used in systems where there aresignificant differences mainline pressure at thevarious laterals take off points.

9. Flow Control Valves:

Are used to regulate the pressure and discharge ofindividual sprinkler and may be helpful unevenness of theground causes an unequal distribution of pressure alongthe lateral.

10. Fertilizer Application:

Soluble chemical fertilizer can be injected in to thesprinkled system and applied to the crop.

When applied with the sprinkler system the fertilizer iseasily placed at the desired depth in a soluble and readilyavailable form to plants without any danger of beingleached away.

The two operations, namely irrigation and fertilizerapplication are done simultaneously, thus saving of labourrequired for fertilizer application.

Type of Sprinkler Systems

Sprinkler systems are of the two major types, on thebasis of the arrangement for spraying irrigation water.

1. Rotating head system

2. Perforated pipe system

Based on portability, sprinkler system are classified into5 types.

3. Portable system

4. Semi- portable system

5. Semi-permanent system

6. Solid set system

7. Permanent system

1. Suited to complete range of topographies and fielddimensions.

2. High irrigation efficiency due to uniform distribution of water.

3. Accurate and easy measurement of water applied.

4. Land leveling is not essential.

5. Soluble fertilizer, herbicides and fungicides can be applied inthe irrigation water

6. No interfere with the movement of farm machinery.

7. Can be used to protect to crop against high temp

8. Easy to operate, operator may be trained quickly.

9. Sprinklers are also used to irrigation high valued plantationcrops like, coffee, cardamom and orchards.

Advantages of sprinkler irrigation:

1. It requires high initial investment.

2. Power requirement is usually high since sprinklersoperate with more than 0.5 kg/cm² water pressure.

3. Fine textured soils that have low infiltration rate cannot be irrigated efficiently in host windy area.

4. Loss of water due to evaporation from the areaduring irrigation.

5. The water must be clean and free of sand, debris andlarge amounts of dissolve salts.

6. Ripening of soft fruit must be protected from thespray.

Limitation of sprinkler irrigation:

Reference/Sources

1.https://lh6.ggpht.com/NySdYH8AK5yTml1-A_CbIz1Y4eh4iBB-VJsdsLgJxHzSFFHaPTSKZMwA8I2HwIultezu6A=s113

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Text book and web sources

- www.agriinfo.in

- ecourses.iasri.res.in

- Soil erosion and Conservation by R. P. Tripathi and H. P. Singh